Frost and Freeze
Protection Workshop
Edited by: Dorota Z. Haman
Sponsored by:
University of Florida
Southwest Florida Water Management District
and
Alafia River Board,
Hillsborough River Board,
Manasota Basin Board
Fact Sheet HS-76
November 1995
Cold Protection by Irrigation: Dew Point and Humidity
Terminology1
Lawrence R. Parsons2
Various forms of irrigation have been used for
frost and freeze protection for many years. When
used properly, water can provide partial or complete
cold protection for a number of crops. On the other
hand, improper use of water can increase cooling or
ice loading and cause greater damage than if no water
were used at all.
Because water can provide
protection in one situation and cause damage in
another, it is important to know what principles are
involved. To better understand what can happen
when using water during a freeze, several commonly
used terms need to be understood. With a knowledge
of these terms, one can better evaluate the risks and
benefits and successfully use irrigation for cold
protection.
Heat of fusion - This is the heat that is released when
liquid water freezes to solid ice. The amount of heat
generated when water freezes is 1200 BTUs/gallon or
80 calories/gram of water frozen. As long as enough
water is continuously applied to a plant, the heat
generated when water freezes generally keeps the
plant at or near 32°F (O°C). This is the principle
used by strawberry, fern, or citrus nursery growers
when they apply high volumes of water by sprinkler
irrigation to protect their plants. At least 0.25
inch/hour or more is required for cold protection.
With very low temperatures, low humidity, or high
winds, more water must be applied to get adequate
protection. Many citrus nurserymen need to apply
water at rates of 0.35 inches/hour or higher.
Heat of vaporization - This is the heat lost when
water changes from a liquid to water vapor. At 32°F,
the heat of vaporization is about 8950 BTUs/gallon or
596 calories/gram of water evaporated. Note that the
heat of vaporization is about seven and one-half times
greater than the heat of fusion. This means that to
maintain a stable situation when both freezing and
evaporation occur, for every gallon of water that
evaporates, seven and one-half gallons of water need
to be frozen to balance out the heat in the grove.
Anything that promotes evaporation, such as low
humidity and high wind speed, will promote overall
cooling.
If the water application rate is high enough on the
trunk of a young tree, it will be protected by the ice
formation. However, on the edge of and outside of
the iced zone, temperatures will not be maintained at
32°F, and those parts will probably be damaged or
killed. Hence, usually the tops of young trees or
branches above the iced zone are more severely
damaged after a freeze.
Humidity - This refers to the amount of water vapor
in the air. There are various ways to express
humidity, but the most commonly used terms are
relative humidity and dew point temperature.
Relative humidity (RH) - This is the percentage or
ratio of water vapor in the air in relation to the
amount needed to saturate the air at the same
1.
This document is Fact Sheet HS-76, a series of the Horticultural Sciences Department, Florida Cooperative Extension Service, Institute of
Food and Agricultural Sciences, University of Florida. First published: October 1986. Reviewed: June 1993. Revised: November 1995.
2.
Lawrence R. Parsons, Professor, Horticultural Sciences, Citrus Research and Education Center, Lake Alfred, FL, Cooperative Extension
Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational
information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national
origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office.
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / Christine Taylor Stephens, Dean
Cold Protection by Irrigation: Dew Point and Humidity Terminology
temperature. Although commonly used, relative
humidity is not the best measure of humidity because
it depends on the air temperature. Warm air holds
more water vapor than cool air. For example, the
relative humidity could be 70% at 40°F or 70% at
90°F, but the amount of water vapor in the air would
be less at the cooler temperature even though the RH
values were identical.
Dew point temperature - This is the temperature at
which dew begins to form or the temperature at
which water vapor condenses to liquid water. It is
also the temperature at which air reaches water vapor
saturation. A common example of condensation is
the water that forms on the outside of a glass of ice
water. This happens because the temperature of the
glass surface is lower than the dew point temperature
of the ambient air in the room. Hence, some of the
water vapor in the surrounding air condenses on the
outside of the cold glass.
When referring to cold protection, dew point is
one of the better ways to describe the humidity or
amount of water vapor in the air. When the dew
point is below 32°F, it is often called the frost point
because frost can form when the temperature is below
freezing. The dew point is important on freeze nights
because water vapor in the air can slow the rate of
temperature fall. With a relatively high dew point on
a cool night, radiant heat losses from a grove are
reduced, and the temperature may be expected to fall
slowly. But if the dew point is quite low, the
temperature may be expected to fall rapidly. Water
vapor absorbs infrared radiation. Water droplets or
fog are an even more effective radiation absorber
than water vapor. Hence, fog can reduce the rate of
temperature drop on a frost night.
In addition to affecting the rate of radiation loss,
the dew point is often a "basement" temperature, and
the air temperature will not go much below it unless
drier air moves in. The reason for this is that when
dew condenses or ice forms, heat is given off. This
heat from condensation is the same as the heat of
vaporization (about 8950 BTUs per gallon or 596
calories per gram of water) because vapor is changing
to liquid water.
This heat release during
condensation slows the rate at which the air
temperature drops. If dew forms, water vapor is
condensed from the air, and the humidity or dew
point of that air is lowered. This is the way that the
evaporation coil in an air conditioner removes water
vapor and dehumidifies the air.
Page 2
Dew point temperatures are commonly higher on
the coasts than they are inland. In the central Florida
citrus belt (e.g. near Lake Alfred), dew point
temperatures on a moderate frost night can be in the
vicinity of 20 to 30°F. On more severe freeze nights,
dew point temperatures can be 10°F or lower. For
example, in the damaging Christmas, 1983 and
January, 1985 freezes, dew point temperatures in
Lake Alfred approached 5°F, which are exceedingly
low for central Florida.
Dry bulb temperature - This is the temperature of the
ambient air. This is the same thing as the normal air
temperature read with a grove thermometer.
Wet bulb temperature - This is defined as the lowest
temperature to which air can be cooled solely by the
addition of water. An example of the wet bulb
temperature is the temperature one would feel when
coming out of a swimming pool on a windy day. As
long as a surface is wet while in the wind, its
temperature will drop to the prevailing wet bulb
temperature of the air.
The wet bulb temperature is between the dew
point and dry bulb temperatures and normally closer
to the dry bulb than the dew point temperature.
When the air is saturated with water vapor, the
relative humidity is 100%, and all three temperatures
(dew point, wet bulb, and dry bulb) are equal.
Psychrometer - A psychrometer is a device used to
determine atmospheric humidity by the reading of two
thermometers, the wet bulb and dry bulb
thermometers. The wet bulb thermometer is kept wet
by a moistened sleeve. With a psychrometer, one
determines how much cooler the wet bulb is than the
dry bulb and then calculates humidity by using
appropriate graphs or tables. "Psychros" comes from
the Greek word meaning "cold," and hence a
psychrometer measures humidity by determining how
much colder the wet bulb thermometer is than the dry
bulb thermometer. An example of a psychrometer is
shown in Figure 1.
For an accurate reading, the wet bulb
thermometer must have air moving over it. With a
sling psychrometer, air flow is created by rotating the
two thermometers through the air by hand. With a
fan ventilated psychrometer, a fan blows air across the
two thermometers. Fan ventilated psychrometers cost
more than sling psychrometers, but they are more
convenient to operate on freeze nights.
Cold Protection by Irrigation: Dew Point and Humidity Terminology
Figure 1. Psychrometer
Sling psychrometers work well at temperatures
above freezing, but are more difficult to operate at
temperatures below freezing. The reason for this is
that at temperatures much below freezing, the water
on the wet bulb freezes, releases its heat of fusion,
and raises the wet bulb temperature to around 32°F.
Eventually, it is possible to get a "frost" wet bulb
temperature if one rotates the sling psychrometer
long enough. A battery powered fan psychrometer
avoids some of the problems of a sling psychrometer
because it may take 20 minutes or more to get a valid
wet bulb temperature when the air is below freezing.
A slightly different chart is used for humidity
calculations when the wet bulb sleeve has ice on it.
Page 3
Wind chill - This refers to the cooling effect of
moving air on a warm body and is expressed in terms
of the amount of heat lost per unit area per unit of
time. Wind chill was developed to estimate heat loss
rate from humans or warm blooded organisms. It
does not apply to plants or vegetation because they
are not warm blooded.
In a windy freeze,
temperature of a dry leaf is usually fairly close to air
temperature. If the leaf is wet and water is not
freezing on it, the leaf can theoretically cool to the
wet bulb temperature. Even though wind chill does
not apply to plants, wind can remove heat from a
grove rapidly. Hence, the length of time a grove will
be at low temperatures can be longer on a windy
night than on a calm one. Thus, more damage can
potentially occur during a windy freeze.
Conclusion
Humidity plays an important role in freezes,
particularly if one is using water for cold protection.
Water is a two-edged sword that can either help or
hurt when used during a freeze. An understanding of
humidity concepts and water principles can help a
person use irrigation for cold protection successfully.
Irrigation systems
Cold and freeze protection in
Florida
• Surface
• Sprinkler
• Microirrigation
Dr. Dorota Z. Haman
Agricultural and Biological Dept.
University of Florida
January 25, 2006
January 25, 2006
Frost versus Freeze
• Frost is a local condition which occurs in
your area on a still night, temperatures
usually go no lower than 29-30 degrees F,
and it warms up again the next day.
This stage can
tolerate 15 - 20 F
temperatures.
• A freeze involves an entire region, has
significantly lower temperatures, and may
last for several days.
This stage can tolerate 25 to 28 F.
January 25, 2006
January 25, 2006
Heat transfer concepts
Latent heat transfer
Conduction – Heat is transferred through the
material, i.e., through the molecules.
Convection – Transfer of heat by the movement of
masses of heated liquid or gas.
Radiation - Transfer of heat from one object to
another without the need for a connecting
medium.
Latent Heat Transfer – heat transfer due to a phase
change.
• Heat of fusion – Heat that is released when
liquid water freezes to ice; For each gram of
water frozen, 80 calories are released;
• Heat of vaporization – Heat lost when
water changes from liquid to vapor. At 32
ºF, the amount lost is 596 calories for each
gram of water.
January 25, 2006
January 25, 2006
1
Irrigation and cold protection
• When used properly, water can provide partial or complete
crop cold protection.
• Improper use of water can increase cooling or ice loading
and cause greater damage than if no water were used at all.
• It is important to know what principles are involved in
using water for cold protection.
January 25, 2006
January 25, 2006
Basics Meteorology
Relative humidity (RH)
This is the percentage or ratio of water vapor in the air in relation to
the amount needed to saturate the air at the same temperature.
Relative humidity depends on the air temperature. Warm air holds
more water vapor than cool air.
January 25, 2006
Dry bulb temperature and wet bulb
temperature
Dry bulb - the temperature of the ambient air. The same
thing as the normal air temperature read with a normal
thermometer.
Wet bulb – the temperature of a wet surface under the
same condition – the temperature is lower due to the
evaporation from the wet surface
Dew point temperature – temperature at which water
condenses is the air – usually, lower than wet bulb
temperature.
January 25, 2006
January 25, 2006
Dew point in cold protection
• Water vapor in the air absorbs infrared radiation
and can slow the rate of temperature fall.
• Fog can provide significant frost protection
• High dew point reduces radiant heat losses from a
plant and the temperature falls slowly.
• Low dew point is associated with rapid reduction
in temperature.
January 25, 2006
2
• Psychrometer - A psychrometer is a device used to
determine atmospheric humidity by the reading of two
thermometers, the wet bulb and dry bulb thermometers.
The wet bulb thermometer is kept wet by a moistened
sleeve. With a psychrometer, one determines how much
cooler the wet bulb is than the dry bulb and then calculates
humidity by using appropriate graphs or tables.
January 25, 2006
January 25, 2006
Psychrometer
Crop temperature
• A typical method of estimating crop temperature
is using the air temperature and adding a "safety
factor" of several degrees.
• This causes systems to be started before they
actually need to be, resulting in excess water and
energy use.
• Knowing exactly when to start and be able to wait
may allow to avoid protection completely.
From R. L. Snyder
January 25, 2006
January 25, 2006
Crop temperature measurements
• Thermocouples are temperature-measuring devices small
enough to be inserted into buds, blossoms or small fruit.
This stage can
tolerate 15 - 20 F
temperatures.
This stage can tolerate 25 to 28 F.
January 25, 2006
January 25, 2006
3
January 25, 2006
January 25, 2006
Inversion
• Happens on clear nights
• Temperatures drop significantly at the surface due
to radiation.
• The temperature in the lower atmosphere inverts
• The temperature increases with altitude to the top
of the air layer.
• The warm air in an inversion is important for
some frost protection
January 25, 2006
Radiation Frost
January 25, 2006
Advective Freeze
Winds less than 5 MPH
Winds higher that 5 MPH
Clear sky
May be cloudy
Cold air mass 30 to 200 ft
Cold air mass 500 to 5,000 ft
The wording of these warnings:
Inversion develops
Cold air in the low spots
•
White or black frost damage
Easier to protect
The National Weather Service (NWS) will issue warnings according to
the forecasted conditions (forecast minimum temperature is for 5 feet
above the ground inside a National Weather Service instrument
shelter).
•
Difficult to protect
•
January 25, 2006
If winds below 10 mph and minimum temperatures above or equal to
32oF are forecasted, a frost warning will be issued.
If winds below 10 mph and minimum temperatures below 32oF are
forecasted, a frost/freeze warning will be issued.
When winds above 10 mph and minimum temperatures below 32oF are
forecasted, a freeze warning will be issued.
January 25, 2006
4
Definition of frost/freeze warnings issued by National Weather Service.
January 25, 2006
Warning
Wind Speed
Air temperature
Frost
Below 10 MPH
Above 32oF
Frost/freeze
Below 10 MPH
Below 32oF
Freeze
Above 10 MPH
Below 32oF
January 25, 2006
Rules of freeze protection with
water
When to start?
To avoid damage under low dew point conditions,
sprinklers should be started at:
• 1.1°C (34°F) if the dew point is -4.4°C (24°F) or above
• 1.7°C (35°F) if the dew point is -6.7 to -5.0°C (20-23°F)
• 2.2°C (36°F) if the dew point is -9.4 to -7.2°C (15-19°F)
• Apply water uniformly
• Apply fast enough to keep ice wet all the
time
• Apply enough water to protect the plant
This recommendation should only be followed when a frost is predicted.
Sprinklers may be turned off when the wet bulb temperature has risen to 1.1°C (34°F).
January 25, 2006
January 25, 2006
Application Rate recommended for Cold protection
Under Different Wind and Temperature Conditions
MIN. TEMPERATURE
EXPECTED
0 to 1
WIND SPEED IN M.P.H.
2 to 4
5 to 8
Application rate (inches per hour)
27oF
0.10
0.10
0.10
27oF
0.10
0.10
0.14
26oF
0.10
0.16
0.30
24oF
0.12
0.24
0.50
22oF
0.16
0.30
0.60
18oF
0.20
0.40
0.70
15oF
0.26
0.50
0.90
January 25, 2006
Uniformity
• High system uniformity is necessary for
effective frost protection.
January 25, 2006
5
Uniformity of sprinkler irrigation
January 25, 2006
Emitter Spacing for High
Uniformity
Overlapping of wetting patterns is critical
for the good uniformity – this is especially
important for frost protection of row crops
like blueberries.
January 25, 2006
Uniformity of microirrigation
January 25, 2006
Sprinkler overlapping
• The overlap is extremely important for
frost/freeze protection
• Higher winds require closer spacing of
sprinklers
January 25, 2006
Inadequate selection of delivery pipe diameters
(submains, manifolds, and laterals.
Reasons for a Low Coefficient of
Uniformity in Sprinkler or
Microirrigation
January 25, 2006
Inadequate selection of sprinkler head and nozzle in
sprinkler irrigation or emitters in microirrigation.
All delivery lines (mains and submains) should be sized
to avoid excessive pressure losses and velocities.
– Excessive pressure losses result in poor application
uniformity.
– Excessive water velocities can create a water hammer
that can damage the delivery lines.
January 25, 2006
6
Ice forming indicator
Ice forming indicator
Wind speed greatly
reduces effectiveness
of sprinkler irrigation
for freeze protection.
The temperature of a
plant covered in ice
will drop below a dry
plant if the ice dries.
January 25, 2006
January 25, 2006
Ice forming indicator
The color of the ice
forming on plants is
very important. If the
system is properly
working, the ice will
be clear.
Primary Applications:
1/2" (13 mm) Full Circle, Brass, Wedge Drive Impact
Sprinkler
Features and Benefits:
•Patented, self-flushing wedge drive
•Durable brass die-cast arm
•Stainless steel springs and fulcrum pin
•Chemically resistant washers
•Two-year warranty
•Wedge drive runs on smaller nozzles and lower
pressures
•Corrosion and grit resistant
•Self-flushing design reduces wear from grit
•Built to last
Models:
•L20VH
Specifications:
•Bearing: 1/2" Male NPT, Plastic
•Trajectory Angle: 10 degrees
•Operating Range: 25-80 psi(1.7-5.5 bar)
•Flow Rate: .56-2.98 GPM(0.13-0.64 m 3 /h)
•Radius: 22-32ft.(6.71-9.9 m)
L20VH
1/2" Brass Impact Sprinkler
Used primarily in undertree permanent systems.
Excellent frost protection sprinkler giving superior
distribution patterns. Exceptionally strong drive due to
the patented self-flushing wedge drive arm which allows
for low precipitation rates required for tight and problem
soils.
January 25, 2006
January 25, 2006
Precipitation Rate for Selected Nozzle Capacity and Sprinkler
Spacing
14VH
1/2" Brass Impact Sprinkler
Used in undertree, overtree, overvine and row crop solid
set and permanent systems. Excellent frostprotection/irrigation sprinkler. Patented self-flushing
nylon "V" wedge makes it a strong-driving sprinkler,
even with the smallest nozzles.
January 25, 2006
Primary Applications:
1/2" Full Circle, Brass, Wedge Drive Impact Sprinkler
Features and Benefits:
•Patented, self-flushing wedge drive
•Durable brass die-cast arm
•Stainless steel springs and fulcrum pin
•Chemically resistant washers
•Two-year warranty
•Wedge drive runs on smaller nozzles and lower
pressures
•Self-flushing design reduces wear from grit
•Corrosion and grit resistant
•Built to last
Models:
•14VH
Specifications:
•Bearing: 1/2" Male NPT, Brass
•Trajectory Angle: 23 degrees
•Operating Range: 20-80 psi(1.4-5.5 bar)
•Flow Rate: .56-2.68(0.14-0.61 m 3 /h)
•Radius: 29-39 ft.(8.85-11.90 m)
Sprinkler
Spacing (ft)
Gallons per minute/sprinkler
2
3
30 X 30
.21
.32
30 X 40
.16
4
5
6
8
10
12
15
.24
.32
40 X 40
.18
.24
.30
40 X 50
.14
.19
.24
.29
50 X 50
.12
.16
.20
.24
.32
.13
.16
.19
.26
.32
.13
.16
.21
.27
.32
.14
.18
.23
.28
50 X 60
60 X 60
60 X 70
.34
January 25, 2006
7
Can microirrigation provide frost
protection??
Microsprayer Frost Protection in Vineyards
by
G. Jorgensen, B.M. Escalera, D.R. Wineman,
R.K. Striegler, D. Zoldoske and C. Krauter
CATI Publication #960803
Targeted microsprinklers were compared to conventional sprinklers in
a commercial vineyard during the springs of 1993 and 1994. Data
collected and presented here indicated that targeted microsprinklers
provided frost protection similarly to conventional sprinklers, but with
80 percent less water used.
January 25, 2006
January 25, 2006
Experimental design
Microsprayers were installed in every vine row
and mounted 0.56 m (22 in) above the cordon on
every other stake, approximately 3.6 m (10.5 ft)
apart. A two-hectare (5 ac) block of microsprayers
was compared to an adjacent sprinkler block. The
sprinkler block included a typical design and
installation for commercial coastal vineyards.
January 25, 2006
Frost Protection at Drip Flow Rates
The PULSATOR (the Wade Rain family of irrigation
products) is an innovative irrigation and frost protection
tool. Its low flow rate makes it the ideal choice for frost
protection in fields with drip irrigation. For new
installations, the PULSATOR's unique features make it a
good choice for both irrigation and frost protection.
January 25, 2006
Frost protection tested to below 26° F.
•Water savings - uses only 12-15 GPM per acre.
•Utilizes existing drip hydraulic system. No separate mains, valves or pumping system required.
•Direct conversion of existing drip system mains and submains possible.
•Water applied continuously to vines, not at 30-45 second intervals as with permenant
sprinklers.
•Reduces soil saturation and run-off.
•Allows access to field for equipment
•Easy installation, variety of mounting options.
•Unique Y-mount allows for irrigation of cover crops between rows.
•Ideal for IPM programs.
January 25, 2006
January 25, 2006
8
Can you change your regular
sprinkler system to provide frost
protection?
• Systems can be designed so that the nozzles or risers can
be quickly changed to allow higher rates of water to be
applied as needed however, these systems may have to
operate with a variable flow rate pumps and have the pipe
system and other components designed for higher flow
rates.
January 25, 2006
Double systems
• Sprinkler system for frost protection
• Drip line under mulch for efficient supplemental irrigation
and chemical delivery to the plant roots.
January 25, 2006
Other freeze protection methods
Heaters
• Trapping the heat that is radiating from the ground with
clear plastic or similar materials,
• Moving the plant to a sheltered location,
• Directly providing heat via light bulbs.
• Anti-transpirant sprays -- Cloud Cover is one brand -- will
provide a couple of degrees of protection if applied at least
a few hours before you expect frost.
• Keeping plants well-watered is important in freezing
weather. Container plants are especially vulnerable to the
desiccating effects of freezing.
January 25, 2006
January 25, 2006
Wind machines
January 25, 2006
Poly houses
January 25, 2006
9
Mulching
Irrigation - Soil Application
• It is estimated that wetting a soil prior to a freeze can
provide 2 degrees F of protection. The water contains heat
that is released, and a wet soil allows heat to be continually
drawn from lower depths during the night.
January 25, 2006
January 25, 2006
Ground maintenance
A weed-free, firm, moist soil can add 1 to 4
degrees of protection during a radiational
frost/freeze event. Soils that are dry, freshly
cultivated, or covered with live or dead grass give
the opposite effect. Growers should make every
effort to properly prepare the orchard floor in early
fall for maximum release of radiant heat during
freeze events. Mowing the orchard floor grass to a
height not exceeding 2 inches is recommended.
January 25, 2006
Sprays for frost protection
(not tested at UF)
• Materials acting as growth inhibitors, penetrating agents,
antitranspirants, nutritional sprays, bactericides, and
hormones have been used with some success in crops, such
as pear, apple and peach. However, the variability in
subtropical tree response to many of these materials has
limited commercial acceptance.
• Bacteria residing on cold-sensitive plants initiate ice
formation. The ice formed in or on plants spreads rapidly,
causing mechanical injury. By controlling the ice nucleating bacteria, tests have shown temperatures may
drop 2-4 ºF below critical levels without plants showing
damage.
January 25, 2006
Chemicals--Cryoprotectants and
Antitranspirants
Several commercially available forms of cryoprotectants
and antitranspirants have been studied for their ability to
provide freeze protection to fruit crops, and none has
proven effective thus far in providing consistent protection.
Some of the latest chemicals to be tested include
formulations of non-ice-nucleating bacteria used in foliar
sprays to nullify the effects of ice-nucleating bacteria.
These latter chemicals have shown promise, but no
recommendations are available yet.
January 25, 2006
Automatic frost monitoring and
alarm systems
Provide early warning by:
text message to your mobile
activation of:
sirens,
strobe lights and
irrigation systems
January 25, 2006
10
Be ready
•
•
•
•
•
•
•
•
Reinstall suction lines and check primers on the pumps.
Test and service the pumping unit
Replace filters and have spare filters available.
Treat diesel tanks for water and algae.
Check lines and sprinklers in the field for leaks and clogged
nozzles and proper rotation.
Check water pressure on ends of distant lines.
Make sure drainage in and around fields is adequate. Make sure
roadways around and through the field will withstand traffic at
night during irrigation ( the soil will be wet!).
Have a high-intensity spotlight ready to plug into the truck to
check sprinkler operation.
January 25, 2006
Be ready
• Have rain suits and boots available for everyone
responsible for checking the irrigation system.
• Have wires or drill bits available to unclog nozzles.
• Have tools and replacement parts that are necessary
to exchange nozzles and/or sprinklers.
• Put shielded minimum thermometers in cold,
average, and warm areas of fields.
• Hang some ribbons on trees or poles around fields to
detect slight breezes.
January 25, 2006
Please visit us at:
Be ready
• Identify a good source of agricultural weather
information and watch it closely.
• Consider subscribing to a weather service that issues
freeze warnings.
• Consider purchasing a monitor that calls you when
the temperature gets low.
• Consider purchasing a hand-held wind meter or
anemometer to measure wind speed.
• Consider purchasing a sling psychrometer to
measure wet bulb and dry bulb temperatures, relative
humidity, and dew point.
January 25, 2006
http/:waterconservation.ifas.ufl.edu
January 25, 2006
Thank you for your
attention
Questions???
January 25, 2006
11
Frost and Freeze Protection
Workshop
MISSION
John Jackson - IFAS Extension
Larry Treadaway – IFAS FAWN
Andy King – IFAS FAWN
To Provide Accurate & Timely
Weather Data to
A Wide Variety of Users
Locations
Current Data
Wind Direction
Select Those You Want
Graph Data
Minimum Temperature Estimator
“Brunt”
Brunt”
COLD PROTECTION TOOL KIT
Wet Bulb Shut Off
What is your critical
temperature?____
For help see Critical temperature determination
Now
SELECT A TOOL
Minimum temperature guidance
Forecast tracker for FAWN sites
Evaporative cooling potential
Wet bulb shut off time
Critical Temperature
Dormancy or Quiescence
Tissue – fruit/leaves
Experience
– Plant condition –
hurricane damage!!
– Variety/Rootstock
– Market
Ownership
Forecasts
National Weather Service
Private
– Media: TV, cable
– Service: web base, one to one
Directly from Internet
Extension service
Pin Point Forecasts
• NWS generated
• For an area 3 miles
square
• Can select by GPS or
point and click
Minimum Temperature Guidance
Select a NWS Fruit Frost Location or a FAWN Site
Select a County
To Obtain a National Weather Service 7 day forecast
The roll over number indicates how many NWS Fruit Frost Stations and FAWN sites are in the county
Seven Day Forecast
Hourly Track
Temperature Forecast Tracker for FAWN Sites
Select a FAWN Site
Evaluate down side risk
Pierson
Forecast Tracker
80
70
What is evaporative
cooling risk
– Wet bulb
– Wind
60
50
NWS
FAWN
Critical Temp
40
30
20
10
0
1
3
5
7
9
11
13
15
17
19
21
23
25
27
29
31
33
35
37
39
41
43
45
47
49
Easy choice when
forecast is below
critical temperature
Difficult when
forecast is for critical
temperature
Time
Evaporative Cooling Risk
Low
Maximum temperature difference between air temp and wet bulb =<1 F
Maximum wind speed =< 5 mph
Moderate
Maximum temperature difference between air temp and wet bulb 1 to 2 F
Maximum wind speed 5 to 8 mph
If wind <5 mph then move to Low: If wind >8 mph then move to Moderate
Strong
Maximum temperature difference between air temp and wet bulb 3 to 4 F
Maximum wind speed 9 to 12 mph
If wind <9 mph move to Mild: If wind > 12 mph move to Strong
High
Maximum temperature difference between air temp and wet bulb 5 to 6 F
Maximum wind speed 12 to 15 mph
If wind <12 mph move to Moderate: If wind >15 mph move to High
Extreme
Maximum temperature difference between air temp and wet bulb >6 F
Maximum wind speed >15 mph
Conclusion
Special thanks to SWFWMD
Cold Protection Tool Kit can save $
$ saved means water conserved!
For additional assistance
County Weather Watch
Provide feed back
– Pros and cons
– Improvements and problems
FAWN (Florida
automated weather network
http://fawn.ifas.ufl.edu
Chill/Frost/Freeze Protection
Bob Stamps, Ph.D.
University of Florida/IFAS
Department of Environmental Horticulture
Mid-Florida Research & Education Center
2725 S. Binion Road, Apopka, FL
rhs@ifas.ufl.edu
Mechanisms of cold damage
Freeze/frost protection
‘Extracellular freezing
‘Plant factors
– Water moves out of cell
• Desiccation ensues
‘Intracellular freezing
– Ice forms inside the cell
‘Environmental factors
• Fatal
Plant physiology
Cultural practices
‘Select more cold tolerant species and
‘Fertilizing
cultivars
– Example: Aglaonemas
• Silver Queen – damaged at 40ºF for 24 hours
• Stars – undamaged
– Complete balanced fertilization
– Reduced application rates in the fall
‘Watering
– Well-watered media prior to cold event
‘Pruning
‘Delay/prevent new growth
– Avoid fall pruning
– Reduce fertilization, especially nitrogen
– Avoid late summer/fall pruning
‘Plant health
– Disease- and pest-free plants
1
Cryoprotectants
Cryoprotectants
‘Any agent added to living tissue that
reduces susceptibility to cold injury, but
does not act as a growth regulator
– Antidesiccants
– Ice nucleation active (INA) bacteria inhibitors
• Pseudomonas erwinia, ice formation at 30ºF
• Treat with P. syringae
Adkar Cloud Cover
– Crystallization inhibitors
– Freezing point depressors
Cryoprotectants
Cryoprotectants
Ficus leaves killed (%)
Treatments
Hibiscus leaves killed (%)
28.4 ºF
26.2 ºF
24.8 ºF
Treatments
28.4ºF
26.2ºF
24.8ºF
Control
0
0
61
Control
0
0
30
Cryo-Tec
0
75
83
Cryo-Tec
0
0
40
Vapor Gard
0
0
22
P. syringae
0
7
67
3-18-18 (1%)
0
63
81
Cryo-Tec + P. syr.
0
0
88
3-18-18 (6%)
0
12
61
P. syr. + Cryo-Tec
0
0
80
Hummel, 1984
Cryoprotectants
Hummel and Teets, 1986
Black currant fruit set
‘DEPEG – dodecyl ether of polyethylene
Mean fruit set (%)
glycol
‘DMSO – dimethyl sulfoxide
‘GLY – glycerol
‘PVP – polyvinylpyrolidone
No frost
Frost
Control
81.2 a
59.0 b
DEPEG
80.7 a
79.8 a0.01
Wilson and Jones, 1980
2
Site selection for cold protection
Freeze/frost protection
‘Location
‘Techniques/methods suitable for use in the
– Good air drainage, no low spots
– Lee side of large water bodies
– Under canopies (oak trees, etc.) for radiation
freezes
– Have wind barriers for advective freezes
(fences, etc.)
– Group plants by cold sensitivity
open and non-enclosed structures (for
example, shadehouses)
‘Freeze/frost protection methods (except
those using heaters) in poly-clad
shadehouses and greenhouses
Heat transfer
Ways to reduce heat loss
‘Convection – Transfer of heat by the
‘Reduce convective heat loss (air flow)
movement of masses of heated liquid or
gas (e.g., air).
‘Radiation - Transfer of heat from one
object to another without the need for a
connecting medium.
‘Conduction – Heat is transferred trough
the material, i.e., through the molecules.
– Windbreaks, shelterbelts and crop covers
Shelter belts and windbreaks
Characteristics of shelter belts and
windbreaks
‘ Shelterbelt
‘Most effective if the porosity is between 48
– Plants (usually trees)
used to reduce wind
flow and possibly
serve as cut crops
• Usually consists of
multiple rows of
vegetation
‘ Windbreak
– Structures that reduce
wind speed
• Fences, walls, etc.
and 55%
– Less density is not as effective and greater
density protects less area
‘Should be 30–50 feet longer than the width
of the area being protected
‘Reduction of wind speed from 50–60%
over a distance of 4 to 7 times the height
on the lee side
3
Shelter belts
‘ Advantage
Windbreaks
‘ Disadvantages
– Reduced wind speeds
– Effective during
advective freezes
– Ability to serve as a
source of revenue
• Cuts, biomass, etc.
– Good longevity
– Ineffective (or
harmful) during
radiational freeze
events
– Loss of growing space
(less of a factor if
generating revenue)
– length
Crop covers/frost blankets
‘ Advantage
‘ Disadvantages
– Reduced wind speeds
– Effective during
advective freezes
– Little loss of space
– Rapid implementation
– Ineffective (or
harmful) during
radiational freeze
events
– Difficulty disposing of
materials
– Reduced longevity
compared to most
shelter belts
– Cost of materials and
construction
Crop covers
‘ Relatively
inexpensive but time
consuming to deploy
‘ Effectiveness depends
on thickness, porosity
and composition
Polypropylene film
Ways to reduce heat loss
‘Liners and coverings
‘Reduce radiational heat loss
– Barrier to radiation loss
• Mulch to protect below ground plant part
– Also provides weed control, moisture retention
• Mulch to protect above ground plant part
– Disease concerns, especially trunks
• Natural barrier using trees
• Crop covers/frost blankets, foams
• Icing of shadecloth
4
Crop covers (weight in oz/yd2)
Temperature
Reduce radiation loss to the sky
45
43
41
39
37
35
33
31
29
27
25
none
0.6 oz
1.5 oz
freezing
8:30
10:30
12:30
2:30
4:30
6:30
Time
Crop covers
Polypropylene film
‘The shorter the crop the easier to protect
‘ Glass and fiberglass
reinforced panels
block radiational heat
loss (95–99%)
‘ However,
polyethylene and
polypropylene do not
(unless water forms a
film on them)
– Wind speeds lower
– Heating less volume
‘If possible, avoid contact between covers
and plant foliage
Water for cold protection
Water for cold protection
‘Water applied using overhead irrigation
‘ Research on reducing water application rates
has been used since the 1960s to protect
crops from cold damage (Harrison et al.,
1974).
needed to cold protect crops in shadehouses was
started in the early 1980s (Stamps and Chase,
1981; Stamps and Mathur, 1982).
5
Heating/cooling values for water
Phase change
BTUs/gallon
Heat capacity
No
8.3/°F
Heat/fusion
Yes
1,200
Heat/vaporization Yes
8,100
Heat/sublimation
9,300
Yes
Using water for cold protection
- disadvantages
‘Requires large amounts of water
‘Can cause large, temporary declines in
water levels
Using water for cold protection
- disadvantages
Using water for cold protection
- disadvantages
‘Leaching of nutrients and pesticides
‘Weight of ice formed may damage plants
‘May cause saturation of soil in crop root
‘If not continuous, may increase damage
zone that leads to root death and disease
development
‘Is not always effective, especially for
protecting immature foliage
Why water?
Cold protection using water
‘ Inexpensive
‘ Easy to implement
‘ Effective in some
situations
‘ Even flowers can be
protected if supported
to prevent breakage
‘ If enough water is
being applied, the ice
will be clear.
Otherwise, the ice
will be cloudy.
6
Water for heating
Water applied in non-crop areas
‘Placement of water
‘Sprinklers and rewetting times
‘Dual irrigation systems for
screenhouses/shadehouses
Dew point in cold protection
Fog
‘ Water vapor in the air absorbs infrared radiation
and can slow the rate of temperature fall
‘ Fog can provide significant frost protection
‘ High dew point reduces radiant heat losses from
a plant and the temperature falls slowly
‘ Low dew point is associated with rapid reduction
in temperature
Fog
When to turn irrigation on?
‘Outside: When the wet-bulb temperature
approaches the critical temperature of the
crop being protected
– Conserves water, saves fuel ($), reduces
equipment wear and tear ($)
‘Inside: Before air temperatures reach the
critical temperature for the crop
7
Psychrometers
When to turn irrigation off?
‘Outside: When the wet-bulb temperature
exceeds the critical temperature of the crop
being protected and/or the ice starts
melting
– Conserves water, saves fuel ($), reduces
equipment wear and tear ($), allows one to get
to bed sooner
‘Inside: When the water is no longer
needed to maintain temperature above the
critical temperature
Temperature sensor placement
Rewetting intervals
‘ Sprinklers should
rewet foliage about
once every 20 seconds
(3 rpm)
‘ Sensor in same location and at same height as
crop canopy
– Radiational, convective and conductive heat tranfers
– Exposure to night sky and wind movement important
– allows use of reduced
water application rates
– alternatively,
maintains higher
temperatures using the
same amount of water
‘ Temperatures 10' above the crop may be up to
8°F higher
– From leaf to air may be 9 °F
– Implications for sensor placement
– Location critical
Rules of freeze protection with
water
‘Apply water uniformly
‘Apply fast enough to keep ice wet all the
time
‘Apply enough water to protect the plant
Dual irrigation system for cold
protection (shadehouses)
‘Benefits
– Obviates the need to buy, put up, take down
and dispose of plastic sheeting
– Only shades the crop temporarily
– Reduced ice formation and damage
– Allows equal cold protection using less water
– Enables the maintenance of higher
temperatures using the same amount of water
– Facilitates the use of intermittent applications
8
Over-the-shade sprinkler system
‘ Impact sprinkler
– Senninger 3023
– 5/32" nozzles
‘ 30 H 60 foot
rectangular pattern
(maximum spacing)
‘ 1.5 feet above the top
of the shadehouse
‘ Pressure of 40 psi
‘ Gravity drain
Infrared temperatures
‘ “Sky” temperatures
through iced
shadecloth are similar
to those through a tree
canopy
– +25ºF, not -5ºF (night
sky can be -94ºF
to -454ºF)
Dual irrigation system
‘Reduced irrigation run-times and water
volumes applied by about 80% compared
to conventional systems (continuous water
application)
No crop cover (irrigation rates
in inches/hour)
Combining sprinkler irrigation
and crop covers
Crop cover – 0.6 oz/yd2
42
42
36
81
5
64
5
73
0
60
0
51
5
43
0
13
0
81
5
73
0
60
0
Time
64
5
51
5
43
0
34
5
30
0
30
21
5
32
30
34
5
34
32
30
0
34
0.12"/hr
0.21"/hr
0.29"/hr
freezing
38
21
5
36
Temperature
40
0.12"/hr
0.21"/hr
0.29"/hr
freezing
38
13
0
Temperature
40
Time
9
Crop cover – 1.5 oz/yd2
Combustion
‘ Charcoal briquets
42
0.12"/hr
0.21"/hr
0.29"/hr
freezing
38
36
34
32
81
5
73
0
60
0
64
5
51
5
43
0
34
5
30
0
21
5
30
13
0
Temperature
40
Time
10
1
READING THE SIMPLIFIED PSYCHROMETRIC CHART FOR FROST
PROTECTION
Ray A. Bucklin and Dorota Z Haman
Agricultural and Biological Engineering
University of Florida
Psychrometrics is the study of the physical and thermodynamic properties of air-water
mixtures. Some terms used in psychrometrics are:
•
•
•
•
•
•
•
•
Dry air - Air not containing any water vapor.
Moist Air - A mixture of dry air and water vapor.
Air Mixture - A mixture of dry air and water vapor.
Dry Bulb Temperature - The temperature measured by an ordinary
thermometer.
Wet Bulb Temperature - The lowest temperature to which an air mixture can be
cooled solely by the addition of water.
Dew Point Temperature - The temperature at which moisture starts to condense
from air cooled at constant pressure and humidity ratio.
Humidity Ratio - Weight of water vapor in pounds per pound of dry air or grains
of water vapor per pound of dry air or kilograms of water vapor per kilogram of
dry air expressed as a decimal. This quantity may also be called Absolute
Humidity.
Relative Humidity (RH) - The ratio of actual water vapor pressure to the vapor
pressure of saturated air at the same dry bulb temperature. RH is expressed as a
percentage.
Reading a Psychrometric Chart
Learning how to read a psychrometric chart will help to understand how relative
humidity and air temperature can be used to predict freezing and frost conditions and how
it can be used as a tool to time the use of frost protection methods. In this introduction to
psychrometrics, you will learn the relationships between dry bulb temperature, relative
humidity, wet bulb temperature and dew point temperature and how to use the chart and
any two of these properties to determine the other two properties.
These properties are needed when you need to calculate: 1) the time to turn your
irrigation system on 2) the time to turn your irrigation system off and 3) the danger of
frost damage. The psychrometric chart is used in many other calculations related to
heating or cooling of the air. For example, it is used for sizing air conditioning
equipment
The composition of atmospheric air is variable, particularly with regard to the
amount of water vapor. The amount of water vapor in the air in Florida varies from
around 1% by volume under cold dry conditions to above 5% during hot, humid summer
months. Dry air has a defined composition of:
2
Substance
% By Volume
Nitrogen
78.09
Oxygen
20.95
Argon
0.93
Carbon Dioxide
0.03
A moist air mixture is defined as a mixture of dry air and water vapor. A
psychrometric chart is a graphical representation of the thermal and physical properties of
moist air. There is no set format for psychrometric charts and charts from different
sources vary in format and in the parameters plotted on the charts. This publication will
deal with a chart specifically developed as an aid for managing the use of frost protection
methods. The frost protection psychrometric chart deals with the relationships among
dry bulb temperature, wet bulb temperature, dew point temperature and relative humidity.
Other charts often deal with additional values including absolute humidity, specific
volume and enthalpy.
Refer to the psychrometric chart shown in Figure 1. Observe the vertical lines on
the chart having numbers along the bottom. These lines represent "dry-bulb"
temperatures, which are the temperatures measured by an ordinary thermometer. All
points falling on a given vertical line will be at the same dry-bulb temperature.
Figure 1. Psychrometric Chart
3
EXAMPLE 1
Locate the vertical line for 340F dry-bulb temperature on a psychrometric chart.
Solution:
Figure 2. Dry Bulb Line.
Relative humidity is defined as the ratio of the partial pressure of water vapor
present in a moist air mixture to the partial pressure of the water vapor that would be
present if the moist air were completely saturated at the same temperature and pressure.
Relative humidity is a measure of the amount of moisture that the air contains compared
to the amount of moisture it would contain if it were saturated.
The curved lines that radiate from the lower left of the chart to the upper right are
the "Relative Humidity" (RH) lines. The uppermost curved line is the 100% RH or
"saturated" line.
4
EXAMPLE 2
Find the intersection of the 34oF dry-bulb temperature vertical line and the 70%
relative humidity curved line.
Solution:
Figure 3. Intersection of 34oF Dry Bulb Line and 70% RH Line.
The wet-bulb temperature is defined as the lowest temperature to which an air
mixture can be cooled solely by the addition of water. The process of cooling an air
mixture with the addition of water and with no removal of heat is called "evaporative
cooling".
The wet-bulb temperature is the temperature you feel when stepping out of a
swimming pool when the wind is blowing. Under this situation, the skin temperature
drops to the prevailing wet-bulb temperature of the air as long as the skin is covered with
a thin film of water while exposed to a breeze.
The wet-bulb temperature is the lowest temperature that air can be cooled to by
evaporative cooling. It is the wet-bulb temperature, not the relative humidity, that
determines to what temperature air can be cooled by the evaporation of water. When
using sprinkler methods for cold protection, it is important to remember that when
sprinklers are first turned on, the air temperature will be cooled to the wet bulb
temperature.
5
On the psychrometric chart, the wet-bulb temperature lines slope upward to the
left. The psychrometric sketch in Figure 4 shows the wet-bulb temperature of a moist air
mixture when the dry-bulb temperature is 34oF and the relative humidity is 70%.
Figure 4 Wet Bulb Line for 34 F Dry Bulb Temperature and 70% Relative Humidity.
The dew point temperature of a moist air mixture is the temperature at which
condensation in the form of frost or dew first begins to form. Condensation will form on
the outside of a glass tumbler filled with iced water because the temperature of the glass
surface is lower than the dew point temperature of the air in the room. Therefore, some
of the water vapor in the moist air mixture condenses on the outside of the glass tumbler.
If the dew point is above freezing, then dew will form and if the dew point is
below freezing, frost will form. The dew point is closely related to the nighttime low
temperature on still nights. When the air temperature drops to the dew point, energy is
added back to the air as frost or dew forms and the temperature stabilizes at the dew point
temperature. The dew point temperature is directly related to the actual quantity of
moisture in the air and does not change much throughout a day unless a weather front
moves through an area and adds or removes large amounts moisture. So the dew point
temperature measured during daytime hours can be used as an estimate of the nighttime
low temperature.
The charts shown in this publication list dew points on the right hand side of the chart.
The dew point temperatures on some charts may be located on the saturation curve of the
psychrometric chart. The psychrometric sketch below in Figure 5 shows the general
location of the properties we have covered.
6
Figure 5. Dry Bulb, Wet Bulb, Relative Humidity and Dew Point Lines on Psychrometric
Charts for Dry Bulb Temperature of 34°F and Relative Humidity of 70%..
EXAMPLE 3
Determine the dew point temperature and wet-bulb temperature when the drybulb temperature is 380F and the relative humidity is 65%.
Solution:
Dew point temperature is 27.80F
Wet-bulb temperature is 33.80F
Many psychrometric charts display a quantity known as the humidity ratio or
absolute humidity. Humidity ratio lines are parallel to dew point temperature lines, so be
careful to read the dew point temperature scale, not the humidity ratio scale.
The four psychrometric properties of dry-bulb temperature, relative humidity,
wet-bulb temperature and dew point temperature have been defined above. Given any
two of these psychrometric properties, the other two properties can be always determined.
It is often necessary to interpolate between lines to estimate the value of a
property of a moist air mixture. You should always be able to interpolate to at least 1 or
2% accuracy on the property values.
7
EXAMPLE 4
Use the chart on the last page of this publication to find the following values:
(a) Find the wet-bulb temperature of a moist air mixture when the dry-bulb temperature is
36oF and the relative humidity is 30%.
(b) Find the wet-bulb temperature of a moist air mixture when the dry-bulb temperature is
36oF and the relative humidity is 90%.
(c) Determine the dry-bulb temperature of a moist air mixture when the relative humidity
is 70% and the wet-bulb temperature is 350F.
(d) Determine the relative humidity of a moist air mixture when the dry-bulb temperature
is 370F and the wet-bulb temperature is 330F.
(e) Determine the dew point temperature of a moist air mixture when the dry-bulb
temperature is 360F and the relative humidity is 80%.
Solution:
(a)
(b)
(c)
(d)
(e)
27.3°F Wet Bulb Temperature
34.9°F Wet Bulb Temperature
38.70F Dry Bulb Temperature
66% Relative Humidity
30.60F Dew Point Temperature
FROST PROTECTION
The dew point temperature and the wet bulb temperature can be used to estimate
the potential for frost and to determine the best time to turn on sprinklers for frost
protection. Unless a weather front is causing cold air to move into an area, the nighttime
low temperature is governed by the heat lost to the sky and the dew point temperature.
The dry bulb temperature falls at night as heat radiates to the sky. If enough heat is lost,
the dry bulb temperature will fall until it reaches the dew point temperature. When the
dew point temperature is reached, the dry bulb temperature stabilizes as moisture starts
condensing from the air in the form of dew or frost. So if you know the dew point, you
can estimate the lowest possible night time low air temperature. However, it is always
important to remember that plants can be cooled to temperatures lower than the air
temperature by radiation losses from plant surfaces particularly on clear nights with low
humidity.
Plants can be protected from frost damage by the use of sprinkler methods, but
sprinklers must be turned on at the correct time to avoid cold damage by evaporative
cooling and to conserve water. When sprinklers are first turned on, the air around the
sprinklers will be cooled to the wet bulb temperature. If the wet bulb temperature is
8
below 32°F, then cold damage can result from the use of the sprinkler system. The
correct method is to start the sprinklers when the wet bulb is at 34°F.
To avoid damage to plants. Sprinklers can be started at wet bulb temperatures above
34°F, but no additional cold protection will be provided and excessive water will be used.
EXAMPLE 5
The chart in Figures 6 illustrates how psychrometric charts can be used to manage
frost protection methods. Figure 6 shows the effects of turning sprinklers on at a dry bulb
temperature of 34°F for two conditions. For both conditions, as heat is lost from the air,
the dry bulb temperature drops along a path following a constant dew point line until
sprinklers are turned on at Points 2 and 5.After sprinklers are turned on, the dry bulb
temperatures drop along constant wet bulb lines until the air is saturated at the 100%
relative humidity line. For the first condition starting at Point 1, the air around the
sprinkler will be cooled to 33.2°F and plants will be safely protected; however for the
second condition, starting at Point 4, the air will be cooled to 31.7°F and plants could
suffer cold damage.
Figure 6. Path on psychrometric chart as temperature drops and sprinklers start.
PSYCHROMETRIC CHART
(Next Page)
9
Fact Sheet HS-121
June 1994
Cold Protection Methods1
L.K. Jackson and L.R. Parsons2
INTRODUCTION TO YOUNG TREE COLD
PROTECTION
Protecting young trees from cold damage is a
difficult task which has been complicated by several
factors in the last decade. These complications
include a significant increase in the number of young
trees planted over the last several years, increases in
cost of fuel, equipment and labor and an increase in
the number and severity of freezes. The problems of
young tree care, a shortage of trees and increasingly
frequent freezes have generated a new interest in
protecting young citrus trees from possible damage by
cold. Since young trees are small and occupy a
relatively small percentage of a planted grove acre,
protection by most active means is not particularly
effective. This is especially true of heating with fossil
fuel sources which are now quite expensive in
addition to being inefficient for young tree protection.
Wind machines could be considered for protection,
but their use is limited to calm nights with
temperature inversions and the cost of acquisition and
operation of this equipment could not be
economically justified for non-bearing groves.
Irrigation for cold protection is a possibility and is
now widely used in many young groves where
properly designed and maintained microsprinkler
systems are in place. Such systems require
uninteruptible power sources to avoid problems of
electrical blackouts. Many young citrus trees are
placed in a situation where active cold protection
measures are difficult, if not impossible, and growers
have to rely upon passive means of cold protection.
Some of the more important passive cold protection
measures include cultivar and rootstock selection, site
selection, clean cultivation, pre-freeze irrigation and
the use of banks and wraps.
COMPARISON OF METHODS
High fuel cost has made grove heating during
freeze nights prohibitively expensive except for high
value crops. Wind machines are effective under some
conditions, but they require maintenance and need a
strong temperature inversion for optimum
effectiveness. Fog can provide protection, but light
winds can blow the fog away from the grove and
obscure nearby roadways. High volume overhead
sprinkler irrigation has been used effectively on limes
and avocadoes in south Florida where temperatures
do not normally go far below freezing. In central and
north Florida, where temperatures are usually colder,
overhead sprinklers should not be used on large citrus
trees because the weight of the ice formed can break
off limbs and cause tree collapse. With overhead
systems, all leaves are wetted and susceptible to
damaging evaporative cooling during low humidity or
windy freezes. Many trees were killed in the windy
1962 freeze when overhead sprinklers were used.
Because of the cost of fuel, microsprinkler irrigation
1.
This document is Fact Sheet HS-121, a series of the Horticultural Sciences Department, Florida Cooperative Extension Service, Institute
of Food and Agricultural Sciences, University of Florida. Revised: June 1993. Publication date: June 1992. Revised: June 1994.
2.
L.K. Jackson, professor, extension horticulturist, Department of Fruit Crops; L.R. Parsons, professor, extension water resource specialist,
Department of Horticultural Sciences, Citrus Research and Education Center, Lake Alfred, Florida, a branch campus of the University of
Florida, Gainesville, Florida.
Trade names, where used, are given for the purpose of providing specific information. They do not constitute an endorsement or guarantee of
products named, nor does it imply criticism of products not named.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational
information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national
origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office.
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / Christine Taylor Stephens, Dean
Cold Protection Methods
Page 2
is rapidly becoming the preferred method for
providing cold protection. This type of irrigation
works particularly well for resets and the lower trunk
and branches of young trees. However, once
microsprinkler irrigation has begun, it must be
continued until the grove temperature rises above
freezing. If irrigation stops before then, the trees will
likely be more damaged than if the irrigation
continued. Banking very effectively provides cold
protection to the trunks of young trees. However,
banks are time consuming to erect and can produce
some pest and cultural problems. The grower can
avoid some of these problems by using tree wraps,
which can be left on for an extended period of time
once installed. While tree wraps as a whole are
effective for cold protection, they are not as effective
as banking. Protection varies greatly depending on the
type of tree wrap used. Table 1 compares energy
requirements for the various methods of cold
protection discussed here.
SOIL BANKING
Soil banking (Figure 1) consists of placing a
mound of soil around the tree’s trunk to protect the
bud union and trunk from cold. It is one of the most
efficient cold protection methods for young trees and
has been used with success for many years.
Banking principles
Since the soil stores heat from the sun during the
day and releases it at night heat deep in the soil
moves up to the surface by conduction and is lost to
the air by radiation. By mounding soil around the
trunk of a tree (banking), heat is conducted through
the soil and into the protected area of the young tree.
Thus, banking protects by conduction and insulation
as well.
When to bank
A definite answer to the question of when to bank
has not been derived. It would be most efficient if
trees were banked the day before a freeze, but the
state of the art of weather forecasting does not permit
this luxury. Growers in much of south Florida do not
bank at all since that area has such a low cold
damage probability. However, growers in the north
and much of central Florida realize the high
probability of cold damage and routinely bank young
trees in the fall as a regular production practice. A
good rule of thumb is to try to have all trees banked
by November 15 for
Figure 1. Soil bank
the northern areas and no later than mid-December
for the rest of the state.
Making the Bank
Banks can be constructed with a shovel or hoe, a
blade on a tractor or similar tool, or with a banking
machine. Build them as high as reasonable, up into
the scaffold limbs whenever possible. Higher banks
afford more protection, but they also require more
labor and expense to build. Use only soil which is free
of weeds, sticks, bags or other trash as these will
invite damage from insects and disease. Watch banks
carefully during the winter since wind and rain may
erode them. Rapid recovery of freeze-damaged trees
will be the payoff for a good banking job.
Unbanking
Trees can be safely unbanked as soon as the
danger of cold weather has passed. In most areas this
will be in mid or late February. If banks remain on
the trees too long in warm weather, disease and insect
problems increase and there is danger of a
Cold Protection Methods
Page 3
physiological bark sloughing disorder (sweating) which
can quickly kill the young tree. Unbanking should be
supervised just as closely as banking to prevent tree
damage from careless equipment operation. Also,
care must be taken to ensure the bank is removed
completely and the soil carefully leveled around the
young tree. Leaving too much soil around the tree
trunk may encourage foot rot in the susceptible scion
portion of the tree.
1. Soil banks must be put up before danger of cold
and removed as soon as possible after the threat
of cold has passed.
2. Labor to build banks is expensive.
3. Hot periods during winter months may necessitate
early removal of at least a portion of the bank
before the danger of cold is over.
4. Construction of banks is often hindered by weeds
or in the case of larger trees, overhanging limbs.
Banking Hazards
Soil Bank Summary
Tree Damage
Careless operation of equipment may break limbs,
skin trunks and even destroy trees. Equipment
operators must be conscientious and well-trained if
the operation is to be a success. Broken limbs and
skinned trunks should be treated with a good
water-repellent pruning paint or fungicide before
being covered with soil. Some mechanical equipment
used for banking removes considerable soil from a
relatively small area, resulting in damage to roots
near the soil surface. The use of such equipment
should be avoided or care should be taken to make
sure damage is minimized.
Diseases and Insects
Fungal disease can sometimes be a problem when
trees are banked. Placing soil on the susceptible scion
portion of the young citrus tree may predispose the
plant to foot rot if conditions are optimal for
development of the fungus. Application of a suitable
fungicide before banking will help reduce the
incidence of foot rot. Ants and termites may
sometimes become a problem in banks, particularly if
there is trash in the soil used to construct the banks.
Problems such as these can be dealt with as they
occur or a preventive insecticide can be sprayed at the
time of banking. Many growers routinely spray trees
with a suitable insecticide-fungicide mixture just
before banking as an insurance measure. One
hundred gallons of spray should treat 400 to 600 trees
if properly applied.
Banking Considerations
These factors should be taken into consideration
before choosing banking as a cold protection method:
Advantages
1. Excellent insulating value (12 - 15° above air
temperature in most cases)
2. Sprout inhibitor
3. Conforms well to large or irregularly shaped trees
4. No cost for material, only labor
Disadvantages
1. Must be constructed and taken down seasonally
2. Difficult to maintain
3. Occasional problems with bark sloughing and foot
rot
4. Moderate insect and disease problems
5. Must be removed after freeze damage to allow
regrowth
6. Labor cost is expensive
TREE WRAPS
Theory of Cold Protection
Tree wraps are most useful in protecting young
citrus trees during mild to moderate freezes or in
traditionally warmer locations within the state. Tree
wraps protect only the trunk, and consequently leaf
loss can occur during moderate or severe freezes.
Wraps work by delaying, but not preventing, heat loss
from the tree trunk as air temperatures decrease.
Temperatures under tree wraps generally are 0° to
6°F higher than air temperatures, depending on the
type of wrap. However, the tree produces and stores
very little heat, and during severe freezes of long
durations the temperatures under most wraps will
approach air temperatures. Wraps are most effective
during freezes of short durations where temperatures
drop rapidly. They are less effective, however, during
freezes where temperatures decrease slowly and
remain low for protracted periods. The effectiveness
of the wrap is related to the insulating value of the
wrap material. Consequently, wrapping trunks with
Cold Protection Methods
thin-walled materials is ineffective for temperature
control, while thicker insulating materials are more
effective.
Wrapping
Most tree wraps, unlike soil banks, can be
attached anytime during the year and left on the tree
throughout the year or even for several years.
However, some types of wraps, like those made of
poor insulating materials or clear plastic, may damage
or even kill the tree due to excessive daytime trunk
temperatures during the summer.
When freeze damage occurs, wraps should be
removed or pushed down to allow for growth of new
shoots. Wraps should be properly positioned and
fastened around the trunk for best results. It is
important to cover the entire lower trunk, especially
at the base.
Page 4
freeze, heat of fusion is released which can generate
considerable heat. When this heat is released within
the confines of an insulating material, and next to the
tree trunk, it can be quite effective.
Types of Tree Wraps
Selection of the proper tree wrap for a particular
grove depends on a number of factors including cost,
ease of installation and probability of freeze damage.
For example, growers in northern regions of the state
should choose wraps with good insulating qualities,
while growers in warmer southern locations may opt
for less costly, thinner wraps. Tree wraps also inhibit
sprouts and protect trunks from herbicide and
mechanical damage. Consequently, no one wrap is
best for all situations.
Fiberglass Wrap
The advantages and disadvantages of fiberglass
wrap (Figure 2) are discussed below.
Heating Effects
Insulating materials are used extensively in most
tree wraps to provide cold protection. Since insulation
holds heat in, protection is provided by slowing down
the loss of heat from young tree trunks, thus making
them warmer. However since there is very little heat
stored in the trunk of a young citrus tree, wraps
utilizing insulation alone have limited effectiveness.
Dormancy Effects
The degree of dormancy of young citrus trees is
a function of environment, and measures to slow the
growth of trees usually results in dormancy and a
better ability to tolerate low temperatures. Insulating
materials in some cases may help to keep tree trunks
cool during daylight hours resulting in greater
dormancy and an increased tolerance to low
temperatures. Though not substantiated by research,
the principle is confirmed by observation. Possible
effects of light on tree dormancy is speculative but
observations support the theory. Trees wrapped with
opaque materials rarely sprout under such wraps
because light is excluded. Sprouting is evidence of
growth and lack of dormancy, so materials which
block light may help to contribute to tree dormancy.
Figure 2. Fiberglass wrap
Use of Liquids
Some wraps utilize pouches of liquid (usually with
an ice nucleator in solution) to furnish additional heat
inside the wrap, next to the tree trunk. When liquids
Cold Protection Methods
Page 5
Advantages
Advantages
1. High insulating value (3 - 6° above air
temperature)
2. Moderately durable
3. Sprout Inhibitor
4. Can be pushed down to allow for regrowth
following a freeze
5. Inert, will not hold water for long periods of time,
rarely causes foot rot problems
6. Moderately inexpensive
7. Conforms well to large or irregularly shaped
trunks
Disadvantages
1. More difficult to install and handle than some
other wraps
2. Moderate ant problems
1. High insulating value (3 - 6° above air
temperature)
2. Moderately durable
3. Sprout Inhibitor
4. Moderately inexpensive
5. Moderately easy to handle and install
6. Conforms well to large or irregularly shaped
trunks
Disadvantages
1. May become waterlogged, particularly if used with
irrigation
2. Sunlight deteriorates some wraps
3. Foot rot is an occasional problem
4. Must be removed after freeze damage to allow
regrowth
Polyurethane foam
The advantages and disadvantages
polyurethane wrap (Figure 3) are listed below.
Figure 3. Polyurethane foam wrap
of
Rigid Polystyrene Foam (Thick-Walled)
Listed below are the advantages and
disadvantages of thick-walled rigid polystyrene foam
(Figure 4).
Cold Protection Methods
Page 6
Figure 4. Thick-walled rigid polystyrene foam
Figure 5. Thin-walled rigid polystyrene foam
Advantages
Advantages
1. Very high insulating value (4 - 8° above air
temperature)
2. Very durable
3. Moderate sprout inhibitor
4. Will not hold water, rarely foot rot problems
5. Easy to handle and install
1.
2.
3.
4.
5.
Low to moderate durability
Sprout inhibitor
Will not hold water, no foot rot problems
Inexpensive
Moderately easy to handle and install
Disadvantages
Disadvantages
1. Expensive
2. Moderate ant problems
3. Must be removed after freeze damage to allow
for growth
4. Subject to loosening by animals, may fit poorly on
irregularly shaped trunks
1. Low insulating value (0 - 2° above air
temperature)
2. Moderate to severe ant problems
3. Must be removed after freeze damage to allow
regrowth
4. Not suited for large, rapidly growing trees, may fit
poorly on irregularly shaped trunks
Rigid Polystyrene Foam (Thin- Walled)
Closed Cell Polyethylene Foam
Below are listed the advantages and disadvantages
of thin-walled rigid polystyrene foam wrap (Figure 5).
Discussed below are the advantages and
disadvantages of closed-cell polyethylene foam
(Figure 6).
Cold Protection Methods
Page 7
5. Easy to handle and install
6. Some models use irrigation water supply tube
inside for extra protection.
Disadvantages
1. Moderately expensive
2. Ant problems are severe in some areas
3. Must be removed after freeze damage to allow
regrowth
4. May cause bark sloughing and fit poorly on large
or irregularly-shaped trunks
MICROSPRINKLER IRRIGATION
Overhead, high-volume sprinklers have been used
successfully in citrus nurseries for years as a means of
cold protection. Recently, there has been interest in
using low-volume microsprinklers to protect young
trees in the field; however, success varies with the
type of system, application rates, type of freeze
(advective vs. radiative), and severity of the freeze.
Theory of Protection
Figure 6. Closed-cell polyethylene foam
Advantages
1. Moderate insulating value (2 - 4° above air
temperature)
2. Moderately durable
3. Sprout inhibitor
4. Inert, will not hold water, rarely causes foot rot
problems.
Water protects young trees by transferring heat to
the tree and the environment. The heat is provided
from two sources, sensible heat and the latent heat of
fusion. Most irrigation water comes out of the
ground at 68° to 72°F, depending on the depth of the
well. In fact, some artesian wells provide water of
80°F or more. As the water is sprayed into the air, it
releases this stored (sensible) heat. However, by the
time the water reaches the tree it has lost most of its
energy, particularly for low volume microsprinkler
systems. Consequently, the major source of heat from
irrigation is provided when the water changes to ice
(latent heat of fusion). As long as water is constantly
changing to ice the temperature of the ice-water
mixture will remain at 32°F. The higher the rate of
water application to a given area, the greater the
amount of heat energy that is applied.
The major problems in the use of irrigation for
cold protection occur when inadequate amounts of
water are applied or under windy (advective)
conditions. Evaporative cooling, which removes 7.5
times the energy added by heat of fusion, may cause
severe reductions in temperature under windy
conditions, particularly when inadequate amounts of
water are used. In addition, most irrigation systems
will not protect the upper portion of the canopy.
Cold Protection Methods
Types of Microsprinkler Systems
A number of low-volume microsprinklers which
can be used for cold protection of young citrus trees
are currently available. As with tree wraps, no one
system is best for a given grove situation. Remember
that microsprinkler irrigation is primarily used to
irrigate trees, and practical irrigation designs may not
necessarily provide optimum cold protection. Again,
cost, ease of operation, and especially probability of
freeze damage should be considered when selecting
an irrigation system. However, the key to successful
cold protection using any microsprinkler system is
providing a continuous and adequate volume of water
directly to the trunk of the tree. This is particularly
true during advective freezes where water may be
blown away from the trunk.
It is generally advisable to place the emitter
northwest of the tree, approximately 1 yard or less
from the trunk. Emitters should be attached to risers
for greatest tree trunk protection.
Improper
placement or inadequate spray coverage will greatly
lessen the effectiveness of the irrigation. A 90° spray
pattern which concentrates the water on the trunk
and lower limbs gives cold protection superior to a
360° or 180° pattern. Inverted cone sprinklers
positioned above the wrap in the tree also give
adequate protection. The volume of water applied
depends on the amount of cold protection required.
Generally, 10 gallons per hour (gph) applied directly
to the trunk in a 90° pattern will provide adequate
cold protection during most freezes.
Wraps Plus Irrigation
This combination of cold protection measures
provides protection by insulation plus heat of fusion
from water freezing on the wrap, and in some cases,
water actually being piped through the wrap to
provide even more protection. Spraying water on
wraps in sufficient volume and without interruption
will theoretically not allow temperatures to fall below
32°F. Furthermore, if ground water is piped through
the wrap prior to spraying it externally, additional
protection could be provided.
When used in combination with adequate
irrigation most tree wraps provide cold protection to
the trunk. However, only wraps with high insulating
characteristics provide protection when irrigation is
discontinued due to a power outage or break in the
irrigation lines. A combination of tree wraps and
Page 8
microsprinkler irrigation provides low cost insurance
against such problems.
COLD PROTECTION USING HEATERS
The greatly increased cost of fuel has practically
eliminated heaters from the growers cold protection
strategy. However, heaters can still be cost effective
when used to protect high-value citrus cultivars.
Using Heaters
Orchard heaters provide heat by direct radiation
and convection. Stack heaters give out 25-30 percent
radiant heat, which moves along a straight line from
the heater to the trees. Air around the immediate
area of the heater is heated by convection; some of
this heat is lost if it rises above the level of the
orchard. Because of the need for fuel-burning
efficiency and pollution reduction, orchard heaters
have evolved to the upright stack design. Vaporizing
pot-type stack heat (for example, jumbo cones and
return stacks) have the advantage of low initial cost,
maneuverability, and versatility. However, fuel can be
lost due to spillage, leakage, and boiling of fuel left in
the heaters after they are extinguished. Labor
requirements for lighting and refueling heaters are
high, and an additional crew is frequently needed to
refuel heaters if several nights of freeze protection are
required. Compared to individual stack heaters,
centralized pressure fuel systems burning diesel fuel
and liquid propane are more fuel-efficient and offer
considerable labor savings. Fuel storage for any
heating system is a big expense and environmental
liability.
Energy Saving Tips
1. Maintain heaters in good working order.
Periodically clean the stacks for most efficient
burning of fuel and to keep emissions within the
standards specified by air pollution laws.
2. Have sufficient thermographs or thermometers
throughout the grove area.
3. Large groves can generally be heated more
efficiently than small groves. To protect grove
borders, additional heaters must be placed along
the edges of the grove, especially on north and
west sides.
4. Calculate temperature drop vs. time throughout
the night to better determine when heating should
be started.
5. It is important to light heaters one to two degrees
above the lethal temperature of leaves or
Cold Protection Methods
blossoms and buds. If fruit is to be protected,
begin protection one or two hours after the
critical freezing temperature of fruit has been
reached, since the fruit has more mass than buds
and cools more slowly or use a thermometer to
determine the internal temperature of the fruit.
6. It is frequently possible to stabilize temperatures
during the initial phase of protection by lighting
every other row of heaters or by lighting central
systems and then turning the pressure down.
Additional heaters can then be lit or line pressure
can be raised slightly to maintain the temperature
in the grove as temperatures drop outside the
heated area.
7. Many small heaters generally provide more
efficient heat distribution than a few large ones.
This point became particularly important with
higher fuel costs. The additional capital outlay of
a greater number of heaters could be returned
through more efficient orchard heating.
8. Be familiar with cold areas in your grove so that
heaters in those areas can be lit first.
Minimizing Heating Requirements
Selecting the proper temperature for lighting
heaters or starting any system of cold protection can
affect fuel savings. For example, using climatic data
for Bartow, Florida, protecting a grove nine out of ten
years at 28°F. would require at least 26 hours of
heating per winter. However, if the crop would
tolerate 24°F., the grower would only have to heat
five hours, using one-fifth as much fuel. Citrus fruit
will withstand temperatures of 28°F for approximately
two hours. But leaves and twigs (fruiting surface) will
often withstand 24°F or lower. With the uncertain
future of fuel supplies, growers may seriously consider
only protecting the fruiting surface of the tree and
allowing the fruit to freeze. The fruit may still be used
for processing if it is harvested within a week to ten
days following the freeze. Leaf freezing points are a
good estimate of the temperature at which leaves
twigs and wood freeze. Often, twigs and leaves will
freeze at or near 24°F in the early fall, but may
withstand 22°F or slightly lower temperatures during
mid-winter.
WIND MACHINES
Wind machines offer some excellent advantages
in cold protection because they minimize labor
requirements, consume less fuel per acre protected
and require less fuel storage than heaters. They are
permanently located in the grove and have a low
Page 9
operational cost per acre. Fuel requirements for wind
machines are about 10 gal/hr or 1 gal/acre/hr
compared to 10-35 gal/acre/hr with heaters. These
advantages must be weighed against the disadvantages
of rather high capital costs and the failure of the wind
machine to provide adequate cold protection under
all conditions. Wind machines are dependent on
having an inversion--that is, warmer air at
approximately 40-50 feet above the orchard. A
temperature inversion of at least 5° difference is
necessary and an inversion of 10-15°F makes the wind
machine very effective. They are most beneficial
when located in low pockets where they mix cold,
heavy air, which settles there, with warmer air above.
In general, one can use the rule of thumb that 10
horsepower is required to protect one acre. Usually,
one wind machine is required for each 10 acre block.
However, the increase in temperatures are highest
nearest the machine and decrease toward the edge of
area protection. Heaters can frequently be used near
the edge of the area protected to remedy this
situation. Start wind machines when temperatures are
two to three degrees above the lethal temperature.
Because of the low cost of running a wind machine,
plus the fact that it can only raise the temperature a
few degrees, it is necessary to start the wind machine
early. It is very important that wind machines be run
Cold Protection Methods
Page 10
at the rpm specified by the manufacturer, since they
provide considerably less protection when operated at
a lower speed.
Helicopters are sometimes used as a
cold-protection device, if they are stationed nearby.
Otherwise, they are too expensive. They are utilized
as a large, moving wind machine. When helicopters
are used effectively, a number of temperature
monitors are required in the grove to determine the
coldest areas and the frequency of passes the
helicopter must make. Monitors should turn on a light
when temperatures reach a critical value. Rapid
refueling or more than one helicopter may be
necessary since protection cannot be halted once
temperatures are below the critical point.
Heating in conjunction with wind machines
provide better protection at lower cost than heaters
alone. For example, an orchard requiring 35 heaters
per acre without the use of wind machines would
require 15 heaters per acre with wind machines.
Heaters plus wind machines and good air temperature
inversions would permit heaters to be used less than
half the time, which would reduce fuel consumption
and increase the heater’s life span.
Table 1. Energy requirements of various cold protection methods for young citrus trees
Fuel Consumption
Method
gal/hr/acre
BTUs/hr/acre (in thousands)
Heaters
20 - 40
2,800 - 5,600
Wind machines
0.5 - 1.5
70 - 210
High volume sprinklers
0.25 - 0.75
35 - 105
Low volume sprinklers
0.10 - 0.25
14 - 35
Source: T. R. Mee
BUL201
Cold Protection for Nursery Crops1
Dewayne L. Ingram, Thomas Yeager, Rita L. Hummel2
develop depends upon the severity of the exposure
and the environmental parameters after the exposure.
Winter temperatures are frequently low enough
Continued cool temperatures and high humidity after
to cause cold injury to tropical, subtropical and
an exposure to cold may slow the symptom
occasionally temperate plants that are produced in
development, while high light intensity and warm
Florida. This publication provides information for
temperatures may accelerate symptom
ornamental plant producers regarding symptoms of
development.
cold injury, plant adaptation to cold, environmental
conditions leading to cold injury and methods to alter
Chilling
the environment to avoid or minimize cold injury.
Many chilling-injury symptoms are common to
Cold injury includes damage from temperatures
other stresses such as drought stress, root rot diseases,
above and below freezing. Many tropical and
phytotoxicity to chemicals, heat stress and light
herbaceous plants do not adapt or harden to withstand
stress. General symptoms of chill injury to plant
freezing temperatures and may be injured by
leaves, stems and fruits are listed below:
temperatures below 10°C (5°F). Injury caused by
low temperatures above freezing is chill injury and
1. Surface lesions, pitting, large, sunken areas and
damage caused by freezing temperatures is freeze
discoloration. These symptoms have been
injury.
reported on several orchids.
Cold Injury Symptoms
Cold injury symptoms usually occur after
exposure to critically low temperatures, not during
the cold exposure. Direct injury is inflicted at a
cellular level and the response of plant tissues to this
injury is revealed through visual or measurable
symptoms. The rate at which these symptoms
2. Water-soaking in tissues results from disruption
of cell structure and release of cell solutes into
spaces between cells, and is commonly followed
by wilting and browning.
3. Internal discoloration (browning) of pulp, pith,
and seed.
1. This document is BUL201, one of a series of the Environmental Horticulture Department, Florida Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida. Original publication date December 2001. Visit the EDIS Web Site at http://edis.ifas.ufl.edu.
2. Thomas H. Yeager, associate professor and extension woody horticulturist, Environmental Horticulture Department, Cooperative Extension Service,
Institute of Food and Agricultural Sciences, University of Florida, Gainesville FL 32611; Dewayne L. Ingram, professor and former extension
horticulturist; Rita Hummell, Associate Scientist, Horticulture, Ornamentals (Research). WSU Puyallup Research and Extension Center, 7612 Pioneer
Way E., Washington State University, Puyallup, WA 98371-4998
The use of trade names in this publication is solely for the purpose of providing specific information. UF/IFAS does not guarantee or warranty the
products named, and references to them in this publication does not signify our approval to the exclusion of other products of suitable composition.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational
information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin.
For information on obtaining other extension publications, contact your county Cooperative Extension Service office. Florida Cooperative
Extension Service/Institute of Food and Agricultural Sciences/University of Florida/Christine Taylor Waddill, Dean.
Cold Protection for Nursery Crops
4. Accelerated rate of senescence (natural death),
but with otherwise normal appearance.
5. Increased susceptibility to attack by fungi and
bacteria not commonly found on the plant.
6. Slowed growth, or limited growth flush. This
symptom may be difficult to detect without
non-chilled plants for comparison or a thorough
knowledge of normal growth rate.
Freezing
Symptoms of freeze injury could include
desiccation or burning of foliage, water-soaked areas
that progress to necrotic spots on leaves, stems or
fruit and death of sections of the plant or the entire
plant. Close examination of woody plants several
days or weeks after freezing may reveal a dead or
weakened root system or split bark on stems or
branches. Obvious symptoms on plant foliage may
not be present until after the plant has been stressed
by warm temperatures. A hot, bright day could
increase transpirational water loss beyond the ability
of injured roots or stem conductive tissue to replace.
Subsequent symptoms might include wilting and/or
desiccation, as caused by direct drought stress.
Plant Response to Freezing
Temperatures
When considering new plant material for use in
the landscape or as a possible nursery crop, cold
hardiness should be determined. Hardiness indicates a
plant's resistance or ability to adjust to cold stress in
order to tolerate freezing temperatures.
The timing and degree of cold hardiness is
determined by environmental conditions and the
genetic makeup of a particular plant. Inherent, genetic
potential is the first limiting factor in development of
hardiness, with plant species and genotypes within
species differing in their tolerance to cold. Most
tropical plants fail to develop hardiness regardless of
preconditioning environmental conditions. Some
species are always killed by freezing while others
tolerate temperatures as low as –196°C (-320°F)
in midwinter.
2
The geographic source of a plant plays an
important role in the timing of hardiness. Northern
plants sensitive to daylength start to cold harden
sooner in autumn than southern plants when grown on
the same site. Therefore, selection of seed or cutting
stock of desired plants from cold hardy genotypes is
an important consideration.
Plant reaction to environmental conditions
leading to increased cold tolerance is called cold
acclimation, and plant reaction to environmental
conditions resulting in less tolerance to cold is called
cold declamation. Although in more northern
climates acclimation occurs primarily in the fall and
declamation in the spring, rapid changes in plant cold
tolerance occur throughout the fall, winter and spring
months in Florida as environmental conditions
change rapidly.
Environmental factors, such as daylength,
temperature, nutrition, water availability, light
intensity and physiological maturity of a plant or
plant part are known to play a role in cold
acclimation. Once the effect of these environmental
factors on cold acclimation is understood, sound
cultural practices directed toward increasing cold
tolerance can be developed.
Generally, plant growth slows or ceases before
cold acclimation begins. Decreasing daylength
provides the primary stimulus or trigger for cold
acclimation in many plants. Phytochrome, a
light-receptive sensory pigment in leaves and bark,
detects decreasing daylengths of autumn and initiates
biochemical and physiological changes that slow
vegetative growth and increase cold acclimation.
Plants that react primarily to daylength slow growth
at approximately the same time each year, regardless
of temperatures. Temperature is the primary stimulus
for cold acclimation in some subtropical plants such
as citrus. However, in most plants there is an
interaction between photoperiod and temperature on
plant growth and development, and the most rapid
cold acclimation is produced by short photoperiods
and low temperatures.
Plants should withstand cold best if fertilized
with a balanced ratio of plant nutrients that produce
optimum growth (Pellet and Carter), but the rate of
fertilization should be reduced slightly in fall and
Cold Protection for Nursery Crops
winter to reflect the lower nutrient requirement during
the cold months. Plants under severe nutrient
deficiencies or plants receiving nutrients at near toxic
levels do not withstand cold or recover from cold
injury as well as plants with properly balanced
nutrition.
Moderate drought stress can result in increased
cold tolerance in some plants by slowing growth and
initiating dormancy. Many plants native to the dry
plains respond to this treatment because in nature the
plants are commonly subjected to dry conditions
before the onset of winter. Although water stress
may increase cold tolerance in some plants, such
stress may result in an unacceptable decrease in
quality of plants such as azaleas.
The maximum cold tolerance level is seldom
reached in Florida, even with temperate plants,
because of the loss of hardiness during extended
warm periods in winter months. Plant metabolic
activity slows during extended cold periods, but a
period of warm temperatures can stimulate rapid
declamation. Temperature seems to be the primary
environmental factor controlling declamation and
plants can reacclimate if a subsequent slow
temperature drop occurs. A rapid temperature drop
following a warm period, a common event in Florida,
may produce injury and death to plants.
Evaluation of cold hardiness is further
complicated because different tissues and organs of
the same plant may display varying degrees of
hardiness. Flower buds are often damaged by freezes
while vegetative buds or stem tissues are uninjured.
Plants grown for their floral display may be hardy, but
if their flower buds are killed every year, they will be
unsatisfactory for landscape use. Leaves of
broad-leaved evergreens may be injured by cold yet
the stems remain unharmed. Root tissues are less
resistant to freezing injury than stem tissues, with
young roots being more sensitive to cold than mature
roots. Stems and leaves of Pyracantha coccinea
'Lalandii' acclimated to –26°C (-15°F), and
mature roots to –17°C (1°F), while young roots
failed to survive below -50C (23°F) (Waist and
Steponkus). Lack of old resistance in roots is not
usually a problem in field production or in the Florida
landscape, however, it may be the limiting factor to
3
winter survival of containerized plants when
prolonged freezing temperatures occur.
Plants that survive freezing temperatures must
either avoid or tolerate the formation of ice in their
tissues. The primary mechanism by which plants
avoid freezing is supercooling. Supercooling occurs
when the plant's temperature drops below its freezing
point without ice formation. Even pure water will
supercool. The lowest subfreezing temperature
recorded before ice formation is the supercooling
point. Unfortunately this point is not a constant value
but varies for repeated tests on the same solution.
Under field conditions supercooling generally allows
nonacclimated plants to avoid freezing when
temperatures in the -1 to –3°C (31 to 26°F) range
occur.
Cold-resistant plants tolerate water freezing in
their tissues as long as the ice crystals form between
cells (extracellular freezing) and not inside them
(intracellular freezing). Extracellular ice formation is
the type of freezing encountered in nature and is
tolerated by hardy plants in their cold-acclimated
state. Intracellular freezing disrupts the cell and is
always fatal. Generally, freezing rates in nature are
too slow to allow intracellular freezing. Cooling rates
of 2°C per minute or faster are required for
intracellular freezing, and slower cooling allows
sufficient time for water to move through the
surrounding membrane and form ice crystals in
extracellular spaces. Thus at the cellular level, the
requirements for freezing resistance in any hardy
plant are the avoidance of intracellular freezing and
the tolerance of extracellular freezing.
Principles of Heat Transfer
Heat loss by plants involves heat transfer which
should be understood before formulating and
evaluating cold protection methods. Heat may be
transferred by conduction, convection or radiation.
Convection
Convection is the process of heat transfer within
a fluid or air that results in mass motion of molecules
in that fluid or air. Heat is transferred from air at the
soil surface to air above the earth by convection. Air
becomes lighter when heated and rises to be replaced
Cold Protection for Nursery Crops
by heavier, cooler air. This mass motion of air is
called convective mixing and explains why air just
above the earth's surface does not become extremely
hot on a sunny, summer day.
Radiation
Radiation is the process of heat transfer from one
object to another without the aid of a transfer
medium. The sun's rays heat the earth's surface and
can burn human skin by radiant heat transfer. The
surfaces in a greenhouse are warmed by absorption of
short wave solar radiation. These surfaces reradiate
heat to the air above them as long wave or infrared
radiation.
Environmental Conditions Leading to
Cold Injury
Cold conditions in Florida are a result of cold air
masses moving down through more northern states
and pushing into Florida. Temperatures associated
with these fronts depend upon where the pressure
systems originate and the rate at which they move
into Florida. Environmental conditions created by
these cold fronts can be categorized into one of two
general types: moist air with considerable cloud cover
or dry air and clear skies. Windy conditions can
accompany either of these conditions, and wind is an
important consideration in developing
plant-protection strategies. Daily temperature
fluctuations are greatest when clear skies exist. Solar
radiation warms the earth's surface during the day
and air temperatures of 16 to 21°C (60 to 70°F) are
common even though the minimum temperatures at
night may be less than –1°C (30°F). This large
fluctuation is primarily due to radiation cooling.
Radiational cooling occurs primarily at night
when the heat absorbed by the earth's surface during
the day is reradiated into the atmosphere. Air at or
near the soil surface is warmest during day and
coldest during night under such conditions. Heat lost
from surfaces by radiational cooling moves away
from the earth's surface by convective mixing. The
soil continues to lose heat until it is colder than the air
just above it; then the soil absorbs heat from the air.
The existing condition is a cold air layer near the
earth's surface with a rapidly cooling soil surface.
The warmest air may be from a few feet to one
4
hundred feet above the soil surface. Plant leaves
close to the ground may sustain freeze injury even
through the temperature a few feet above the leaves is
above their freezing point. This condition is called a
“temperature inversion" because it is an inversion of
normal daytime conditions where the warmer air is
near the ground.
Moist air and cloud cover reduce the fluctuation
of daily air temperatures. Cloud cover reduces the
amount of solar radiation reaching the earth's surface
during the day, and heat radiating from surfaces on a
calm night is absorbed by clouds and reradiated back
to the earth's surface. The primary cooling process of
plants and other objects during cloudy, calm, cold
weather is conduction of heat from the leaf to the
colder air surrounding it. Leaf temperature generally
is not lower than the air temperature in these
conditions.
Wind increases the rate of temperature drop. A
5- to 10-mph wind on a cold, cloudy night constantly
replaces the warmer air on leaf surfaces with cold air,
and this accelerates the rate of heat loss from the leaf.
A temperature gradient will develop between the leaf
and the air if the air is calm and the rate of heat loss
from the leaf would be reduced slightly. Wind on a
clear night prevents or reduces the formation of an
inversion layer by mixing the warmer air above the
crop with colder air at the crop surface, thus slowing
the rate of leaf temperature drop.
The lowest temperatures occur in Florida when
cold, dry (clear skies) air masses move rapidly across
the United States and into Florida. Generally, as such
a cold front moves from north Florida to south
Florida, the temperature of the air mass increases, but
the amount of temperature increase depends upon
how fast the front is moving. Clear skies at night
greatly increase the chance of crop injury by allowing
considerable radiational heat loss from the crop
environment.
The terms frost injury and freeze injury are often
confused. The injury mechanism in both is the
freezing of cellular water, but freeze injury can take
place even if frost is not present. Frost occurs when
the dewpoint of the air (the temperature at which air
is saturated with water) is reached at freezing
temperatures. Air can hold less water vapor as it gets
Cold Protection for Nursery Crops
colder. When the dew point occurs at freezing
temperatures, the water vapor in the air changes to ice
crystals on exposed surfaces. When the air contains a
lot of moisture, the dew point may be reached before
freezing temperatures occur and water vapor will
condense as a liquid on exposed surfaces. Dew may
freeze after it has condensed on leaf surfaces if the air
temperature drops below freezing, but this type of ice
formation is less damaging to plants.
When the humidity is very low, freeze injury can
occur without frost. Freezes without frost are often
called “black frosts."
Measuring Environmental
Conditions
Proper measurement of environmental conditions
is essential for predicting or assessing plant response
and for optimum management of plant protection
systems. Temperature is the most important
environmental parameter to be measured by the
nursery manager. However, temperature
measurements related to relative humidity and wind
speed and direction can provide the manager with
more insight into current and expected conditions.
Relative humidity is the quantity of water vapor
present in the atmosphere, expressed as a percentage
of the quantity which would saturate the air at the
same temperature. Relative humidity helps to
determine how rapidly temperatures drop. It also
affects plant response to cold temperatures and
influences how well specific protection systems
work. Air with a high relative humidity will resist
temperature change more than dry air, and plant water
lost to desiccating winds will be lessened by high
relative humidity. Relative humidity sensors and
recorders are available in a wide range of prices and
accuracies. One practical way of measuring relative
humidity at a given temperature is with a wet-bulb
thermometer. A wet-bulb thermometer has a bulb
covered with a moist muslin bag, thus lowering the
measured temperature by loss of latent heat through
evaporation. The lower the wet bulb temperature
compared to the dry bulb temperature, the lower the
relative humidity.
5
Care should be taken to purchase high quality
thermometers which should be routinely calibrated in
an ice water bath. Electronic sensors such as
thermocouples and thermisters can be purchased or
made to sense a temperature at a particular point.
Microprocessors are available that can be
programmed to scan a large number of temperature
sensors at predetermined time intervals and record the
temperatures.
Methods Used for Cold Protection
Water Used for Cold Protection
The unique physical properties of water as a
vapor, liquid or solid make it a primary factor in plant
protection from freezing or chilling temperatures. As
water cools at temperatures above freezing, "sensible"
heat is released. Actually, 1000 calories of heat
energy are released as 1 liter of water is cooled from
3°C to 2°C (8.3 BTUs /gallon/°F). A BTU,
British Thermal Unit, is defined as the heat required
to raise the temperature of 1 pound of water 1°F or
to raise 816 grams of water 1°C. As water cools
from 16°C (60°F) to a liquid at 0°C (32°F) 16
kcal per liter (232 BTU's per gallon) of water are
released into the surrounding environment. Fogging,
flooding and sprinkling (at temperatures above
freezing) use the sensible heat in water to moderate
temperature drop in the nursery.
When water changes from a liquid to solid state
(ice), a tremendous amount of energy is released.
This energy is called the "heat of fusion" and is equal
to 80 kcal per liter or 1200 BTU's per gallon.
Sprinkling when air temperatures are below or
approaching 0°C (32F) is sometimes called icing
and uses the heat of fusion to provide cold protection
for plants.
Sprinkling
Sprinkling for cold protection is becoming
increasingly popular in Florida nurseries. It can be
used to moderate temperatures above freezing
because of sensible heat in water and can maintain
plant leaf temperature at 1 to 2°C degrees or more.
Sprinkling should continue until after thawing or the
wet bulb temperature rises above freezing especially
if windy, dry conditions prevail. Evaporative cooling
Cold Protection for Nursery Crops
occurs because heat energy is lost to the atmosphere
as water changes from a liquid to a vapor. Do not
rely on a household window thermometer to monitor
leaf and air temperatures.
The water must be delivered uniformly with
allowances for changes in wind velocities and
direction. Wind adversely affects the sprinkler
distribution pattern and causes the heat from the heat
of fusion to be lost by evaporation. This means that
up to seven times the amount of water used for a
freeze on a calm night must be applied to compensate
for heat loss due to evaporation and conduction when
a 5- to 10-mph wind exists.
The greatest disadvantage of sprinkling is
breakage of plant limbs due to ice weight. Easily
broken container plants may be placed on their sides
and iced to prevent breakage, but should be placed
upright as soon as possible after the freeze.
Sprinkling for freeze protection can be used
effectively in Florida. Plants do not have to be
repositioned, and there are no structures to erect,
therefore the reduced labor requirements for
sprinkling is an advantage. However, frequent
sprinkling may leach nutrients and/or cause
waterlogged soils or container media which may
result in plant stunting or death. The large amount of
water required for this practice could be a limiting
factor in some areas of Florida.
Water applied to aisles of shade structures or
greenhouses increases the moisture content of the air
and soil surrounding the plants (increases wet-bulb
temperatures), thus slowing the rate of temperature
drop. The water absorbs heat during the day which is
released slowly at night. The water should be applied
in late afternoon of a warm day. Sides of adequately
constructed shade houses can be covered with ice by
sprinkling on freezing nights to reduce the effect of
wind.
Fog
Fog also retards the loss of heat from soil and
plant surfaces to the atmosphere. Natural fogs create
a barrier to radiant heat loss much like clouds,
although their effectiveness varies with the size of the
suspended water particles. Fog can provide up to
6
4°C (8°F) of protection outdoors during
radiational cooling. Applying ground water with an
average temperature of 21°C (70°F) to a shade
house or greenhouse can create a ground fog if the
ground surface is several degrees cooler than the
water. This applied water adds heat to the plant
environment and/or buffers temperature change by
increased humidity. Fogging is most effective in an
enclosed structure such as a greenhouse or partially
enclosed structure such as a saran house but must be
uniformly distributed. Temperatures can be elevated
as much as 5°C (9°F) in these unheated structures.
High pressure, low volume systems are the best
means available to create a uniform fog. A low
volume system dramatically reduces water
requirements compared to sprinkling.
Air Movement for Cold Protection
Wind machines have been used for many years
in citrus and vegetable industries and recently in the
ornamental industry as a means of cold protection.
Wind machines are only effective in the advent of
radiational freezes characterized by winds less than 5
miles per hour.
Denser cold air settles in low areas resulting in
temperature strata with warm air above the cold.
Wind movement can disturb this inversion existing
on calm nights. This forced air movement will mix
the cold and warm air resulting in warmer air
surrounding the plant. Air movement also helps
distribute and circulate heat added by orchard heaters
or other sources.
Cold Protection Structures
Structures for cold protection are used to prevent
plant desiccation caused by winds associated with
severe freezes, to trap heat present and to contain
supplied heat energy. They should be constructed to
withstand high winds and minimize heat loss. These
structures are expensive because of construction
materials and required labor for movement of plants
in and out as the conditions or seasons change.
Florida nursery operators should analyze nursery
production systems for each plant species, the risk of
cold damage and the projected dollar return before
investing in structures for cold protection. Certain
Cold Protection for Nursery Crops
high value crops warrant structures specifically for
cod protection, but in other cases, dual purpose
structures should be considered. A structure used for
shading in summer often can be used for cold
protection during winter.
Structures can be constructed of wood,
galvanized pipe, conduit, PVC pipe, or concrete
reinforcing rods and cost will be the overriding factor
in determining which to use. Sizes of structures
depend on size of plants to be protected, growing bed
width and length, and the production system used.
Detailed plans for various greenhouses and cold frame
structures can be obtained from the Extension
Agricultural Engineer through your local cooperative
extension agent.
A common winter protection structure is the
quonset type constructed of bent galvanized pipe.
Half-inch pipe joints are used for bows which are
placed inside larger pipe studs that protrude 15 cm (6
inches) from the ground. This results in a house 4.3
meters (14 feet) wide and 1.8 meters (6 feet) tall. The
bows are usually 61 cm (2 feet) apart and the house is
usually long enough to accommodate common-size
polyethylene. One purlin down the center is adequate
for support. Similar construction with PVC pipe bows
also has become popular due to reduced costs of PVC
pipe. The house should be oriented in a north-south
direction to distribute the light uniformly within the
structure. A clear polyethylene covering (4 to 6 mil)
is usually pulled over the ends and secured along the
sides. A door at one or both ends facilitates entry.
Venting may be done by raising the plastic on the
side opposite prevailing winds and closing during
cold weather.
Frames for such permanent structures are usually
built on a portion of the container production area and
plants from areas adjacent to the quonsets are
crowded into these structures to reduce the number of
houses needed. The irrigation system for the
container production area is usually not flexible
enough to use to irrigate plants enclosed in these
quonsets and expensive hand watering may be
required.
One may elect to construct small, lightweight,
portable structures which can be placed over beds of
cold-sensitive plants during cold weather and
7
removed during warm weather. Such portable
structures may be small quonsets constructed of
conduit, PVC pipe or concrete reinforcing rods
covered with concrete reinforcing wire for support.
They should be wide enough to span a bed 1.8 to 3.0
meters (6 to 10 feet) long. Quonsets made to stack on
top of each other will facilitate storage. Polyethylene
coverings can be attached to wooden strips at the
bottom of each side. A small piece of plastic may be
secured over ends of the structures and opened during
the day for ventilation. Portable quonsets have
definite advantages to the nonportable galvanized
pipe structures since plants are not repositioned and
the structure may be removed for watering.
Winter temperatures in Florida are not
consistently low enough to warrant placing plants on
their sides in structures and covering with
Styrofoam™ or polyethylene material for the entire
season. Nursery operators might consider placing
high-value container plants on their sides in the event
of a severe freeze. The plants may then be covered
with 1 or 2 mil of polyethylene, Styrofoam™ or other
insulating material supported just above the plants so
the cover is not in direct contact with the foliage. The
insulating material should be removed and the plants
placed upright after the freeze.
Shade structures are most effective in providing
protection during cold weather with little air
movement. Saran structures may raise the ambient
temperature under them 1 to 2°C (2 to 4°F) by
reradiated heat radiating from the ground and objects
within the structure. Lath houses are less efficient
than saran structures at reradiating heat, but both
provide some cold protection. Sides of shade
structures may be covered with water during freezing
conditions since the ice forms a windbreak. Care
should be taken that ice loads do not crush the
structure. Some shade structures are designed so they
can be covered with polyethylene film during winter
months or when cold weather is expected.
Plants may be placed in cold frames for
protection from rapid temperature fluctuations.
Small plants, such as liners, can be set upright in the
frame while larger plants (1 gal. etc.) may be placed
on their side. Placing larger plants in the frame is an
expensive operation, and one should contemplate
Cold Protection for Nursery Crops
placing the plants on their sides and covering in the
field rather than transporting to a cold frame. In
either case, the plants should be placed upright as
soon as cold weather passes. Frames may be
economical for protection of liners which were
propagated and/or held in the frame through the
winter. Cold frames may be covered with
polyethylene, Styrofoam™ or other insulating
materials placed over the plants to trap radiant heat.
Polyethylene or other light transmitting covers with a
southern exposure permit solar radiation to warm the
structure. The frame covering should be removed on
warm days to prevent excessive heat build up.
Supplementary Heat
Air temperatures inside unheated quonsets made
of a single layer of plastic are usually about 3°C
(5°F) warmer than outside air on a cold night. This
small temperature differential can be critical to the
survival of many plants, however, supplemental heat
ensures that plant environment temperatures are
above critical levels. Adding heat to an enclosed
structure is more feasible than heating an outside
growing area with orchard heaters, but energy costs
may prohibit the heating of any woody ornamental
growing area. If outside heating is used, it should be
combined in combination with other protection
techniques such as wind movement, fog or some
barrier created to reduce radiant heat loss. The
decision to add supplemental heat must be based on
crop value and rate of return on investment.
Heat sources for structures include solar
radiation, well water and boilers fired by oil, gas or
wood. Solar heating systems, warm water units and
unheated well water circulation offer the greatest
potential for Florida woody ornamental growers.
Nursery operators should consult their Water
Management District personnel before installing
unheated well water circulators. Several passive solar
heating designs are available that collect the sun's
rays and store this heat energy in some medium like
stone or water. The heat collected during the day is
then circulated in the structure at night.
Circulation of warm water (43 to 54°C, 110 to
130°F), not hot water, in enclosed growing and/or
storage areas has gained popularity in recent years.
Warm water is more economical than forced-air heat
8
to keep plant environments above a critical minimum
temperature. This water often is circulated through
PVC pipe installed in some material like sand or
concrete under plant containers.
Well water temperatures in Florida during winter
months range from 20 to 24°C (68 to 75°F). A
system has been designed and tested at the
Agricultural Research Center in Monticello, Florida
that circulates unheated well water through PVC pipe
in a propagation cold frame. This system kept the
minimum cold frame soil temperature above 12°C
(54°F) and the minimum air temperature above
8°C (46°F) while outside temperatures were below
–7°C (20°F) for several hours. Minimum soil and
air temperatures in cold frames without this water
circulation system were 4°C (40°F) for the same
time period.
The decision to add heat to the plant growing
environment must be based on economics. Costs and
returns for a given heating system, structure and crop
plant must be known or estimated before a system is
constructed or an existing system is used.
Treatment of Plants After Cold Stress
The environment to which plants are subjected
after cold stress affects the degree of injury and rate
of symptom development. Importance of
post-exposure environment varies with the severity of
cold stress. Plants exposed to temperatures below
their cold tolerance level will not recover, however,
damage to plants exposed to near critical
temperatures may be influenced by post-stress
handling.
Intense light, low humidity and high
temperatures following chilling of some tropical
plants result in increased water loss through
transpiration. Extreme water stress can develop if the
chill exposure has disrupted water absorption,
temporarily or permanently.
Root systems of plants in field production are
seldom frozen in Florida, but roots of
container-grown plants can be frozen for several
consecutive hours. Clear skies are common when
extremely low temperatures occur in Florida. Sunny
conditions on mornings after night freezes can result
Cold Protection for Nursery Crops
in rapid transpiration (water vapor loss) as leaves are
warmed, but the soil/root mass may be frozen and
unable to provide ample water to leaves, resulting in
excessive water stress and leaf desiccation.
Symptoms may not occur for several days and may be
manifest as marginal leaf scorch or overall browning.
Watering container-grown plants can thaw the
growing medium/root mass and allow water
absorption and transport to the leaves. Excessive
water, however, can leach nutrients and cause root
injury by waterlogging the growing medium.
Cold injury to roots may not be evident until
spring when plants are stressed by high temperatures.
Failure to initiate a spring growth flush may be the
only visual symptom of winter injury and little can be
done to minimize the effect of winter injury at this
time. Weakened or injured plants are more
susceptible to disease attack, so growers should
increase frequency of inspection and implement a
preventative fungicide program if justified. Increased
shade may also reduce heat or water stress during
recovery periods. Justification of such efforts should
be determined on an economic basis.
References
Harrison, D., J. F. Gerber, and R. E. Choate.
1974. Sprinkler irrigation for cold protection.
University of Florida, IFAS, Technical Circular
348, p. 19.
Levitt, J. 1980. Response of plants to
environmental stress. Volume I. Chilling, freezing
and high temperature stresses. Academic Press,
NY, NY. p. 23-290.
Pellett, H. M. and J. V. Carter. 1981. Effect of
nutritional factors on cold hardiness of plants.
Horticultural Reviews 3: 144-171.
Wiest, S. C. and P. L. Stephonkus. 1976.
Acclimation of Pyracantha tissues and differential
thermal analysis of the freezing process. J. Amer.
Soc. Hort. Sci. 101: 273-277.
Wright, R. D. 1977. Physiology of plant tops
during winter. Proc. Int. Plant Prop. Society. Vol.
27: 287-290.
9
ENH1
Cold Protection of Ornamental Plants1
Dewayne L. Ingram and Thomas H. Yeager2
Winter temperatures in Florida are frequently
low enough to cause cold injury to tropical,
subtropical, and occasionally temperate plants not
adapted to Florida climatic conditions. Tropical
plants and summer annuals do not adapt or harden to
withstand temperatures below freezing and many are
injured by temperatures below 50°F (10°C).
Subtropical plants can harden or acclimate (become
accustomed to a new climate) to withstand freezing
temperatures and properly conditioned temperate
plants can withstand temperatures substantially below
freezing. Freezing conditions occur annually in north
and central Florida, while below freezing
temperatures are rare for south Florida. Freeze
probabilities for various locations in Florida are
published in IFAS Bulletin 777, Freeze Probabilities
in Florida.
Freezes can be characterized as radiational or
advective. Radiational freezes or frosts occur on
calm, clear nights when heat radiates from the
surfaces of objects into the environment. These
surfaces can become colder than the air above them
due to this rapid loss of heat or long wave radiation.
When the air is moist, a radiant freeze results in
deposits of ice or frost on surfaces. Dry radiational
freezes leave no ice deposits but can cause freeze
damage. Plant damage from a radiational freeze can
be minimized by reducing radiant heat loss from
plant and soil surfaces.
Advective freezes occur when cold air masses
move from northern regions causing a sudden drop in
temperature. Windy conditions are normal during
advective freezes. Although radiant heat loss occurs
during an advective freeze, the conditions are quite
different from a radiational freeze. Plant protection
during advective freezes is more difficult.
The ability of plants to withstand freezing
temperatures is affected by temperature fluctuations
and day lengths prior to a freeze. A gradual decrease
in temperature over a period of time increases the
ability of plants or plant parts to withstand cold
temperatures. A sudden decrease in temperature in
late fall or early winter usually results in more
damage than the same low temperature in January of
February. Short durations of warmer temperatures in
midwinter can deacclimate some plants resulting in
bud break or flowering. Deacclimated plants are more
prone to freeze injury. Preconditioning of tropical
plants to withstand chilling temperatures has not been
well documented.
1. This document is ENH1, one of a series of the Environmental Horticulture Department, Florida Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida. Original publication date June 1990. Reviewed October 2003. Visit the EDIS Web Site at
http://edis.ifas.ufl.edu.
2. Dewayne L. Ingram, professor and former extension horticulturist; Thomas H. Yeager, associate professor and extension woody horticulturist,
Environmental Horticulture Department, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville FL
32611.
The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and
other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex,
sexual orientation, marital status, national origin, political opinions or affiliations. U.S. Department of Agriculture, Cooperative Extension Service,
University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry
Arrington, Dean
Cold Protection of Ornamental Plants
Cold injury can occur to the entire plant or to
plant parts such as fruits, flowers, buds, leaves,
trunks, stems or roots. Many plant parts can adapt to
tolerate cold, but fruits and roots have little ability to
acclimate or develop cold tolerance. Cold injury to
roots of plants in exposed containers is a common
occurrence and usually is not evident until the plant is
stressed by higher temperatures. Leaf and stem tissue
will not survive ice formation inside the cells (result
of rapid freeze) but many plants can adapt to tolerate
ice formation between cells.
One type of winter injury is plant desiccation or
drying out. This is characterized by marginal or leaf
tip burn in mild cases and totally brown leaves in
severe cases. Desiccation occurs when dry winds and
solar radiation result in the loss of more water from
the leaves than can be absorbed and/or transported by
a cold or frozen root system. Root systems in the
landscape are seldom frozen in Florida, but potting
media in small containers in north Florida can be
frozen for several consecutive hours.
WHAT TO DO BEFORE THE
FREEZE
Homeowners can take steps to help acclimate
plants to cold temperatures and to protect plants from
temperature extremes. These steps range from
selection of a proper planting site to alteration of
cultural practices.
Planting Site Selection
The microclimate of a location is determined by
factors such as elevation, landform, surface
reflectivity, soil properties, degree of canopy cover,
proximity of structures or plants, and the general
solar heat exchange model. Temperature fluctuation
can differ from one location to another, even within a
residential landscape. Thus, existing microclimates
and/or possible modifications of microclimates
should be considered when choosing the planting site
for cold sensitive plants.
Tender plants should be planted in a site with
good air drainage, and not in a low area where cold
air settles. Arranging plantings, fences, or other
barriers to protect tender plants from cold winds
improves cold protection, especially from advective
2
freezes. Poorly drained soils result in weak, shallow
roots which are susceptible to cold injury.
Proper Plant Nutrition
Plants grown with optimal levels and balance of
nutrients will tolerate cold temperatures better and
recover from injury faster than plants grown with
suboptimal or imbalanced nutrition. Late fall
fertilization of nutrient deficient plants or fertilization
before unseasonably warm periods can result in a late
flush of growth which is more susceptible to cold
injury. Plants in Florida landscapes should be
fertilized four times per year. Landscape plants in
north and north central Florida should be fertilized in
March, June, September, and December. Plants in
south and south central Florida should be fertilized in
February, May, August, and November. One to 1 1/2
pounds (454 to 681 grams) of 6-6-6 or 8-8-8, or 1/2
pound (227 grams) of 12-4-8 or 16-4-8 should be
applied per 100 square feet (9 square meters) of
planting area for the first three applications per year.
A decrease in the amount of fertilizer applied in the
fall is necessary because plant nutrient consumption
declines during the colder season. Plants grown in
colder portions of the state require one-third to
one-half the standard fertilization rate in the fall, and
two-thirds the standard rate should be applied in the
warmer sections of Florida.
Shading
Tree canopy covers can reduce cold injury
caused by radiational freezes. Plants in shaded
locations usually go dormant earlier in the fall and
remain dormant later in the spring. Tree canopies
elevate minimum night temperature under them by
reducing radiant heat loss from the ground to the
atmosphere. Shading from early morning sun may
decrease bark splitting of some woody plants. Plants
that thrive in light shade usually display less winter
desiccation than plants in full sun. But plants
requiring sunlight that are grown in shade will be
unhealthy, sparsely foliated, and less tolerant of cold
temperatures.
Windbreaks
Fences, buildings, and temporary coverings, as
well as adjacent plantings, can protect plants from
Cold Protection of Ornamental Plants
cold winds. Windbreaks are especially helpful in
reducing the effects of short-lived advective freezes
and their accompanying winds. Injury due to
radiational freezes is influenced little by windbreaks.
The height, density, and location of a windbreak will
affect the degree of wind speed reduction at a given
site.
Water Relations
Watering landscape plants before a freeze can
help protect plants. A well watered soil will absorb
more solar radiation than dry soil and will reradiate
heat during the night. This practice elevated
minimum night temperatures in the canopy of citrus
trees by as much as 2°F (1°C). However,
prolonged saturated soil conditions damage the root
systems of most plants.
Other Cultural Practices
Avoid late summer or early fall pruning which
can alter the plant hormonal balance resulting in
lateral vegetative budbreak and a flush of growth.
This new growth is more susceptible to cold injury.
Healthy plants are more resistant to cold than
plants weakened by disease, insect damage, or
nematode damage. Routine inspection for pests and
implementation of necessary control measures are
essential. Contact your County Extension Office for
information on pest identification and recommended
controls.
Methods of Protection
Plants in containers can be moved into protective
structures where heat can be supplied and/or trapped.
Containers that must be left outdoors should be
protected by mulches and pushed together before a
freeze to reduce heat loss from container sidewalls.
Leaves of large canopy plants may be damaged if
crowded together for extended periods.
Heat radiating from soil surfaces warms the air
above the soil or is carried away by air currents.
Radiant heat from the soil protects low growing
plants on calm cold nights, while tall, open plants
receive little benefit. Radiant heat loss is reduced by
mulches placed around plants to protect the roots. For
perennials, the root system is all that needs to be
3
protected since the plants die back to the ground
annually.
Coverings protect more from frost than from
extreme cold. Covers that extend to the ground and
are not in contact with plant foliage can lessen cold
injury by reducing radiant heat loss from the plant
and the ground. Foliage in contact with the cover is
often injured because of heat transfer from the
foliage to the colder cover. Some examples of
coverings are: cloth sheets, quilts or black plastic. It
is necessary to remove plastic covers during a sunny
day or provide ventilation of trapped solar radiation.
A light bulb under a cover is a simple method of
providing heat to ornamental plants in the landscape.
WHAT TO DO DURING A FREEZE
Ornamental plants can be protected during a
freeze by sprinkling the plants with water. Sprinkling
for cold protection helps keep leaf surface
temperatures near 32°F (0°C) because sprinkling
utilizes latent heat released when water changes from
a liquid to a solid state. Sprinkling must begin as
freezing temperatures are reached and continue until
thawing is completed. Water must be evenly
distributed and supplied in ample quantity to maintain
a film of liquid water on the foliage surfaces.
Irrigation for several days may water soak the soil
resulting in damaged root systems and/or plant
breakage due to ice build up. Consult Extension
Circular 348, Sprinkler Irrigation for Cold Protection,
for more technical information on this subject.
WHAT TO DO AFTER THE FREEZE
Water Needs
Plant water needs should be checked after a
freeze. The foliage could be transpiring (losing water
vapor) on a sunny day after a freeze while water in
the soil or container medium is frozen. Apply water
to thaw the soil and provide available water for the
plant. Soils or media with high soluble salts should
not be allowed to dry because salts would be
concentrated into a small volume of water and can
burn plant roots.
Cold Protection of Ornamental Plants
Pruning
Severe pruning should be delayed until new
growth appears to ensure that live wood is not
removed. Dead, unsightly leaves may be removed as
soon as they turn brown after a freeze if a high level
of maintenance is desired. Cold injury may appear as
a lack of spring bud break on a portion or all of the
plant, or as an overall weak appearance. Branch tips
may be damaged while older wood is free of injury.
Cold injured wood can be identified by examining the
cambium layer (food conducting tissue) under the
bark for black or brown coloration. Prune these
branches behind the point of discoloration.
Florida homeowners enjoy a vast array of plant
materials and often desire a tropical or semitropical
appearance to their landscapes. Plants are often
planted past their northern limit in Florida, although
microclimates differ dramatically. Tropical and
subtropical plants can be used effectively in the
landscape, but they must be protected or replaced
when necessary. A combination of tender and hardy
plants should be planted in order to prevent total
devastation of the landscape by extremely cold
weather.
4
Cold Protection of Leatherleaf Fern Using Irrigation
Dr. Robert H Stamps
Mid-Florida REC - Apopka
2725 South Binion Road
Apopka, FL 32703-8504
Essentially all commercial irrigation of leatherleaf fern is done using overhead irrigation systems
that use impact sprinklers because these systems can be used to cold protect this subtropical crop
during freezes. There are several methods and techniques that can be used to minimize the
amount of water necessary for cold protecting this crop. See Reference 23 for additional cold
protection information not covered below.
Sprinklers
Frost protection impact sprinklers having faster rotation rates (2 to 3+ revolutions per minute
[rpm]) than conventional impact sprinklers (1 rpm) have been shown to provide equivalent cold
protection using about 50% less water than when using conventional sprinklers (Reference 24).
Dual Irrigation Systems
Shadehouses equipped with two irrigation systems, one to apply water to the shade cloth and one
to apply water to the crop, can be used to decrease the amount of water needed to cold protect
leatherleaf fern during freeze events and to reduce the amount of crop damage during advective
freezes. The over-the-shade cloth irrigation system is run just long enough to wet the cloth
sufficiently so that ice can form and seal the openings. Icing of the shade cloth reduces advective
and radiational heat losses.
Determining When to Start/Stop Irrigation
During mild radiation freezes where temperatures drop slowly over the course of the night,
growers can watch for the onset of frost formation on the crop and start irrigating when frost first
starts to develop. An additional technique growers can employ is to monitor immature fronds and
start irrigating when the tender exposed fronds at the top of the crop canopy first begin to stiffen
up, but before ice forms that causes plant damage. Fronds located in the coldest parts of the
fernery should be monitored. (See Additional Factors to Consider for another water-saving
technique to use during mild, calm freeze events.)
During more severe freezes, irrigation water applications for cold protection should start when
wet bulb temperatures in the shadehouse or hammock reach 34°F (1°C) and stop when wet bulb
temperatures rise to that same temperature, or slightly higher if it is windy.
Additional Factors to Consider
1
During moderate to severe freezes, irrigation water is usually applied continuously to the crop;
however, intermittent water application can successfully be used to cold protect leatherleaf fern
during mild radiation freezes when ambient temperatures stay in the upper 20s (above -3°C).
Careful monitoring of leaf temperatures and/or leaf surfaces for unfrozen water can be used to
determine when to apply additional irrigation water. As long as there is a significant amount of
water on the foliage in the liquid state, additional water application is unnecessary. As the water
turns to ice, heat energy is released. When the supply of liquid water on the foliage gets low due
to ice formation, additional water is applied. This technique is most practical for growers with
one fernery or only a few ferneries located near one another.
Windbreaks and shelterbelts, when used in conjunction with irrigation water, can be beneficial in
reducing cold damage during windy (advective) freezes by reducing air movement. However,
temperatures inside shadehouses (prior to being cold protected) are often colder than
temperatures outside shadehouses during radiation freezes. Most freeze events in Florida are the
radiation type where lack of air mixing is the problem. Under these calm conditions, windbreaks
and shelterbelts can make the temperature inversions caused by the stagnant air movement
worse. Therefore, windbreaks that can be opened and closed are preferred so that they can be left
open as long as possible during radiation freezes and closed up prior to advective freezes.
2
ENH841
Chilling Injury in Tropical Foliage Plants: I.
Spathiphyllum1
J. Chen, L. Qu, R. J. Henny, C. A. Robinson, R. D. Caldwell, Y. Huang2
The genus Spathiphyllum, commonly known as
Peace Lily, has about 36 species. Most species
originate in Central and South America, but two have
their native habitat in the Malay Archipelago.
Because of their elegant white spathes, deep green
foliage, and ability to tolerate low light, Peace Lily
has become one of the most popular ornamental
foliage plants.
Spathiphyllum, like most other foliage plants
with tropical origin, is sensitive to chilling
temperatures. A chilling temperature can be any
temperature that is cold enough to cause injury but not
cold enough to freeze the plant, usually ranging from
just above 32oF to 59oF. Chilling injury has been a
significant cause of losses in foliage plant production,
transportation, and interiorscaping.
A general consensus among growers nowadays
is that Spathiphyllum is relatively chilling resistant
compared to other foliage plants such as Aglaonema.
As a result, comparatively little attention has been
given to maintaining appropriate greenhouse
temperatures for this plant in winter and early spring.
Also, limited information is available on how
Spathiphyllum responds to chilling temperatures.
A systematic evaluation of foliage plants in
response to chilling temperatures has recently been
initiated in our program. Here is a summary of
chilling responses of 15 Spathiphyllum cultivars with
emphases on chilling injury symptoms and cultivar
differences in chilling resistance.
Chilling Injury Symptoms
Chilling injury initiated from leaf tips and
edges and progressed inwardly, with injured leaf
areas becoming necrotic, then turning black, and,
finally, drying up completely. Symptoms appeared
within 24 hours for 'Mini' and 5 days for '5598' after
initial exposure to 38oF. Degree of the visual injury
ranged from minor necrotic lesions on leaf tips or
edges to the complete death of plants, depending on
cultivars, when exposed to 38oF.
In general, mature leaves were more sensitive to
chilling than young leaves. Prolonged exposure to
either 38oF or 45oF caused more injury than a shorter
1. This document is ENH841, a series of the Enviromental Horticulture Department, Florida Cooperative Extension Service, Institute of Food and
Agricultural Science, University of Florida. First published: September 2001. Please visit the EDIS website at http://edis.ifas.ufl.edu
2. Jianjun Chen, Assistant Professor, Plant Physiologist, Luping Qu, former Postdoctoral Research Associate, R. J. Henny, Professor, Plant Geneticist,
Cynthia A. Robinson, former Biological Scientist, and Russell D. Caldwell, Biological Scientist; Yingfeng Huang, visiting Professor at Environmental
Horticulture Department, Institute of Food and Agricultural Sciences, University of Florida, Mid-Florida Research and Education Center, Apopka, FL
32703.
Authors appreciate Agri-Starts, Inc. and Twyford Plant Laboratories, Inc. for providing plant materials for this evaluation.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational
information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin.
For information on obtaining other extension publications, contact your county Cooperative Extension Service office. Florida Cooperative
Extension Service/Institute of Food and Agricultural Sciences/University of Florida/Christine Taylor Waddill, Dean.
Chilling Injury in Tropical Foliage Plants: I. Spathiphyllum
period of exposure at these temperatures, and injury
was more pronounced at 38oF than at 45oF.
Therefore, chilling injury can be lessened if
preventative actions are taken to reduce either the
severity or the duration of chilling, or both, during
production.
No visual injury occurred in plants exposed to
52 F. However, when growth indices (Growth index
= [(canopy's widest width + width perpendicular) ÷
2] x plant height) were measured 45 days after
chilling, they were found to be smaller than those of
the controls (Table 1), suggesting that plant growth
was actually affected by chilling exposure. This
could explain why some cultivars recover slowly in
spring even when temperature becomes optimal and
nutrient supply is adequate.
o
Table 1. Growth index of eight Spathiphyllum cultivars 45
o
days after exposure to 52 F for 5 or 10 days in contrast to that
of control plants.
Days Being Chilled
2
Cultivar Differences in Chilling
Resistance
Distinct chilling resistance exists among
Spathiphyllum cultivars with leaf area injury ranging
from 2.5% to 100%. Based on the percentage of
injured leaf areas, resistant cultivars are '5598',
'Annette', '0597-3', and 'Debbie'; moderately
resistant: 'Viscount' and 'Classic Viscount';
intermediately resistant: 'Little Angel', 'Petite', and
'Connie'; sensitive: 'Vicki Lynn', 'Starlight', and
'Lynise'; and highly sensitive: 'Mini', 'UF576-14',
and 'UF474-1' (Table 2).
In summary, chilling injury in Spathiphyllum can
be either visible or invisible. Visible injury ranging
from necrotic lesions to complete plant death
occurred when plants were exposed to 38oF or 45oF
for 5 days. The reduction or delay in plant growth
mainly reflects invisible injury when Spathiphyllum is
exposed to 50oF. Cultivars differ significantly in
resistance to chilling. Using resistant cultivars may
reduce the chance of chilling injury in production and
transport as well as decrease greenhouse heating costs.
y
5
10
5598
370.5
223.8
200.2
Annette
283.1
227.8
181.7
Debbie
276.3
249.9
200.6
Viscount
350.6
301.9
181.5
Little Angel
265.5
215.3
199.5
Cultivar
Connie
502.9
332.9
308.5
5598
2.5
Lynise
474.8
424.9
387.1
379.8
194.5
202.3
Annette
0597-3
3.0
6.1
Debbie
9.0
Cultivar
0
Mini
z
2
Growth index (cm )=[(plant width 1 + plant width 2)
÷2] x plant height.
y
Control plants grown in a shaded greenhouse with a
temperative range of 18°C to 32°C (64.4°F to
89.6°F).
Compared to Aglaonema cultivars, which are
often visibly injured at 50oF, Spathiphyllum cultivars
indeed appear to be more resistant to chilling
temperatures. Growers, however, should be
particularly aware of the invisible effect of chilling in
Spathiphyllum, which may be wrongly diagnosed as
insufficient fertilization or other culture practices.
Table 2. Classification of chilling responses of 15
Spathiphyllum cultivars based on the percentage of injured
o
leaf area three days after exposure to 38 F for five days.
% injured
leaf area
Classification of
chilling resistance
Resistant
Viscount
16.0
Moderately
Classic Viscount
16.8
resistant
Little Angel
34.4
Intermediately
Petite
Connie
35.7
35.9
resistant
Vicki Lynn
Starlight
56.7
58.3
Sensitive
Lynise
60.2
Mini
95.5
UF576-14
100.0
UF474-1
100.0
Highly
sensitive
ENH843
Chilling Injury in Tropical Foliage Plants: II. Aglaonema 1
Jianjun Chen, Richard W. Henley, Richard J. Henny, Russell D. Caldwell, and Cynthia A. Robinson2
Aglaonema, commonly called Chinese
Evergreen, is a member of the family Araceae and
comprises 21 known species that are native to
southeast Asia where they grow in the humid, heavily
shaded tropical forest (Huxley, 1994). Cultivated in
the East for centuries, Aglaonema was believed to
bring fortune to life and probably was introduced to
the western world in 1885 (Brown, 2000). Currently,
Aglaonema are among the most popular tropical
ornamental foliage plants because of the attractive
foliar variegation, low light and humidity tolerances,
and few pest problems.
A major limitation in the production of
Aglaonema is chilling injury when plants are exposed
to temperatures from just above 32°F to 59°F
(Chen, et al., 2001; Fooshee and McConnell, 1987;
Henley, et al., 1998; Hummel and Henny, 1986).
Chilling injury to Aglaonema may also occur during
shipment, retail display, and interior decoration
(Blessington and Collins, 1993; Griffith, 1998;
Joiner, 1981).
With recently increased release of Aglaonema
cultivars, hybrids with different variegation patterns,
showy petiole and stem colors, and varying growth
habits have become available (Cialone, 2000).
However, the response of these new cultivars to
chilling temperatures is largely unknown. This report
intends to summarize chilling injury symptoms in
Aglaonema and cultivar differences in resistance to
chilling temperatures.
Twelve Aglaonema cultivars were grown in 8”
pots under greenhouse conditions. After attaining
marketable sizes, these plants were chilled at 35°,
45°, and 55°F for 24 hours in walk-in coolers.
Chilling injury symptoms were characterized and
percentages of injured leaves were determined daily
for the next 10 days. Since Aglanomena's aesthetic
appearance is directly related to foliage color and
quality, any damage on leaves, regardless of severity,
can greatly reduce its ornamental value in the market
place (Chen, et al., 1998; Henley, et al., 1998).
Therefore, the percentage of injured leaves was the
primary parameter used to determine the sensitivity
of cultivars to chilling temperatures.
Chilling Injury in Aglaonema
Dark and greasy patches appeared between
midvein and leaf margin on the upper surface of
leaves two days after chilling at 35°, 45°, or 55°F.
Injured areas or individual patches were irregular,
1. This document is ENH843, a series of the Environmental Horticulture Department, Florida Cooperative Extension Service, Institute of Food and
Agricultural Science, University of Florida. First published: October 2001. Please visit the EDIS website at http://edis.ifas.ufl.edu
2. Jianjun Chen, Assistant Professor, Plant Physiologist; Richard W. Henley, Professor Emeritus; Richard J. Henny, Professor, Plant Geneticist; Russell
D. Caldwell, Biological Scientist; and Cynthia A. Robinson, former Biological Scientist at the University of Florida, Institute of Food and Agricultural
Sciences, Environmental Horticulture Department and Mid-Florida Research and Education Center, Apopka, FL 32703.
The authors appreciate the Sunshine Foliage World, Zolfo Springs, FL and the Butler's Nursery, Miami, FL for providing initial plant materials, and also
Verlite Co., Tampa, FL for providing Vergo Container Mix A.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational
information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin.
For information on obtaining other extension publications, contact your county Cooperative Extension Service office. Florida Cooperative
Extension Service/Institute of Food and Agricultural Sciences/University of Florida/Christine Taylor Waddill, Dean.
Chilling Injury in Tropical Foliage Plants: II. Aglaonema
varying from 10% to 80% of the entire leaf area. If
injured areas totaled less than 40%, leaves could stay
alive for months; whereas if more than 40%, leaves
could became yellow and finally abscised. The
number of leaves injured continuously increased for
up to 10 days, but no further injured leaves appeared
10 days later.
Leaves of different maturity expressed dissimilar
responses to chilling temperatures. Mature and old
leaves appeared to be much more sensitive to chilling
than young leaves. Among the injured leaf totals,
mature leaf injury ranged from 45% to 100% and old
leaves from 14% to 53%, but young leaf injury was
only 0% to 6% depending on cultivar (Table 1).
2
chilling, with 30, 43, and 68% of leaves injured at
55°, 45°, and 35°F 10 days after chilling. 'Maria',
a cultivar well known for its chilling resistance, was
not the most resistant one tested. Ten days after
chilling at 35°F, 32% of 'Maria's' leaves were
injured, but there was no discernable injury on
'Emerald Star', 'Stars', or 'Jewel of India'. 'Emerald
Star', 'Stars', and 'Jewel of India' were the most
chilling resistant cultivars. In addition, 'Black
Lance' and 'Green Lady' appeared to be slightly
better than or at least equal to 'Maria' in chilling
resistance. Cultivation of the resistant cultivars will
reduce chilling injury and energy used for heating
during production.
Table 2. Percentages of injured leaves of 12 Aglaonema
cultivars 10 days after 24-hour chilling at 35°F, 45°F, or
55°F.
Table 1. Percentages of leaf injury categorized by leaf
z
maturity 10 days after 24-hour chilling at 35°F.
Leaf maturity
Cultivar
Chilling temperature
Young
Mature
Old
Cultivar
Emerald Star
0.0
0.0
0.0
Emerald Star
0.0
0.0
0.0
Stars
0.0
0.0
0.0
Stars
0.0
0.0
0.0
Jewel of India
0.0
0.0
0.0
Jewel of India
0.0
0.0
0.0
Black Lance
0.0
100.0
0.0
Maria
0.0
66.6
33.4
Black Lance
18.3
12.0
0.0
Green Lady
1.0
46.6
52.4
Maria
32.0
8.3
0.0
Green Majesty
0.0
60.2
39.8
Green Lady
34.0
9.0
0.0
Royal Queen
0.0
46.9
53.1
Green Majesty
50.0
17.3
4.7
Moonshine
2.5
50.1
47.4
Silver Queen
4.7
54.4
40.9
Royal Queen
51.0
34.1
14.0
Manila Pride
0.0
85.6
14.4
Moonshine
54.0
10.0
2.0
Silver Frost
6.5
45.4
48.1
Silver Queen
68.3
37.7
29.7
Manila Pride
73.0
15.3
13.3
Silver Frost
80.0
4.7
0.0
z
Young: the most recently fully expanded leaves up to
and including the newest unfurled leaf; mature: leaves
immediately below the young leaves down to the old
leaves; and old: about three to four basal leaves. The
sum of the percentage of injured young, mature, and old
leaves equals to 100%.
Cultivar Differences in Chilling
Resistance
Cultivars were significantly different in
resistance to chilling temperatures (Table 2). 'Silver
Queen', one of the most popular cultivars in the
foliage plant industry, was extremely sensitive to
35°F
45°F
55°F
Cultivars also differ in their sensitivity to critical
chilling temperature, i. e. a temperature at which
chilling injury occurs (Table 2). For example, 10
days after chilling, 'Silver Frost' had no injury at
55°F and only 5% leaf injury at 45°F, but 80% of
the leaves were injured when exposed to 35°F. A
similar pattern occurred in 'Maria' and 'Green Lady'.
In contrast, 'Silver Queen' and 'Royal Queen' had
30% and 14% injured leaves, respectively, 10 days
after chilling at 55°F. Implications are that critical
Chilling Injury in Tropical Foliage Plants: II. Aglaonema
chilling temperatures of 'Silver Frost', 'Maria', and
'Green Lady' are around 45°F, whereas critical
temperatures of 'Silver Queen' and 'Royal Queen'
are above 55°F. Critical chilling temperature
distinctions are potentially important in Aglaonema
production because growers will be able to manage
their greenhouse temperatures based on
cultivar-dependent chilling temperature sensitivity
ranges.
In summary, chilling injury in Aglaonema was
characterized by dark and greasy-appearing patches
on injured leaves. Young leaves appeared to be more
resistant to chilling temperatures than either mature
or old leaves. Significant chilling resistance exists
among Aglaonema cultivars, a genus considered
extremely sensitive to chilling temperatures.
'Emerald Star', 'Stars', and 'Jewel of India'
withstood exposure to 35°F without injury, whereas
'Silver Queen' was injured at 55°F. In addition,
cultivars differed in their sensitivity to critical
chilling temperatures. Use of resistant cultivars may
greatly reduce the chance of chilling injury during
production and transportation and also conserve
energy used for greenhouse heating. However, if
chilling-sensitive cultivars must be grown,
greenhouse facilities should allow for maintaining
temperature above 60°F.
Literature Cited
Blessington, T.M. and Collons, P.C. 1993.
Foliage Plants: Prolonged Quality, Postproduction
Care and Handling. Ball Publishing, Batavia, IL.
Brown, D. 2000. Aroids: Plants of the Arum
Family. Second Edition. Timber Press. Portland.
OR.
Chen, J., Henley, R.W., Henny, R.J., Caldwell,
R.D., and Robinson, C.A. 1998. A simple leaf-assay
method for evaluating Aglaonema sensitivity to
chilling temperatures. Proc. Fla. State Hort. Soc.
111:43-46.
Chen, J., Henny, R.J., McConnell, D.B., and
Nell, T.A. 2001. Cultivar differences in interior
performances of acclimatized foliage plants. Acta
Horticulturae 543:135-140.
3
Cialone, J. 2000. New Chinese evergreen
cultivars for the interiorscape. Ohio Florists' Assoc.
Bull. No. 847:1, 9-10.
Fooshee, W.C. and McConnell, D.B. 1987.
Response of Aglaonema 'Silver Queen' to nighttime
chilling temperatures. HortScience 22:254-255.
Griffith, L.P. 1998. Tropical Foliage Plants: A
Grower's Guide. Ball Publishing, Batavia, IL.
Henley, R.W., Henny, R.J., and Chen, J. 1998.
Chilling injury on twenty Aglaonema Cultivars. Proc.
Southern Nursery Assoc. Conf. 43:117-121.
Hummel, R.L. and Henny, R.J. 1986. Variation
in sensitivity to chilling injury within the genus
Aglaonema. HortScience 21:291-293.
Huxley, A. 1994. The New Royal Horticultural
Society Dictionary of Gardening. The Macmillon
Press Ltd, London.
Joiner, J.N. 1981. Foliage Plant Production.
Prentice-Hall, Englewood Cliffs, NJ.
ENH-92
Treating Cold-Damaged Palms1
Alan W. Meerow2
Cold weather slows down the growth of palms,
reduces the activity of the roots, and often weakens
the plant to the point where a disease can become
active and kill the palm. Severe cold damage from
frost or freezing temperatures destroy plant tissues
and may severely reduce water conduction in the
trunk for years. Often the only above-ground portion
of a cold-damaged palm that is still alive is the
protected bud. As warmer weather returns, primary
or secondary plant pathogens often attack weakened
plants through damaged tissue.
Possible Preventative Action
The secondary plant pathogens that cause death
of the bud soon after freeze damage are, in most
cases, bacteria that are present on healthy palm tissue
at low levels, but become a problem only after the
damage is received. Consequently, there may be
value in applying a preventative spray of fungicidal
copper before freezing temperatures are reached in
order to reduce these bacteria populations to the
lowest levels possible. This strategy has not been
tested, however, under controlled conditions.
It is evident that palm tissue deficient in one or
more essential plant nutrients is less tolerant of
exposure to freezing temperatures. Thus, it is
important that palms receive a balanced fertilization
in late summer or early fall to insure that foliar
nutrient levels are near optimum as winter approaches.
Protecting the Damaged Palm While
Waiting for Warm Weather
Figure 1. Severe cold damage.
To avoid attacks by primary or secondary plant
pathogens, it is important that steps be taken to insure
protection of the healthy bud until active growth
resumes.
1. This document is ENH-92, one of a series of the Environmental Horticulture Department, Florida Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida. Original publication date December 1992. Reviewed October 2003. Visit the EDIS Web Site at
http://edis.ifas.ufl.edu.
2. Alan W. Meerow, associate professor, Palm and Tropical Ornamentals Specialist, Environmental Horticulture Department, Ft. Lauderdale Agricultural
Research and Education Center (AREC), Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville
FL 32611.
The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and
other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex,
sexual orientation, marital status, national origin, political opinions or affiliations. U.S. Department of Agriculture, Cooperative Extension Service,
University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry
Arrington, Dean
Treating Cold-Damaged Palms
Remove the cold-damaged portion of the leaves.
Leaves should not be completely removed if they are
green (even if they are spotted from the cold). The
green intact portions of the palm are important to
assure adequate photosynthesis during the recovery
stage.
Disease Control
1. Immediately after pruning, spray the palms with
a fungicide containing copper at the
recommended rate. The use of fungicide is
recommended only for palms not bearing
edible fruit. Include a spreader sticker.
2. Repeat the copper spray 10 days after the first
treatment or use another broad spectrum
fungicide. Contact your county agent for current
fungicide recommendations. In all cases, these
sprays must cover the damaged tissue and
healthy bud thoroughly. Copper sprays should
not be repeated more than twice because of the
possibility of copper phytotoxicity.
3. Palms growing in containers may benefit from a
soil drench of fungicides that suppress root
diseases. Contact your county agent for current
recommendations of available fungicide
formulations.
4. Occasionally, cold damage is so severe or
disease has already progressed to the point where
the spear leaf becomes loose and pulls out easily.
With these palms, there is still a chance of
recovery if the meristem (growth point) is alive.
To treat these palms, remove as much dead and
decaying material from around the bud as
possible so it can dry out. Drench with a copper
fungicide in the bud using the force of the
sprayer to clean out the bud as much as possible.
Follow up ten days later.
Nutrients
Warmer weather promotes rapid growth and this
helps the palms recover. After the two initial sprays, a
monthly application of soluble nutrients should be
applied to the leaves. The following formulation has
been tested by the Institute of Food and Agricultural
Sciences. Other products containing similar nutrients
should work equally as well:
2
• 1/4 to 1/2 teaspoon per gallon S.T.E.M.® (Peter's
Soluble Trace Element Mix) Spreader sticker
Cryptic Cold Damage
Palms that were severely damaged during the
winter should be watched carefully during the
subsequent spring and summer seasons. Damage to
embryonic leaves within the bud may not show up
until those leaves emerge (as much as six months to
one year after the freeze). If leaves emerging during
the spring and summer months appear deformed,
partially, browned or otherwise abnormal, this may
be indicative of this type of damage. In most cases,
the palm will grow out of this later in the season.
Figure 2. deformed, partially, browned or otherwise
abnormal.
Freeze damage to conducting tissue in the trunk
may limit the ability of the palm to supply water to
the canopy of leaves. Unlike typical broad-leafed
trees, palms have no ability to regenerate conducting
tissue in the trunk. Sudden collapse of some (or even
all) of the leaves in the crown during the first periods
of high temperature in the spring or summer after a
winter freeze may indicate that this type of trunk
damage has occurred. Unfortunately, there is nothing
that can be done to remedy this, and loss of the palm
will be inevitable.
Conclusion
The above steps will help reduce loss from cold
damage and speed up recovery. Nutrient sprays
should continue into the summer if the plants are
young or newly established in the landscape. Older
palms will benefit from a soil application of a
Treating Cold-Damaged Palms
granular palm fertilizer in the spring that is repeated
every three to four months.
3
ENH80
Low Temperature Damage to Turf1
L.E. Trenholm2
Injury to warm-season turfgrasses often occurs
when temperatures drop below 20°F (-6.7°C). In
general, major winter injury to turfgrass is caused by
the following: 1) tissue desiccation, 2) direct low
temperature kill, 3) diseases, and 4) traffic effects.
For example, damage from the 1989-90 freeze can
probably be attributed to poor cultural practices
which weakened turf and made it more susceptible to
injury or death from low temperatures. Subsequent
damage may also have resulted from effects of traffic
on frozen turf.
Reasons for Temperature Damage
Most warm-season grasses have very poor cold
tolerance ratings when compared to cool-season
grasses. Due to lower fall temperatures and reduced
daylengths, warm-season grasses enter a state of
dormancy, evidenced by brown, dead shoot tissue,
which is maintained throughout the winter in north
Florida. In central Florida, a growth and metabolism
reduction, rather than an actual dormancy, may be
seen. This death of shoot tissue or lack of growth
does not generally indicate that the grass is not going
to recover; instead, this is a natural state and provides
protection for the grass when faced with cold
temperatures. In cases of severe freezing
temperatures, some grasses may suffer irreversible
damage, and use of these grasses should be limited to
warmer climates. For instance, St. Augustinegrass,
which generally exhibits poor cold tolerance, is not
used as extensively in north Florida as other grasses,
and is used less as you progress into northern
Georgia.
Cultural factors that tend to promote cold injury
include: poor drainage (soil compaction), excessive
thatch, reduced lighting, excessive fall nitrogen
fertilization, and a close mowing height. The weather
pattern preceding a severe and sudden cold wave also
influences a turf's low temperature tolerance. In
general, if turf has had several frosts prior to a drastic
temperature drop, it has been better `conditioned' to
survive. The 1989-90 cold snap in much of north and
central Florida was preceded by three to five frosts.
These helped increase carbohydrates and proteins in
plants that enabled crown tissue to withstand cold
temperatures without severe membrane disruption.
The freezes that occurred in the early 1980s did not
have these preconditioning periods, resulting in
severe damage. In this case, grasses were still green,
and protective crown tissue was succulent and
therefore susceptible to cold temperatures.
Shaded areas may suffer more intense cold
damage. Shade (low light intensity) prevents normal
daytime soil warming; therefore, these areas stay
colder for longer periods of time, and more low
1. This document is Fact Sheet ENH-80, a series of the Environmental Horticulture Department, Florida Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida. First published: May 1991. Revised: May 2000. Please visit the EDIS web site at http://edis.ifas.ufl.edu.
2. L.E. Trenholm, Assistant Professor, Extension Turfgrass Specialist, Environmental Horticulture Department, Institute of Food and Agricultural Sciences,
University of Florida, Gainesville, FL.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational
information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin.
For information on obtaining other extension publications, contact your county Cooperative Extension Service office. Florida Cooperative
Extension Service/Institute of Food and Agricultural Sciences/University of Florida/Christine Taylor Waddill, Dean.
Low Temperature Damage to Turf
2
temperature damage may occur. Shade also reduces
the plant's ability to produce carbohydrates needed
for increased cold tolerance.
Regardless of turfgrass species selected, the
following management practices can help minimize
cold temperature damage.
Traffic (foot or vehicular) may further increase
injury to cold damaged turf. Traffic should not be
allowed on frozen turf until the soil and plants have
completely thawed. Syringing the area lightly prior
to allowing traffic on it will help reduce frozen turf
injury associated with traffic.
Recently planted (sprigged, sodded, or seeded)
grasses can expect to be more severely damaged by
cold. Because roots are less developed and shoot
tissue more tender, overall stress tolerance is reduced
in just-planted grasses. To minimize cold damage,
particularly in north Florida, delay fall planting of
grasses until spring or early summer. In south
Florida, grasses may be planted year-round, but care
should be taken to protect immature turf from
occasional cold temperatures.
Assessing the Extent of Injury
Symptoms of direct low temperature damage
includes leaves that initially appear wilted. They may
subsequently take on a water-soaked look, turning
whitish brown and then progressing to a dark brown.
Damaged leaves are not turgid and tend to mat over
the soil, often emitting a distinct putrid odor. Areas
hardest hit are usually poorly drained ones such as
soil depressions. If you suspect your grass has
experienced cold damage, take several 4 to 5 inch
diameter plugs from suspected areas and place them
in a warm area for regrowth. A greenhouse or warm
windowsill should suffice. Observe these for 30 days
or until growth resumes. If good regrowth occurs,
then little damage is assumed. If regrowth is absent
or sporadic, then some degree of damage was
sustained.
Selection of Cold-Hardy Grasses
Within the warm-season grasses, the most
cold-hardy species is zoysiagrass, followed in
descending order by bermudagrass, bahiagrass,
centipedegrass, seashore paspalum, carpetgrass, and
St. Augustinegrass. Within these species, there are
different degrees of cold tolerance between cultivars.
For instance, centipedegrass cultivars 'Oklawn,'
'Tifblair,' and especially 'TennTurf' have good cold
tolerance. In St. Augustinegrasses, 'Raleigh,'
'Bitterblue,' 'Seville,' and 'Jade' generally exhibit
the best cold tolerance, while 'Floratam,'
'Floralawn,' and 'Floratine' are more susceptible to
cold temperatures.
Management Practices to Minimize
Cold Damage
Fertility can also influence cold tolerance. Late
season (late September in north Florida, after mid
October in the central and southern regions)
application of nitrogen will promote shoot growth in
the fall, when the grass growth and metabolism are
slowing down. Fall shoot growth will deplete
carbohydrate reserves, which help the grass regrow
from any stress, and tender shoots are less able to
tolerate adverse conditions such as cold. Therefore,
late-season application of nitrogen is not
recommended.
Potassium fertility in the fall has been shown to
enhance cold tolerance and promote earlier spring
greenup of grass. Application of potassium at the
rate of 1/2 to 1 lb. per 1000 square feet is
recommended for the last fertilization of the year.
Effects of shade can increase cold damage.
Because shaded areas do not become as warm as
areas in full sun, injury in these areas may be more
severe. Compacted soils also remain cooler than
well-drained areas, which increases the probability of
cold temperature damage. See Environmental
Horticulture and Soil and Water Science Department
factsheets for more information on relieving soil
compaction.
Increasing mowing height can reduce cold injury
in a number of ways. First, it will promote deeper
rooting, which is one factor always associated with
greater stress tolerance. It will also allow for
production and storage of more carbohydrates late in
Low Temperature Damage to Turf
the summer. In addition, higher mowing heights can
create a warmer micro-environment due to extra
canopy cover provided by longer leaf tissue.
Because cold damage may initially resemble
drought stress, people sometimes feel that additional
water may be needed. Overall, correct irrigation
practices can alleviate many stresses faced by turf,
but as the grass goes into dormancy, water needs are
reduced.
Spring Greenup
Unless your turfgrass has been subjected to
unusually cold or freezing temperatures for long
periods, or your management practices have
augmented the effects of the temperatures, your grass
should begin to green up as temperatures and day
lengths increase in the spring. At this time,
recommended fertility, irrigation, and mowing
practices should be resumed for the best health of
your lawn all season.
3
HS968
Protecting Blueberries from Freezes in Florida 1
P.M. Lyrene and J.G. Williamson 2
Dormancy and Cold Hardiness
Most temperate zone plants, including blueberry,
enter a dormant period during late fall and winter
characterized by no growth and greatly reduced
metabolic activity of above ground parts. This
dormant condition is a defense mechanism which
enables plants to survive cold. The development of
dormancy and cold hardiness is a gradual process
which begins in late fall or early winter in Florida. In
response to shorter days and lower temperatures
during the fall, growth of blueberry plants slows,
dormancy begins to develop, and cold hardiness
increases. Even before cold temperatures occur,
blueberries develop a certain amount of cold
hardiness. Exposure to cool temperatures greatly
accelerates dormancy development and increases cold
hardiness. Later in the winter, as temperatures
continue to drop, cold hardiness continues to
increase. Fully dormant blueberry plants are quite
cold hardy and seldom suffer serious damage from
cold weather in Florida.
Once fully dormant, a blueberry plant must be
exposed to a period of cool temperatures before it will
break dormancy and grow normally the following
spring. This is a result of its chilling requirement.
Each cultivar has its own characteristic chilling
requirement. The amount of chilling that blueberry
plants receive in Florida varies considerably from
year to year. Temperatures needed to satisfy the
chilling requirement are generally considered to be
between 32°F and 45°F. However, keeping track of
chill hours is more complicated than merely
recording the number of hours between 32°F and
45°F. Exposure to one hour of temperatures either
slightly above or below the optimum chilling
temperature can result in some chill accumulation.
The farther from the optimum temperature, the
smaller the fraction. At temperatures below freezing,
no chilling accumulation occurs. Loss of accumulated
chilling can occur with exposure to very warm
temperatures. Temperatures between 32°F and
45°F appear to be most effective at satisfying the
chilling requirement of blueberries, but temperatures
between 45°F and 55°F contribute something to
chilling and temperatures above 70°F between
mid-November and mid-February probably negate
some chilling.
Another factor which can affect chill
accumulation in blueberries is the presence of leaves
during chilling. Blueberry plants in Florida often
retain some of their leaves throughout much of the
winter, especially in southern Florida. These plants
1. This document is HS968, one of a series of the Department of Horticultural Sciences, Florida Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida. Original publication date: May 2004. Please visit the EDIS Web site at http://edis.ifas.ufl.edu.
2. P.M. Lyrene, professor, and J.G. Williamson, professor, Horticultural Sciences Department, Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida, Gainesville, FL 32611.
The Institute of Food and Agricultural Sciences (IFAS) is an Equal Employment Opportunity - Affirmative Action Employer authorized to provide
research, educational information and other services only to individuals and institutions that function without regard to race, creed, color, religion,
age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For information on obtaining other extension
publications, contact your county Cooperative Extension Service office. Florida Cooperative Extension Service / Institute of Food and Agricultural
Sciences / University of Florida / Larry R. Arrington, Interim Dean
Protecting Blueberries from Freezes in Florida
will not accumulate chilling as quickly as defoliated
plants.
Once chilling is satisfied, warm temperatures
cause vegetative and floral buds to initiate growth. In
Florida, most blueberry cultivars initiate flower bud
growth before vegetative bud growth. As flower buds
pass through the developmental stages from dormant
buds to fully open flowers, they become more
susceptible to cold. Experiments suggest that swollen
rabbiteye flowers (Spiers stage 3) can withstand
temperatures as low as 25°F, but some may be killed
at 21°F, or lower. Buds which had opened to the
point where individual flowers were distinguishable
can be killed by temperatures of 25°F, and buds
with expanded corollas are often damaged or killed at
28°F. Even a slight freeze can cause severe damage
to fully open flower buds.
Freezes and Freeze Protection
Freezes during February, March, and April are a
much greater problem for Florida blueberry growers
than was anticipated 20 years ago. With a shift toward
earlier-ripening cultivars, both rabbiteye and southern
highbush blueberry crop losses from freezes have
increased significantly. This is compounded by the
fact that many blueberry plantings have been
established in cold pockets to take advantage of soils
that are higher in organic matter. Today, freezes are
probably the number one production problem for
Florida blueberry growers.
Several factors affect the severity of damage to
blueberry plants, flowers, and fruit in particular
freezes. Some of these factors are fairly well
understood; others have received little study.
Temperature, Wind Speed, and Dew Point
A low dew point is always worse than a high
dew point. Dry air loses heat faster after sunset.
Water vapor in all levels of the atmosphere radiates
heat to the earth's surface and partially off-sets the
heat being lost by radiation from the ground. Moist
air increases the amount of frost formed and increases
the amount of latent heat released in the field at night.
If the air is very dry and there is not much wind,
flowers can become several degrees colder than the
air, and low humidity increases evaporative cooling
2
when irrigation is run. Wind can be bad or good. If
overhead irrigation is being applied, wind is a serious
problem, because it increases evaporative cooling,
removes heat from the field, and interferes with the
even distribution of the water. If water is not being
applied, the wind is beneficial. It prevents formation
of a cold pool of air near the ground beneath the
inversion and it prevents the flowers and berries from
becoming colder than the air that surrounds them. On
still nights, flowers and berries lose heat faster than
the air and become colder than the air. Open
blueberry flowers have sometimes survived
temperatures as low as 26°F when the wind blew
steadily through the night with no calm periods,
whereas flowers in the same stage of development
are often killed at 28°F when there is little or no
wind and the dew point is below 26°F.
Plant Tissue and Stage of Hardiness
Young blueberry plants are sometimes damaged
in field nurseries during late fall and winter if they
have not been properly hardened. New spring
vegetative flushes can be killed by the same
temperatures that kill open flowers and fruit.
Completely dormant branches and flower buds are
very cold-hardy in midwinter. However, any January
warmth promotes growth and expansion of the flower
buds, and some loss of hardiness accompanies each
subsequent stage of flower bud development. Styles,
ovary tissue, ovules, corollas, and pedicels are similar
in their freezing points, but some marginal freezes
may kill the styles but not the corollas, or the ovules
but not the ovaries. The relative sensitivity of these
organs seems to vary from one freeze to another. A
partial crop can sometimes be rescued by spraying
gibberellic acid on the ovaries of flowers whose
styles or ovules have been killed by marginal freezes.
However, GA-rescued fruit develop slower, produce
smaller berries, and ripen later than fruit with viable
ovaries which makes the rescued crop much less
profitable for Florida growers who rely on high prices
for early-market fresh fruit. If the dew point is high,
and the temperature is only slightly below freezing,
open blueberry flowers may be heavily coated with
frost with no damage to any flower parts. On the
other hand, if the air is dry, flowers may be killed
with no frost on the plants, even on still nights.
Protecting Blueberries from Freezes in Florida
Physical Conditions in the Field
Pine bark mulch culture can lower the air
temperature at flower level in blueberry fields by as
much as 5°F on calm nights with low dew points.
Pine bark beds on hillsides with excellent air drainage
would probably be less problematic. If the dew point
is high, the pine bark has less cooling effect. The
effect of thoroughly wetting the pine bark the
afternoon before the freeze has not been studied. Dry
soil and any weeds, alive or dead, lower the
temperature in the field. Any object in the field on
which frost can form, might be expected to lower the
temperature of the blueberry plants by contributing to
the dehydration of the air during the night. Dry soil
lowers the temperature compared to wet soil by two
mechanisms. First, dry soil provides little moisture to
replenish the water vapor that is lost from the air by
frost formation. This allows the temperature and dew
point in the field to continue to fall after dew and frost
begin to form. Second, dry soil conducts heat poorly
from the warm depths of the soil to the cold surface.
Wet soil has been reported to have a temperature
conductivity approximately 8 times greater than that
of dry sand. The lay of the land, with respect to
elevation and air drainage patterns, greatly affects
field temperatures on calm nights with low dew
point, but is less important as the wind and/or dew
point increase.
Weather Conditioning Before the Freeze
The ability of citrus leaves and stems to harden
in response to several weeks during which night
temperatures fall below about 50°F before a freeze
is well known. Experienced blueberry growers are
convinced that blueberry flowers and flower buds at
all stages of development also have some ability to
increase their cold hardiness in response to cold days
preceding the freeze. This phenomenon merits
further study in blueberry.
Blueberry Variety
It has long been known by growers that flowers
and developing flower buds of rabbiteye blueberry
(Vaccinium ashei Reade) are less cold hardy than
highbush buds and flowers at the same stages of
development. Among southern highbush cultivars,
which are advanced-generation interspecific hybrids
3
between a deciduous, northern blueberry species
(Vaccinium corymbosum from New Jersey) and an
evergreen blueberry species from the Florida
peninsula (V. darrowi Camp), there appears to be
wide variation in flower bud cold tolerance. Just
prior to anthesis, the range in killing temperatures of
flowers of different varieties at similar stages of
development in the field appears to be on the order of
2 or 3°F.
Passive Freeze Protection
Several strategies can be used to reduce crop loss
from freezes. Some are more practical than others.
Cultivar Selection
The freeze risk to a blueberry crop can be
reduced greatly by planting cultivars that flower late.
For example, late-flowering rabbiteye cultivars such
as Powderblue, Brightwell and Tifblue seldom suffer
significant crop damage from freezes at the research
farm in Gainesville. However, over a period of many
years at the same location, Sharpblue, Misty and other
early-flowering southern highbush cultivars averaged
losses over 60% unless they were protected with
overhead irrigation. Unfortunately, late-flowering
cultivars tend to ripen later than those that flower
early, and, at present, cultivars that ripen before
prices fall around May 20 usually flower before the
last hard freezes on north Florida farmland.
Site Selection
Both advective and radiation freezes have
damaged blueberry fruit and flowers in Florida.
Advective freezes occur when a cold air mass moves
rapidly into Florida, usually accompanied by
moderate to high winds. During an advective freeze,
temperatures at a particular latitude in Florida tend to
be similar from farm to farm. The exception might be
for areas located immediately down wind from a
large lake, which could be a few degrees warmer than
surrounding areas. During an advective freeze,
temperatures are gradually warmer the farther
southeast you are in peninsular Florida. The farther
south and east, the less likely it is that temperatures
will be low enough to freeze berries while the wind is
blowing. This is important because the combination
Protecting Blueberries from Freezes in Florida
of wind and freezing temperatures is the hardest
situation to combat when cold protecting blueberries.
Radiation freezes occur on clear, cold nights
when there is little or no wind. Heat is radiated from
the Earth to the open sky. Under these conditions,
large temperature differences can develop over short
distances due to differences in elevation. Hill tops
may be 5 to 10 degrees F warmer than low ground at
the same latitude. Hilltops in the northern part of
peninsular Florida may be warmer than cold pockets
200 miles farther south. Traditionally, Florida
blueberry farms have been planted on low, cold land
because soils there are high in organic matter. There
is increasing interest in planting blueberries on high
ridge land, circumventing the soil problem either by
growing plants in pine bark, or by adding enough peat
to the soil to obtain good growth.
4
indicated by the table. In Alachua county, blueberry
crops have occasionally been lost between February
20 and March 20, even in fields protected with
overhead irrigation at a rate of 0.2 inches per hour.
Temperatures of 26°F combined with 15 mph winds
and low humidities exceed the protection capabilities
of such a system, even though the same amount of
water would protect flowers down to 18°F with no
wind.
Pruning
If done at the right time of the year, pruning can
delay flowering by 1 to 2 weeks. Growers who often
lose their crops in freezes can delay flowering by
pruning immediately after harvest and providing
conditions that promote vegetative growth during
summer and fall. Flower buds produced on vigorous
shoots that result from pruning in late May and early
June will mature later in the fall and flower later the
following spring.
Freeze Protection Methods
Overhead irrigation systems, designed for freeze
protection with diesel, rather than electric, pumps are
the most widely used and practical method of
reducing blueberry fruit losses to freezes in Florida
(Fig. 1). Large volumes of water must be pumped to
get good protection. The number of gallons per
minute needed to protect one acre depends on the
temperature, wind speed, relative humidity, and the
design of the system. Table 1 adapted from Gerber
and Martsolf (Circular 287, Florida Agricultural
Extension Service) attempts to describe the
relationships between minimum
temperature/windspeed combinations and water
application rates needed for protection during a
freeze. However, this table does not consider the
water vapor content of the air. With unusually dry
air, higher water application rates will be needed than
Figure 1. Blueberry field protected with overhead irrigation
during a freeze.
Some growers have designed systems that can be
quickly altered to deliver 0.4 inch of water per hour
by changing riser heads. A practical system might be
able to deliver 0.25 inch per hour over 10 acres or 0.4
inch per hour over 6 acres. In most years, the entire
10 acres could be protected. In years with severe late
freezes, 4 acres could be allowed to freeze so that the
other 6 acres could be given maximum protection.
Before installing an irrigation system, seek advice
from an irrigation specialist.
Protecting Blueberries from Freezes in Florida
Best use of an irrigation system for freeze
protection requires experience and close attention to
the weather. Blueberry flowers and fruit will not
freeze if temperatures in a weather bureau shelter
located alongside the plants at the same height as the
flowers stay 32 degrees F or above. Frost on the
grass between the rows does not necessarily mean
that flowers are damaged since on humid nights, frost
can form when temperatures in the weather shelter
are as high as 36 degrees F. With a clear sky and no
wind, a thermometer placed open to the sky will read
about 2°F colder than the same thermometer at the
same height in a weather shelter. By placing several
thermometers throughout a blueberry field, one can
learn a lot about the temperature distribution patterns
in that field during radiation freezes.
If or when to turn on the irrigation system during
a cold night can sometimes be difficult decisions to
make. The answer depends on such factors as the
capabilities of the irrigation system, state of
development of the crop, relative humidity,
temperature, and wind speed. Some of these factors
can not be predicted with certainty. The following
guidelines should be helpful in most but not
necessarily in every situation. First, the system
should not be used on nights where the
temperature-wind combination produces conditions
more extreme than the system was designed to
handle. Refer to a reliable forecast and Table 1 to
determine whether or not the system should be used.
Calm Nights
If there is no wind predicted and a decision is
made to run the system, it is usually turned on when a
thermometer, hung under the open sky from a bare
branch in the coldest part of the field reaches 32°F.
However, if the dew point temperature is below
25°F, the system should be turned on at 34°F,
which will probably be only about half an hour before
the temperature reaches 32°F. The temperature has a
great tendency to fall to within 1 degree of the dew
point on clear, calm nights. If the dew point is 26°F
or lower and frost forms on flowers or berries, they
will be killed. If the dew point temperature is 30°F
or above and frost forms on flowers or berries, they
may not be damaged. During the morning following
5
the freeze, if there is no wind and the sun is shining
brightly, the irrigation can be turned off when icicles
are falling rapidly from the plants and have been
falling for more than half an hour. Never turn off the
irrigation before icicles are falling no matter what the
temperature. If the dew point temperature is below
20°F, continue running irrigation until the shaded air
temperature rises to 40°F. If it is windy and the dew
point is 26°F or below, do not turn off the irrigation
until most of the icicles have fallen.
Windy Nights
For windy freezes, the decisions about whether
or not to run irrigation become complicated. Table 1
provides guidelines for determining the amount of
water required to protect fruit at various
temperature/wind speed combinations. However, the
values in Table 1 assume normal relative humidity. If
relative humidity is very low, as sometimes happens
when a cold dry air mass moves into Florida, the
values in Table1 may underestimate the amount of
water needed for adequate freeze protection. Paying
attention to the dew point temperatures during
various nights of freeze protection will help take the
mystery out of why crops are sometimes saved when
it seemed too cold and windy and why crops may be
lost when it seemed they should have been saved.
Overhead Irrigation the Afternoon or
Evening Before a Freeze
Experienced fruit growers have long known that
irrigating their fields the afternoon before an expected
freeze can sometimes reduce the damage caused by
the freeze. There are four situations in which this
practice is potentially useful to blueberry growers.
First Situation
It is a calm afternoon, and minimum
temperatures are forecast to be on the borderline
between damaging and safe. A wet ground may allow
Protecting Blueberries from Freezes in Florida
the grower to avoid having to turn on the system
during the night. In such situations, even minimum
overhead irrigation during the night should be
effective in preventing damage, but there are
disadvantages to irrigating on frost nights, and being
able to avoid a run is highly desirable. If the
temperature does become critical during the night, a
wet ground will reduce the probability that damage
will occur before the system is turned on.
Second Situation
The dew point is low and the wind speed is
expected to be erratic during the night. Or,
temperatures are expected to fall to or below the
damaging point with light winds, with a rising wind
expected later in the night. Even though a rising wind
in the night is frequently bringing in colder, drier air
behind a secondary cold front, the effect may be to
raise the temperature of the blueberry flowers, as cold
surface air is mixed with warmer air above the
inversion and the wind raises the flower temperature
to the temperature of the surrounding air. On some
occasions, growers may be able to protect the crop
with overhead irrigation before the wind increases,
but lose the crop due to evaporative cooling after the
wind begins. On the other hand, dry plants might
have survived the cold wind without damage, but
could not survive the lower temperatures that
occurred before the wind broke the inversion. On
some such nights, fields that have been thoroughly
wet late in the afternoon before the freeze have
escaped damage because a higher temperature was
maintained before the wind began, whereas crops
were lost in dry fields that were not irrigated at all and
in fields in which irrigation was run throughout the
night.
Third Situation
The grower lacks sufficient pumping capacity to
protect the entire acreage against a freeze of the
expected severity. A decision is made to change the
sprinkler heads to a larger orifice diameter in half of
the field and close off the valve to the other half. It
may be possible to reduce damage in the half that
cannot be irrigated during the night by thoroughly
wetting the soil during the afternoon before the
freeze.
6
Fourth Situation
This may be the most common situation in
which growers could improve their crops by adopting
a practice that is seldom being used at present.
Frequently, during January and early February, after
blueberry flower buds have begun to swell in
response to warm periods in the winter, a freeze will
occur in which the dew point is so low, the air so
cold, and the probability of some wind during the
night so high that no experienced grower would
choose to run the irrigation at night for fear of
causing massive damage from evaporative cooling,
frozen emitters, broken branches, and uprooted
plants. Furthermore, many of the flower buds may
still be quite dormant, and will survive if nothing is
done. Frequently, in late January, the flower buds
may show a wide range of developmental stages. For
example, 20% of the buds might be killed if the
temperature falls to 24°F, an additional 20% will be
killed if it falls to 21°F, an additional 20% will be
killed at 18°F, and 20% would survive 16°F. A
low-risk strategy for the grower would be to
thoroughly wet the ground the afternoon before
freezing temperatures began, with the goal of raising
minimum temperatures in the field by two or three
degrees and reducing the fraction of the crop lost.
Because fruit prices are often higher in years with
light crops, and because blueberry plants can
sometimes partially compensate losses in fruit
number by increasing fruit size, saving part of the
crop could be quite rewarding for the grower.
Alternative Freeze Protection
Methods
Wind machines and helicopters have been used
to some extent to protect blueberry crops from freezes
in Florida. Both are based on the fact that on clear
calm nights a strong temperature inversion develops,
in which temperatures within 6 feet of the ground
may become much colder than temperatures 50 to
100 feet above the ground. By mixing these air
layers, wind can raise the temperature near the ground
by about 4°F, the exact amount varying with the
strength of the temperature inversion and the
effectiveness of the air mixing. On nights with wind,
wind machines and helicopters cannot warm an
orchard because no temperature inversion develops.
Protecting Blueberries from Freezes in Florida
Many windy freezes occur in Florida during January
and February which usually coincides with the
southern highbush bloom period.
A single wind machine will normally provide a
maximum heating of about 5°F over an area of
about 10 acres. The cost of installing and maintaining
a wind machine is fairly high, but the cost of running
one is comparatively low, about eight gallons of
gasoline per hour. A single helicopter can provide a
similar degree of heating over an area of about 40
acres, so long as it is constantly flying. A problem
with helicopters, apart from their high cost, is how to
keep them continuously in the air on freeze nights.
Scheduling problems, pilot fatigue, mechanical
breakdowns, and the need to refuel can interrupt
service. Wind machines are commonly used in some
fruit producing areas of the World. They are seldom
used on blueberries in Florida but might be practical
at the south end of the production area in Florida.
Orchard heaters, which burn fuel oil, were
widely used to protect citrus trees from freezing
before fuel prices became prohibitive. They are also
effective, but expensive, in protecting blueberry
crops. On a still night, 24 grove heaters per acre will
heat a citrus grove by about 5°F. These heaters burn
a gallon of fuel per hour and fuel costs can be over
$300 per acre for a single night. High fuel costs have
prevented widespread use of grove heaters for freeze
protection in Florida blueberries.
7
Protecting Blueberries from Freezes in Florida
8
Table 1. Suggested overhead irrigation application rates for cold protection of blueberries under different wind and
temperature conditions.
Minimum
Temperature
Expected
Wind Speed in M.P.H.
0 to 1
2 to 4
5 to 8
10 to 12
Application Rate (inches/hour)
1
27 °F
0.10
0.10
0.10
0.1
26 °F
0.10
0.10
0.14
0.2
24 °F
22 °F
0.10
0.12
0.16
0.24
0.3
0.5
0.4
0.6
20 °F
0.16
0.3
0.6
0.8
18 °F
0.20
0.4
0.7
1.0
15 °F
0.26
0.5
0.9
---
Dry air accompanied by wind will require higher application rates than indicated for a given temperature/wind speed
combination. From Gerber and Martsolf, Extension Circular 287, Florida Agricultural Extension Service.
HS911
Frost and Freeze Protection for Vegetable Crops Grown
in Florida in the BMP Era1
Eric Simonne and George Hochmuth2
This publication is one of a series entitled
Fertilizer and Irrigation Management in the BMP Era.
This series is divided into nine principles described in
the Introduction Chapter (HOS-897). This
publication is part of Principle 5, "Irrigation Amount
Must Reflect Crop Water Use ... No More, No Less."
BMP implementation requires a global approach to
production management. However, for presentation
purposes, each aspect of vegetable production is
described in a separate publication.
Protecting crops from occasional frost and
freezes has been a continual problem for vegetable
growers throughout Florida. Several options are
available including sprinkler irrigation, soil banking,
row covers, raising water table, and cover crops.
Working Definition
Frost and freeze protection consists of various
methods to protect plants from damage from freezing
temperatures.
Frost and Freeze Protection - Things
to Do
• Water freezes at 32° F, while most fleshy plant
tissue freezes at lower temperatures
(approximately 28° F). Know the temperature
threshold below which crop damage will occur.
• The best method of frost-freeze protection is
proper site selection. Visualizing the flow of
cold air, as if it were water, and its possible
buildup in low spots or behind cold air dams is
the most effective site selection method. If a site
has good cold air drainage, then it is likely a
good production site as far as frost-freeze
damage is concerned.
• Use soil banking (covering the crop with soil)
for crops such as potatoes which have large
energy reserves in the seed piece to grow out
from the soil covering.
• Early melons, sweet corn, or beans may be
planted in a small trench to protect seedlings
from frost by the warm surrounding soil mass.
1. This document is HS911, one of a series of the Horticultural Sciences Department, Florida Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida. Publication date: January 2003. Please visit the EDIS Web site at http://edis.ifas.ufl.edu.
2. Eric Simonne, assistant professor, George Hochmuth, center director, NFREC-Quincy, Horticultural Sciences Department, Cooperative Extension Service,
Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational
information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin.
For information on obtaining other extension publications, contact your county Cooperative Extension Service office. Florida Cooperative
Extension Service/Institute of Food and Agricultural Sciences/University of Florida/Christine Taylor Waddill, Dean.
Frost and Freeze Protection for Vegetable Crops Grown in Florida in the BMP Era
• When possible, use high beds to prevent the
crop from being exposed to water logging
conditions when overhead irrigation is used for
frost protection.
• Row covers (crop covers) can be used for frost
and freeze protection. Depending on the
thickness of the cover, protection to 25° F is
possible.
• Sprinkler irrigation uses the heat released when
water changes from liquid to ice to warm the air
around the plant leaves. Water must be
continuously provided to protect the plant.
Enough water must be used to compensate for all
heat losses. Sprinkler irrigation is less effective
under windy conditions.
• Monitor crop nutritional status after repeated
frost protection since mobile nutrients may leach
or their oxidation state may have changed.
• Follow weather advisory reports; know air
temperature and wind factor when deciding to
freeze protect.
Irrigation System Design for
Freeze/Frost Protection - Things to
Do
• Design the irrigation system for high uniformity
of water application. Wind adversely affects the
uniformity of water applied through an overhead
sprinkler system.
• The sprinkler pattern should overlap the
adjacent sprinkler pattern down the laterals and
between the laterals about 50% for adequate
coverage for winds up to 13 km/hr (8 miles/hr).
The spacing of the sprinklers must be closer in
the areas where high winds may be frequent.
• Sprinklers should rotate at least one revolution
per minute.
• Brass sprinklers should be used since they
operate with greater consistency and less
susceptibility to stoppage due to freezing.
• Use diesel pumps since electric pumps are prone
to power outages.
• Temperature, humidity, and wind velocity
affect the rate of water application necessary for
cold protection. Water applied at too low a rate
can cause more damage than no protection
during a freeze. The system must be designed to
apply a sufficient rate of water to protect the
plant. Table 1 gives guidelines regarding water
application rates as related to wind velocity and
temperature. Table 2 gives guidelines regarding
precipitation rate for selected nozzle capacity
and sprinkler spacings. By using Table 1 and
Table 2, you can find the appropriate sprinkler
size and spacing for adequate water application
for certain climatic conditions.
Things to Avoid: Potential Pitfalls
• Do not run frost/freeze protection when
temperatures are so low that water may freeze
inside the irrigation system.
• Do not run the injection system intermittently
for frost protection.
Other Considerations
• Formation of a “milky-white” ice without
icicles and incomplete coverage of the plant
indicate too low a rate of water application.
Clear ice and icicles are usually an indication of
successful cold protection.
• Row covers can also be used to promote early
maturity and increase yields of vegetables in
cooler production areas.
• Row covers will cost about $750 to $1000 per
acre, but most materials can be used for at least
two or more seasons.
Additional Readings
Row Covers for Commercial Vegetable Culture
in Florida, Circ. 728, Fla. Coop. Ext. Ser., IFAS,
Univ. of Fla. http://edis.ifas.ufl.edu/CV201
Row Covers for Growth Enhancement, HS716,
Fla. Coop. Ext. Ser., IFAS, Univ. of Fla.
http://edis.ifas.ufl.edu/CV106
2
Frost and Freeze Protection for Vegetable Crops Grown in Florida in the BMP Era
Cold Protection by Irrigation: Dew Point and
Humidity Technology, HS76, Fla. Coop. Ext. Ser.,
IFAS, Univ. of Fla. http://edis.ifas.ufl.edu/CH054
Uses of Water in Florida Crop Production
Systems, Circ. 940, Fla. Coop. Ext. Ser., IFAS, Univ.
of Fla. http://edis.ifas.ufl.edu/AE036
Principles and Practices of Irrigation
Management for Vegetables, AE260, Fla. Coop. Ext.
Ser., IFAS, Univ. of Fla.
http://edis.ifas.ufl.edu/CV107
Sprinkler Irrigation for Cold Protection, Circ.
348, Fla. Coop. Ext. Ser., IFAS, Univ. of Fla.
3
Frost and Freeze Protection for Vegetable Crops Grown in Florida in the BMP Era
4
1
Table 1. Application rate recommended for cold protection under different wind and temperature conditions.
Expected Minimum
Temperature
Wind Speed (MPH)
0 to 1
2 to 4
5 to 8
Application rate (inches/hour)
27 F
0.10
0.10
0.10
26 F
0.10
0.10
0.14
24 F
0.10
0.16
0.30
22 F
0.12
0.24
0.50
20 F
0.16
0.30
0.60
18 F
0.20
0.40
0.70
15 F
0.26
0.50
0.90
1
Ext. Circular 287, Florida Agricultural Extension Service, by Gerber and Martsolf
Table 2. Precipitation rate for selected nozzle capacity and sprinkler spacings.
Sprinkler Spacing (ft.)
Gallons Per Minute/Sprinkler
2
3
30 x 30
.21
.32
30 x 40
.16
.24
.32
40 x 40
.18
.24
.30
40 x 50
.14
.29
.24
.29
50 x 50
.12
.16
.19
.24
.32
.13
.16
.19
.26
.32
.13
.16
.21
.27
.32
.14
.18
.23
.28
50 x 60
60 x 60
60 x 70
4
5
6
8
10
12
15
.34
HS-931
Microsprinkler Irrigation for Cold Protection of Florida
Citrus1
L. R. Parsons and B. J. Boman2
Introduction
Millions of boxes of fruit and thousands of acres
of citrus trees have been lost in freezes and frosts.
Oranges are usually damaged when the fruit are
exposed to temperatures of 28°F or lower for 4
hours or more. As the temperature gets colder or
durations below 28°F get longer, damage to fruit,
leaves, twigs, and eventually large branches
increases. More than nearly any other factor, freezes
have caused some of the most dramatic changes in
fruit supply, availability, and price. Thus, any method
that provides some cold protection can be of major
importance to citrus growers.
Many cold protection methods have been used
over the years. These methods include heaters, wind
machines, fog generators, high volume over-tree
irrigation, and low volume undertree microsprinkler
irrigation.
High fuel cost has made grove heating during
freeze nights prohibitively expensive except for high
value crops. Wind machines are effective, but they
require maintenance and need a strong temperature
inversion for optimum effectiveness. Fog can
provide cold protection, but light winds can blow the
fog away from the grove and obscure nearby
roadways. In south Florida where temperatures do
not normally go far below freezing, high volume
over-tree sprinkler irrigation has been used
effectively on limes and avocados. In central and
north Florida, temperatures are usually colder, and
over-tree sprinklers should not be used on large citrus
trees because the weight of the ice formed can break
off limbs and cause tree collapse. With overhead
systems, all leaves are wetted and susceptible to
damaging evaporative cooling during low humidity
or windy freezes. Many trees were killed in the
windy 1962 freeze when overhead sprinklers were
used because of evaporative cooling of wetted
leaves.
Low volume undertree microsprinkler irrigation
is an alternative method for partial frost protection
and can be more affordable than other methods (Fig.
1). Microsprinklers have proven effective during
several freeze nights in central Florida tests. In
addition to frost protection, microsprinklers can
provide effective year-round irrigation.
Microsprinklers, or spray jets, are small, low volume
irrigation sprinklers that discharge 5 to 50
1. This document is Fact Sheet HS-931, one of a series of the Horticultural Sciences Department, Florida Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida. Publication date: November 2003. Please visit the EDIS Web site at http://edis.ifas.ufl.edu.
2. L. R. Parsons, professor, Horticultural Sciences Department, Citrus Research and Education Center, Lake Alfred and B. J. Boman, associate professor,
Agricultural and Biological Engineering Department, Indian River Research and Education Center, Ft. Pierce. Cooperative Extension Service, Institute of
Food and Agricultural Sciences, Gainesville, FL 32611.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational
information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin.
For information on obtaining other extension publications, contact your county Cooperative Extension Service office. Florida Cooperative
Extension Service/Institute of Food and Agricultural Sciences/University of Florida/Christine Taylor Waddill, Dean.
Microsprinkler Irrigation for Cold Protection of Florida Citrus
gallons/hour. In citrus groves, the most commonly
used spray jets discharge from 5 to 25 gallons/hour
and cover a diameter of 5 to 21 feet. Usually 1 or 2
microsprinklers per tree are installed at the ground
level or on short risers. Unlike overhead sprinklers,
microsprinklers do not commonly wet leaves and
branches above a height of about 3 feet and do not
usually cause serious limb damage.
2
evaluate the risks and benefits and successfully use
irrigation for cold protection.
Heat of fusion
The heat that is released when liquid water
freezes to solid ice is called the heat of fusion. The
amount of heat generated when water freezes is 1200
BTUs/gallon or 80 calories/gram of water frozen. As
long as enough water is continuously applied to a
plant, the heat generated when water freezes can keep
the plant at or near 32°F (0°C). This is the
principle used by strawberry, fern, or citrus nursery
growers when they apply high volumes of water by
sprinkler irrigation to protect their plants. At least
0.25 inch/hour or more is required for cold
protection. With very low temperatures, low
humidity, or high winds, more water must be applied
to get adequate protection. Many citrus nurserymen
need to apply water at rates of 0.40 inches/hour or
higher.
Heat of vaporization
Figure 1. Ice on young citrus as a result of microsprinkler
irrigation for freeze protection.
How Irrigation Works for Cold
Protection
Various forms of irrigation have been used for
frost and freeze protection for many years. When
used properly, water can provide partial or complete
cold protection for a number of crops. On the other
hand, improper use of water can increase cooling or
ice loading and cause greater damage than if no water
were used at all. Because water can provide
protection in one situation and cause damage in
another, it is important to know what principles are
involved in cold protection. To better understand
what can happen when using water during a freeze,
several commonly used terms need to be understood.
With a knowledge of these terms, one can better
The heat lost when water changes from a liquid
to water vapor is called the heat of vaporization. At
32°F, the heat of vaporization is about 8950
BTUs/gallon or 596 calories/gram of water
evaporated. Note that the heat of vaporization is
about 7.5 times greater than the heat of fusion. This
means that to maintain a stable situation when both
freezing and evaporation occur, for every gallon of
water that evaporates, 7.5 gallons of water need to be
frozen to balance out the heat in the grove. Anything
that promotes evaporation, such as low humidity and
high wind speed, will promote overall cooling.
If the water application rate is high enough on
the trunk of a young tree, it will be protected by the
ice formation. However, on the edge of and outside
of the iced zone, temperatures will not be maintained
at 32°F, and those parts will probably be damaged or
killed. Therefore, usually the tops of young trees or
branches above the iced zone are more severely
damaged after a freeze.
Dry bulb temperature (Tdb)
The dry bulb temperature is the temperature of
the ambient air, which is the same thing as the
normal air temperature read with a grove thermometer.
Microsprinkler Irrigation for Cold Protection of Florida Citrus
3
Wet bulb temperature (Twb)
The wet bulb temperature is defined as the
lowest temperature to which air can be cooled solely
by the addition of water. An example of wet bulb
temperature is the temperature one feels when coming
out of a swimming pool on a windy day. As long as a
surface is wet while in the wind, its temperature will
drop to the prevailing wet bulb temperature of the air.
The wet bulb temperature is between the dew
point and dry bulb temperatures and normally closer
to dry bulb than the dew point temperature. When the
air is saturated with water vapor, the relative
humidity is 100%, and all three temperatures (dew
point, wet bulb, and dry bulb) are equal (Fig. 2).
Humidity
Humidity refers to the amount of water vapor in
the air. There are various ways to express humidity,
but the most commonly used terms are relative
humidity and dew point temperature.
Relative humidity (RH)
Relative humidity is the percentage (or ratio) of
water vapor in the air in relation to the amount
needed to saturate the air at the same temperature.
Although commonly used, relative humidity is not
the best measure of humidity because it depends on
the air temperature. Warm air holds more water
vapor than cool air.
Example:
Compare the amount of water vapor in air at
40°F and 70% RH to air at 90°F and 70%
RH.
Using Fig. 2, enter the graph at 40°F on the
horizontal axis and go straight up until midway
between the 60% and 80% RH curves. Then
read the weight of water vapor in 1 lb of dry
air on the left vertical axis - about 25 grains.
Following the same procedure for 90°F and
70% RH, the weight of water vapor in 1 lb of
dry air is about 150 grains, or 6 times as much.
Hence, the warmer air in this example holds
about 6 times more water vapor than the
cooler air even though both have the same RH.
Figure 2. Psychrometric chart showing effects of relative
humidity and dry bulb temperatures on dew point
temperatures.
Dew point temperature (Tdp)
The dew point temperature is the temperature at
which dew begins to form or the temperature at
which water vapor condenses to liquid water. It is
also the temperature at which air reaches water vapor
saturation. A common example of condensation is
the water that forms on the outside of a glass of ice
water. This happens because the temperature of the
glass surface is lower than the dew point temperature
of the ambient air in the room. Thus, some of the
water vapor in the surrounding air condenses on the
outside of the cold glass.
When referring to cold protection, dew point is
one of the better ways to describe the humidity or
amount of water vapor in the air. When the dew
point is below 32°F, it is often called the frost point
because frost can form when the temperature is below
freezing. The dew point is important on freeze nights
because water vapor in the air can slow the rate of
temperature fall. With a relatively high dew point on
a cool night, radiant heat losses from a grove are
reduced, and the temperature may be expected to fall
slowly. But if the dew point is quite low, the
temperature can be expected to fall rapidly. Water
vapor absorbs infrared radiation. Water droplets or
fog are even more effective radiation absorbers than
water vapor. Therefore, fog can reduce the rate of
temperature drop on a frost night.
In addition to affecting the rate of radiation loss,
the dew point is often a "basement" temperature, and
Microsprinkler Irrigation for Cold Protection of Florida Citrus
4
the air temperature will not go much below it unless
drier air moves in. The reason for this is that when
dew condenses or ice forms, heat is given off. The
amount of heat from condensation is the same as the
heat of vaporization (about 8950 BTUs per gallon or
596 calories per gram of water) because vapor is
changing to liquid water. This heat release during
condensation slows the rate at which the air
temperature drops. If dew forms, water vapor is
condensed from the air, and the humidity or dew
point of that air is lowered. This is how the
evaporation coil in an air conditioner removes water
vapor and dehumidifies the air.
Dew point temperatures are commonly higher on
the coasts than they are inland. In the central Florida
citrus belt (e.g. near Lake Alfred), dew point
temperatures on a moderate frost night can be in the
vicinity of 20 to 30°F. On more severe freeze
nights, dew point temperatures can be 10°F or
lower. For example, in the damaging Christmas 1983
and January 1985 freezes, dew point temperatures in
Lake Alfred approached 5°F, which is exceedingly
low for central Florida.
Figure 3. Wet bulb, dry bulb, and dew point temperature
o
relationships for wet bulb temperatures less than 40 F.
When wet bulb and dry bulb temperatures are
known, Fig. 3 and Fig. 4 can be used. (Fig. 4 for Twb
< 40°F and Fig. 5 Twb 40-60°F). Locate Tdb on
the bottom axis and go up to the curve with the
measured Twb. The dew point temperature (Tdp) is
read from the vertical axis value from the intersection
of the Tdb and Twb lines.
Example:
For the following temperatures, determine the
dew point temperature.
Tdb = 60°F
Twb = 46°F
Using Fig. 4, locate 60°F on the bottom axis
and go up to the Twb = 46°F curve. The value
of Tdp can be read from the vertical axis as
30°F.
If Tdb and the relative humidity (RH) are known,
Tdp can be determined from Fig. 5. Locate Tdb on
the bottom axis and go up to the line that represents
the measured relative humidity. Tdp is read from the
Figure 4. Wet bulb, dry bulb, and dew point temperature
o
relationships for wet bulb temperatures from 40-60 F.
vertical axis value from the intersection of the Tdb
and RH lines.
Microsprinkler Irrigation for Cold Protection of Florida Citrus
5
Psychrometer
A psychrometer is a device used to determine
atmospheric humidity by reading the wet bulb and
dry bulb temperatures. The wet bulb thermometer is
kept wet by a moistened cotton wick or sleeve. With
a psychrometer, one determines how much cooler the
wet bulb is than the dry bulb and then calculates
humidity by using appropriate graphs or tables.
"Psychros" comes from the Greek work meaning
"cold," and hence a psychrometer measures humidity
by determining how much colder the wet bulb
thermometer is than the dry bulb thermometer.
Figure 5. Relationships between dry bulb temperature,
relative humidity, and dew point temperature.
Example:
For a dry bulb temperature of 45°F with a
relative humidity of 50%, determine the dew
point temperature.
For an accurate reading, the wet bulb
thermometer must have air moving over it. With a
sling psychrometer, air flow is created by rotating the
two thermometers through the air by hand (Fig. 6).
The sling psychrometer consists of two
liquid-in-glass thermometers. One thermometer
measures the air (dry bulb) temperature while the
other one measures the wet-bulb temperature. After
the wick is dipped in distilled water, the sling
psychrometer is whirled around using the handle.
Select Fig. 5 (for relationships between Tdb, RH,
and Tdp). Enter the figure on the bottom axis at Tdb =
of 45°F and go straight up until the RH = 50% line is
intersected. Read the Tdp from the vertical axis as
Tdp= 32°F.
Wind chill
Wind chill refers to the cooling effect of moving
air on a warm body and is expressed in terms of the
amount of heat lost per unit area per unit of time.
Wind chill was developed to estimate heat loss rate
from humans and other warm blooded organisms. It
does not apply to plants or vegetation. Even though
wind chill does not apply to plants, wind can remove
heat from a grove rapidly. In a windy freeze,
temperature of a dry leaf is usually fairly close to air
temperature. If the leaf is wet and water is not
freezing on it, the leaf can theoretically cool to the
wet bulb temperature. Thus, the length of time a
grove will be at low temperatures can be longer on a
windy night than on a calm one. Thus, more damage
can potentially occur during a windy freeze.
Figure 6. Operation of sling psychrometer to obtain wet
and dry bulb temperatures.
Water evaporates from the wick on the wet-bulb
thermometer and cools the thermometer due to the
latent heat of vaporization. The wet-bulb
thermometer is cooled to the lowest value possible in
a few minutes. This value is known as the wet-bulb
temperature. The drier the air is, the more the
thermometer cools and thus, the lower the wet-bulb
temperature becomes. With a fan ventilated
psychrometer, a fan blows air across the two
thermometers. Fan ventilated psychrometers cost
Microsprinkler Irrigation for Cold Protection of Florida Citrus
6
more than sling psychrometers, but they are more
convenient to operate on freeze nights.
Sling psychrometers work well at temperatures
above freezing, but are more difficult to operate at
temperatures below freezing. The reason for this is
that at temperatures much below freezing, the water
on the wet bulb freezes, releases its heat of fusion,
and raises the wet bulb temperature to around 32°F.
Eventually, it is possible to get a "frost" wet bulb
temperature if one rotates the sling psychrometer long
enough. A battery powered fan psychrometer avoids
some of the problems of a sling psychrometer but it
may take 20 minutes or more to get a valid wet bulb
temperature when the air is below freezing. A
slightly different chart is used for humidity
calculations when the wet bulb sleeve has ice on it.
Effectiveness of Microsprinklers
Microsprinkler irrigation is more effective for
cold protection when high volumes of water are used.
A system that delivers the maximum amount of water
per acre and is practical or affordable is best for frost
protection. Irrigation rates of 2000 gallons/acre/hour
or 33.3 gallons/acre/minute are recommended. This
can be accomplished with one 20 gallon/hour jet or
two 10 gallon/hour jets per tree in a grove with 100
trees/acre (Fig. 7). If there are 200 trees per acre,
then one 10 gallon/hour jet is adequate. Rates below
this level will provide some protection but not as
much as higher rates. Application rates of 3000
gallons/acre/hour or more are more effective at lower
temperatures.
At high application rates, average warming with
spray jets at a 4-foot height is only 2 to 3°F. Spot
readings have occasionally shown temperature
increases of 4°F or more, but 1°F or less is also
common during freeze nights. At heights greater than
8 feet, warming is usually less than 1°F. When
compared to a non-irrigated area, lower volume
systems provide slight warming, but higher volume
systems (2000 gallons/acre/hour) provide more
warming. Low irrigation rates can provide some,
although not much, protection on calm frost nights.
While a small amount of water can provide a little
protection, it is generally best not to go below 10
gallons/hour/tree. Emitters that put out less than 10
Figure 7. Gallons per acre per hour of water available for
freeze protection based on microsprinkler discharge rate
(ranging from 8-20 gph per tree) and tree planting density.
gallons/hour usually have small orifices that can plug
easily. If a jet next to a tree is plugged, that tree will
suffer more damage or may be killed.
Example:
Determine the gallons per hour per acre
available for freeze protection for a block that
has 1 microsprinkler/tree that discharges 12
gph. Trees are planted at a 12 ft in-row and 24
ft across-row spacing.
Determine the number of trees/acre
trees/ac = 43,560 ft2/ac x 1 tree/(12 ft x 24 ft)
= 151 trees/ac
Calculate gal/hr/ac
gal/hr/ac = 151 trees/ac x 12 gal/hr/tree = 1812
gal/hr/ac
or, use Fig.7
Enter the figure on the bottom axis at a density
of 151 trees/ac and go straight up until the 12
gph line is intersected. Read the per acre
application rate from the vertical axis as 1800
gal/hr/ac.
Microsprinkler Irrigation for Cold Protection of Florida Citrus
Microsprinklers can provide some protection to
leaves and wood, particularly on the lower and inner
part of the canopy. A dense canopy tends to retain
heat from the soil and provide better protection than a
thin canopy. Damage will commonly be seen on the
outer and upper parts of the tree after severe freezes.
Since fruit is more sensitive to cold temperatures than
leaves or wood, microsprinklers generally do not
protect the fruit. At higher volumes, spray jets will
help protect fruit a little better than no irrigation, but
generally microsprinkler irrigation is best for tree
protection rather than fruit protection.
There is a limit to the effectiveness of
microsprinklers. Factors such as tree health,
rootstock, and cold acclimation affect tree survival.
Depending on volume of water applied, the lower
limit of effectiveness for microsprinkler irrigation is
around 17°F. The lower parts of young trees have
been protected to even colder temperatures, but
damage usually increases as it gets colder.
Young Tree Protection with
Microsprinklers
Microsprinklers have been effective in
protecting the bud union and lower portion of young
trees. In young trees, the microsprinkler protects the
lower trunk by the direct application of water. When
water freezes, it releases heat. If the application rate
is high enough, the freezing water will maintain the
trunk at a temperature near 32°F. The spray jet must
be close enough to the young tree so that water sprays
directly on the trunk and lower part of the tree.
Recommended distances between the trunk and the
jet are 1 to 2.5 feet. If the jet is too far away from the
young tree, wind can blow the water away. If the
water freezes before it hits the tree, milky white ice
can form on the tree. Protection under milky ice is
usually not as good as under clear ice. During most
Florida freeze nights, the wind comes from the north
or northwest making it best to put the jet on the north
or northwest (upwind) side of the tree. In this
position, the wind will carry the water into the tree
and not blow it away from the tree.
It is common for protection to be seen only in
the iced zone on young trees. Damage commonly
occurs, particularly in severe freezes, above the iced
7
zone or where no water was run. Figure 8 shows a
row of green trees where microsprinklers were run
during a freeze and a freeze-damaged brown tree
where no water was run during the freeze. Young
trees are usually more sensitive to cold and do not
retain heat as well as mature trees. Therefore,
protection down to 17°F cannot always be assured
even if the tree is in good health.
Figure 8. Green trees protected with microsprinkler
irrigation run during a freeze night. The dead tree received
no irrigation during the freeze.
Insulating tree wraps placed around the trunks of
young trees slow the rate of temperature fall. Tree
wraps alone provide some trunk protection. Tree
wraps in combination with microsprinkler irrigation
provide even better cold protection insurance. If the
irrigation system fails during the night, the tree wrap
(particularly if it is a good insulator or has enclosed
water pouches) can slow the temperature drop and
protect the tree longer.
Operation of Microsprinklers on
Freeze Nights
For cold protection, a microsprinkler system
must be designed to provide water to the entire block
or grove all at once. An irrigation system that can
apply water only to smaller zones is not satisfactory
for freeze protection. Hence, a system designed for
freeze protection is initially more expensive to install
because it handles a larger volume of water.
Compared to higher pressure overhead systems,
microsprinklers operate at relatively low pressures of
20 to 25 psi. Because of this low pressure and the
small openings in the emitters, spray jets can freeze
Microsprinkler Irrigation for Cold Protection of Florida Citrus
up if they drop below 32°F before the water is turned
on. Hence, the water should be turned on before the
temperature reaches 32°F so the jets do not freeze.
On frost nights, it is recommended that
microsprinklers be turned on when the air
temperature reaches 36°F. Be careful of
thermometer placement, because in low-lying cold
pockets, the ground surface can be below 32°F when
it is 36°F at the thermometer location. When spray
jets ice up, they are very difficult to thaw and usually
the emitters must be replaced in the field.
Once the microsprinklers are turned on, the
system must keep running all night. If the system
stops or fails when the temperature is below 32°F, it
will be very difficult to restart the system because the
emitters can rapidly freeze up. The situation becomes
especially critical for young trees if the system fails
when the temperature is below freezing. Shortly after
the water stops spraying on a young tree, the trunk
temperature can drop to the wet bulb temperature.
If the irrigation system stops, the trunk can
rapidly drop below the actual air temperature by
evaporative cooling. Thus, more tree damage can
occur than if the jets were never turned on. Even if
the system has only stopped for a short time, many of
the emitters probably will have frozen and it will be
difficult to get the entire system fully operational
again. Because of evaporative cooling, damage can
be greater to trees in the area where a system stopped
during a freeze night than where the system kept
operating continuously. Do not turn off a system
when the temperature is near the critical tree killing
temperature. If the pumping system is unreliable or if
the pump is electrically driven in an area that
commonly has electrical interruptions on freeze
nights, it may be wise to convert to a more reliable
system.
In the morning, when the temperature warms up,
it is not necessary to wait until all the ice has melted
before turning off the system. When the wet bulb
temperature is above 33°F, the system can be safely
turned off with no damage to the trees. At
temperatures below freezing, it is possible for the wet
bulb to read 32°F and actually be warmer than the
dry bulb thermometer. This occurs because the water
in the wick freezes, releases the heat of fusion, and
8
raises the wet bulb temperature to 32°F. Therefore,
one should wait until the wet bulb temperature
reaches 33°F before turning off the irrigation
system.
Generally speaking, if the air temperature (dry
bulb) has risen to 40 or 45°F, the irrigation system
can be turned off safely. If the grove contains only
mature hardened off trees, the system can be turned
off at 40°F. Under the most adverse conditions of
low dew point and high wind, the grower may want
to wait until the temperature is above 45°F. If it is a
two-night freeze and the daytime temperature never
gets above 40°F, then the system should be run
continuously throughout the day and into the second
night. If drainage or water conservation is of major
concern, the system can possibly be turned off
slightly before 40°F under less severe non-windy
conditions, but that increases the freeze damage
risk.
Why Microsprinklers Provide Cold
Protection
Several factors contribute to the cold protection
effectiveness of microsprinkler systems. Most well
water in Florida is around 68 to 70°F. This warm
water contributes a small amount of sensible heat to
the grove at it drops from the initial temperature to
32°F. When temperatures drop below freezing, the
latent heat of fusion is released when the water
freezes. Depending on the amount of ice that forms,
the heat released can raise temperatures in the lower
part of the canopy. Water has a high heat capacity
and can store a fair amount of heat. Therefore, a
moist soil can hold more heat than a dry soil.
Microsprinklers can also raise the dew point or
frost point temperature in the grove. When the
temperature drops to the frost point, heat is released
as the water vapor is converted to ice crystals. When
the grove air temperature reaches the dew point
temperature, the rate of cooling slows down because
heat is released as the water vapor in the air
condenses. It has been suggested that microsprinklers
can provide some protection above the spray zone
because moist air rises and condenses higher up in the
canopy. The heat of condensation (8950
BTUs/gallon) may help warm the upper canopy and
protect more of the tree.
Microsprinkler Irrigation for Cold Protection of Florida Citrus
Depending on the dew point temperature,
microsprinklers can sometimes create fog on cold,
calm nights. Fog is beneficial for frost protection and
if the fog is dense enough and the droplets are of the
proper size, the rate of cooling can be slowed since
fog can act like a blanket and reduce the rate of
radiation loss. Similarly, clouds (which consist of
water droplets) can act like a blanket and slow
radiation loss.
Operation on Windy Nights
Like other methods used for citrus cold
protection, microsprinklers are less effective during
windy or advective freeze nights. They provide little
or no protection to mature trees. There is a risk when
using water during windy or low humidity freezes.
When dew point temperatures are low and winds are
high, high evaporation rates can occur and cool the
wetted part of a tree below the air temperature. This
happened in the windy 1962 freeze. Where overhead
sprinklers were used, evaporative cooling occurred
and trees were killed. The heat lost by evaporative
cooling is approximately 7.5 times greater than the
heat gained by ice formation.
The irrigation application rate on the wetted area
influences the level of protection. A higher
application rate can protect trees to lower
temperatures. One way to increase the application
rate is to reduce the spray pattern size. This can be
done by changing a 360° full circle spray pattern to
a half circle (180°) or quarter circle (90°) pattern.
This essentially doubles or quadruples the application
rate by concentrating the amount of water on a
smaller area. With a higher application rate, the
protection level is better. Because of changing
winds, a half circle cap may do a better job of
directing water into the young tree than a quarter
circle cap.
Summary
Undertree microsprinkler irrigation is an
affordable alternative to other forms of cold
protection. It does not provide complete protection
and generally will not protect fruit. Weak trees will
receive little or no protection. On calm nights,
microsprinklers have given partial protection to
healthy and well-hardened trees down to 17°F. On
9
windy severe freeze nights, little if any protection will
be provided for mature trees, and only higher volume
systems will provide protection for the lower portion
of young trees.
References
Buchanan, D. W., F. S. Davies, and D. S.
Harrison. 1982. High and low volume under-tree
irrigation for citrus cold protection. Proc. Fla. State
Hort. Soc. 95:23-26.
Parsons, L. R., T. A. Wheaton, N. D. Faryna, and
J. L. Jackson. 1991. Elevated microsprinklers
improve protection of citrus trees in an advective
freeze. HortScience 26(9):1149-1151.
Parsons, L. R. and T. A. Wheaton. 1987.
Microsprinkler irrigation for freeze protection:
Evaporative cooling and extent of protection in an
advective freeze. J. Amer. Soc. Hort. Sci.
112:897-902.
Parsons, L. R., T. A. Wheaton, D.P.H. Tucker,
and J. D. Whitney. 1982. Low volume microsprinkler
irrigation for citrus cold protection. Proc. Fla. State
Hort. Soc. 95:20-23.
HS120
Cold Damage Symptoms on Citrus1
L.K. Jackson2
TREE COLD INJURY
The extent of cold injury to citrus depends on a
number of factors, and its expression may occur over
an extended period of time. Factors involved include
type and severity of freeze, location, tree dormancy,
tree vigor, scion and rootstock, crop load and grove
and soil conditions. Citrus is essentially not injured
unless ice is formed in the tissues. Ice formation is
usually accompanied by disruption of cell membrane
which produces the damage. Ice in the leaves is
indicated by dark water-soaked areas on the surface.
Such areas may or may not turn brown after thawing.
Completely frozen, killed leaves appear bleached
brown in color. New succulent growth, when frozen,
will often turn blackish in color instead of brown
upon thawing. Leaf fall within a few days indicates
that the wood is likely not killed, while leaf retention
on the twigs usually indicates wood kill. Wood
damage can be checked by scraping the outer layer of
bark. Apparently green tissue in most (but not all)
cases indicates live wood, and brown tissue, dead
wood. Totally frozen trees will be covered with
curled and brown leaves within 2 to 3 days of the
freeze. Ice may also occur in wood and result in bark
splits, particularly in young trees. Such splits may be
extensive in larger trees resulting in serious trunk
injury. Trees may also develop freeze cankers -- local
areas of bark killed on the limbs and trunks. Scraping
the bark away will reveal dead phloem areas. Such
cankers will frequently appear in the crotches and on
exposed limbs particularly following advective
freezes. In future years such limbs may die or break
off as a result of the canker. New growth developing
on freeze-damaged trees following freezes will often
collapse as the wood behind the growth dies. Hence
the recommendation not to initiate pruning operations
until the extent of the damage is determined.
Varieties such as Pineapple oranges and Murcotts,
when heavily laden with fruit, will often sustain
severe freeze damage while similar trees when lightly
cropped or without fruit following harvest, will
tolerate freezes well.
dark water-soaked areas.
1. This document is Fact Sheet HS-120, a series of the Department of Horticultural Sciences, Florida Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida. Publication date: June 1992. Reviewed: June 1994.
2. Larry K. Jackson, professor, extension horticulturist, Department of Horticultural Sciences, Citrus Research and Education Center (CREC), Lake Alfred, a
branch campus of the University of Florida, Gainesville, Florida.
The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and
other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex,
sexual orientation, marital status, national origin, political opinions or affiliations. U.S. Department of Agriculture, Cooperative Extension Service,
University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry
Arrington, Dean
Cold Damage Symptoms on Citrus
2
freeze and shortly after, cutting the fruit progressively
from the outside to the inside starting at the stem end
will show the amount of ice formed and its location.
The deeper the ice, the greater the severity of injury.
The frozen area will eventually dry out leaving the
injured fruit partially hollow and lighter in weight
than sound fruit of comparable size. Juice loss occurs
over a period of several weeks with the extent of loss
being dependent on damage severity and weather
conditions following the freeze. The formation of
white crystals of hesperidin on the membranes of the
fruit is also a symptom of freeze injury but in no way
detracts from its eating quality.
Leaf fall.
bark splits.
Temples.
freeze cankers.
FRUIT INJURY
Fruit severely injured during a freeze may drop
over time but usually its external appearance is not
significantly changed. Temples and grapefruit are
particularly susceptible to drop, while oranges are
often retained on the tree for longer periods. Certain
cultivars such as Murcotts and grapefruit may show
dark or reddish-brown depressions, pockets, or pitting
on the peel surface. Blemishes in the form of pitting
may occur on the peel of grapefruit as a result of low,
but nonfreezing temperatures, known as "chilling"
injury. Following severe freezes, fruit usually show
extensive internal injury which progresses with time.
Thin-skinned fruit usually show greater internal
injury than thick-skinned cultivars such as grapefruit.
The first evidence of freeze injury is the presence of
water-soaked areas on the segment membranes with
the juice sacs or vesicles in injured areas
subsequently becoming dry and collapsed. During the
pitting.
extensive internal injury.
INFRARED PHOTOGRAPHY
At the present time aerial color infrared (ACIR)
photography can detect freeze damage in both foliage
and fruit. Knowing the amount of freeze damage in
foliage can help the experienced citrus grower
estimate the amount of limb dieback. Using ACIR
Cold Damage Symptoms on Citrus
hesperidin.
photography after a freeze to determine the location
and amount of freeze damaged fruit can be extremely
important to a grower. Within hours, instead of days,
a grower can determine in which groves or sections
of groves fruit must be picked quickly for processing.
Fruit in the remaining sections of the groves can be
held on the trees for a better price.
3
HS935
Chilling Injury of Grapefruit and its Control1
Mark A. Ritenour, Huating Dou and Greg T. McCollum2
Chilling injury (CI) is a physiological disorder
that is occasionally reported on fresh citrus shipments
from Florida. It is most often characterized by areas
of the peel that collapse and darken to form pits (Fig.
1). Pitting is not targeted to the oil glands. Less
severe symptoms may show up as circular or arched
areas of discoloration or scalding. Symptoms of CI
are typically more pronounced after fruit are warmed
to room temperature following exposure to the
chilling temperature. CI symptoms generally require
at least 3 to 6 weeks to develop at low (e.g. 40oF)
shipping and storage temperatures. Chilled fruit are
also more susceptible to decay than are non-chilled
fruit. CI is often confused with another physiological
disorder called postharvest pitting (PP) that is caused
by low-oxygen concentrations (< 9%) within waxed
fruit and is visible as collapsed oil glands. PP requires
only 2 to 4 days for symptom development after
waxing and appears in fruit held at warm (> 50oF)
temperatures.
Packers and shippers should keep in mind several
factors that influence if and to what degree grapefruit
develop CI.
Figure 1. Symptoms of chilling injury on grapefruit.
Temperature Effects on CI
Depending on other predisposing factors,
grapefruit storage and shipment below 50oF can
cause severe CI. Studies show that CI is most severe
when fruit are stored at temperatures from 38 to 40oF
compared with storage at higher or lower
temperatures. Though holding fruit at temperatures
above 50oF greatly reduces the potential for CI, it can
1. This document is HS935, one of a series of the Horticultural Sciences Department, Florida Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida. Publication date: July 2003. Please visit the EDIS Web site at http://edis.ifas.ufl.edu.
2. Mark A. Ritenour, assistant professor, IRREC-Ft. Pierce, CREC-Lake Alfred, Cooperative Extension Service, Institute of Food and Agricultural Sciences,
University of Florida, Gainesville, 32611; Huating Dou, Florida Department of Citrus, Lake Alfred Fla.; Greg T. McCollum, USDA, Ft. Pierce, Fla.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational
information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin.
For information on obtaining other extension publications, contact your county Cooperative Extension Service office. Florida Cooperative
Extension Service/Institute of Food and Agricultural Sciences/University of Florida/Christine Taylor Waddill, Dean.
Chilling Injury of Grapefruit and its Control
also lead to the development of severe PP in waxed
fruit. Thus, storage of waxed grapefruit at 45oF may
often represent the best compromise to minimize the
occurrence of both disorders. Preconditioning fruit
for 7 days at 60oF can greatly reduce CI, but this may
promote severe PP if fruit are preconditioned after the
wax application. The conditions fruit experience
during degreening can reduce grapefruit susceptibility
to CI.
Time of Season
In Florida's climate, fruits are most susceptible
to CI early (October-December) and late
(March-May) in the season. The fruit usually become
more resistant to CI during mid-season
(December-March), but the specific time of year
when the fruit become resistant fluctuates from
season to season.
2
“breathe” (e.g. carnauba). However, too little gas
exchange leads to off flavors (anaerobic respiration)
and increased PP. Waxing also reduces water loss,
thus slowing the development of CI symptoms.
Fungicide
Fungicides such as thiabendazole (TBZ),
benomyl, and imazalil reduce CI in citrus fruit. These
generally have less of an effect on reducing CI
development than waxing or use of modified
atmospheres.
Canopy Position and Sun Exposure
Fruit from the sun-exposed, exterior canopy are
more susceptible to CI than the shaded fruit from
inside the canopy. Even the sun-exposed side of
exterior fruit is more susceptible to CI than the
shaded side of the same fruit.
Heat Treatments
Intermittent Warming
Though intermittent warming (e.g. warming fruit
to room temperature 1 day a week) has been reported
to reduce CI development, it is usually not practical
with large quantities of fruit under commercial
conditions.
Relative Humidity
High relative humidities (e.g. > 95%) reduce the
development of CI symptoms by reducing water loss
from the fruit. Water loss dehydrates the cells
resulting in their collapse and the development of
pitting associated with CI.
Waxing and Modified Atmospheres
Storing citrus fruit in low O2 (possibly not
effective for grapefruit) or high CO2 concentrations
(e.g. 10%) reduces CI. Increased CO2 generated by
the use of semipermeable film packages sometimes
reduce CI, but the effect disappears on grapefruit
harvested after the trees bloom. Waxing reduces CI,
but the effect appears to depend on the gas
permeability of the wax and the CO2 buildup within
the fruit. Waxes that restrict gas exchange (e.g.
shellac) reduce CI more than do waxes that
Heat treatments, such as dips or sprays in hot
water, have been shown to reduce CI. A range of
treatments involving longer exposure to relatively
cooler temperatures (e.g. 2 minutes at 127oF) or
shorter exposure to higher temperatures (e.g. 15
seconds at 140oF) have been tested. However, fruit
response to heat treatments (e.g. temperatures
resulting in injury vs. CI resistance) has not yet been
determined under Florida conditions.
What Packers Can Do to Reduce CI
• Do not hold fruit at chilling temperatures.
However, when PP is a potential problem on
waxed fruit, storage and shipping temperatures
of 45oF should be considered as a compromise to
minimize the occurrence of both CI and PP.
• Be particularly cautious of holding grapefruit at
low temperatures early and late in the season
when grapefruit are most sensitive to CI.
• Remember that use of more “breathable”
waxes (e.g. carnauba) may reduce the CI
protection commonly observed when using less
gas-permeable waxes (e.g. shellac). On the other
hand, use of waxes with lower gas permeability
may result in the development of PP.
Chilling Injury of Grapefruit and its Control
• Be more cautious of holding organic or
“chem-free” fruit at low temperatures because
potential CI protection from TBZ and/or imazalil
will be absent.
• Maintain relative humidity at 85% to 90%. At
relative humidities above 90%, fiberboard
cartons deteriorate. If fruit are stored in plastic or
wood bins, maintain relative humidity between
90% and 98%.
3
HS982
Dwarfing and Freeze Hardiness Potential of Trifoliate
Orange Rootstocks1
James J. Ferguson and Jose Chaparro2
Commercial nurseries, growers, and homeowners
who produce or grow citrus trees north of Gainesville
should use a freeze hardy rootstock such as trifoliate
orange (Poncirus trifoliata) or one of its hybrids.
Trifoliate orange is in the same taxonomic group as
other types of citrus like oranges (Citrus sinensis) and
grapefruit (Citrus paradisi) but it is in a different
genus. It is not really an "orange" in the common
sense of the word.
on trifoliate orange rootstock are also reduced in size,
especially when Flying Dragon, a selection of
trifoliate orange, is used. While Flying Dragon has
shown some promise as a freeze hardy and dwarfing
rootstock, questions often arise about its rootstock
genetics, rootstock/scion interactions, and cultural
practices that can affect tree size. Accordingly, the
purpose of this fact sheet is to discuss these issues as
they relate to trifoliate orange rootstocks, especially
Flying Dragon.
Other commercial citrus rootstocks also have
trifoliate orange parentage. Carrizo citrange is a
hybrid of Washington navel and trifoliate orange.
Swingle citrumelo is a hybrid of Duncan grapefruit
and trifoliate orange. The leaves of trifoliate orange
and of other rootstocks with trifoliate orange
parentage have "trifoliate" or compound leaves
divided into three leaflets or sections (Fig. 1).
Trifoliate orange trees produces seedy, round,
golf-ball size fruit (Fig. 2) that are not edible. These
trees have the potential to survive low temperatures
and, when used as a rootstock, can impart some
freeze hardiness to scion trees grafted onto it.
Trifoliate orange is also deciduous, shedding its
leaves in the winter. Most citrus varieties propagated
Figure 1. Compound leaves of trifoliate orange and related
rootstocks are divided into three leaflets.
1. This document is HS982, one of a series of the Horticultural Sciences Department, Florida Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida. Publication date: October 2004. Please visit the EDIS Web site at http://edis.ifas.ufl.edu.
2. James J. Ferguson, professor, Jose Chaparro, assistant professor, Horticultural Sciences Department, Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida, Gainesville, 32611.
The Institute of Food and Agricultural Sciences (IFAS) is an Equal Employment Opportunity - Affirmative Action Employer authorized to provide
research, educational information and other services only to individuals and institutions that function without regard to race, creed, color, religion,
age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For information on obtaining other extension
publications, contact your county Cooperative Extension Service office. Florida Cooperative Extension Service / Institute of Food and Agricultural
Sciences / University of Florida / Larry R. Arrington, Interim Dean
Dwarfing and Freeze Hardiness Potential of Trifoliate Orange Rootstocks
2
California, where dwarfing citrus rootstocks have
been widely used, 8-to-12-year-old navel and
Valencia oranges, Minneola tangelos, and Dancy
tangerines on Rubidoux trifoliate orange rootstock at
different sites ranged from 10 to 14 feet tall.
Comparable trees on Pomeroy trifoliate rootstock
were predicted to be 15-20% taller, about 12 to 17
feet.
Using an interstock between the rootstock and
the scion, done more successfully with apples than
with citrus, can also reduce tree size. However, in
some cases, interstock effects on tree size can be
counteracted by rootstock effects. For example, a
Hamlin sweet orange scion on a Flying Dragon
interstock on a vigorous Volkamer lemon rootstock
was 30% larger than Hamlin with a Flying Dragon
interstock on a less vigorous rootstock, Carrizo
citrange.
Figure 2. Seedy fruit and stem of a trifoliate orange
selection called Flying Dragon. Find the dragons in the twig.
How Does Dwarfing Occur?
For commercial growers and homeowners, a
dwarf tree is simply one defined by convention as
being about 5 to 6 feet tall at maturity. This particular
tree height is somewhat arbitrary and is based mostly
on comparison to a standard-sized tree which is
usually about 15 to 20 feet tall at maturity. In other
words, a common grapefruit or sweet orange tree on
"standard" or non-dwarfing commercial rootstocks
will reach a 15 to 20-foot height at maturity, while a
tree on Flying Dragon may grow to only "dwarf"
size.
Commercial citrus growers have been interested
in dwarfing rootstocks for citrus because small tree
size allows a higher tree number or density per acre,
fewer if any hedging and topping (pruning) costs, and
easier fruit harvesting on smaller trees. A number of
factors can influence tree size, but for most
commercial citrus cultivars, research has shown that
tree size can be greatly modified by selecting an
appropriate rootstock. For example, trifoliate orange
rootstock selections have been generally divided into
two groups based on flower size. Trees on
small-flowered trifoliate orange selections like
Rubidoux are typically 15 to 20% smaller than trees
on large-flowered selections like Pomeroy. In
Environmental factors like soil type, mild
drought, soil salinity or alkalinity, marginal growing
sites, or climate can affect tree size. Trees on
trifoliate orange rootstock are often smaller-sized
when grown in deep, sandy soils even with irrigation.
On better soils, these trees grow slowly but reach
standard size. In Australia, deliberately infecting
trees with the citrus exocortis viroid, a virus-like
pathogen, has dwarfed trees but this procedure has not
been widely adapted on a commercial scale because
of potential risks and uncertainties related to using
this pathogen. For example, in Florida, trees
deliberately infected with citrus exocortis sustained
greater freeze damage than comparable trees that
were not infected.
Growth hormones like auxins and cytokinins
have been used to reduce tree size and have decreased
internode length (the gap between successive points
of attachment of leaves on a stem) and increased
lateral shoot development. Other cultural practices
like growing otherwise standard rootstock/scion
combinations in small pots will result in a dwarfed
tree, but if such trees were transplanted into the home
landscape or a commercial citrus grove, they could
grow into standard size trees. To cite another
example, bonsai, usually ranging from 2 inches to
about 3 fee tall, are not genetically dwarfed plants but
are standard size plants kept small by pruning
Dwarfing and Freeze Hardiness Potential of Trifoliate Orange Rootstocks
branches and roots, repotting, and pinching off new
growth. Small-leafed ornamentals like camellias are
commonly used for bonsai but Flying Dragon
rootstock, with its downcurved thorns and sinuous or
winding growth habit, might also make an interesting
bonsai specimen.
Characteristics of Trifoliate Orange
Rootstock
Trifoliate orange is suitable for use in cool
climates where maximum freeze hardiness can be
developed. Freeze hardy rootstocks, like trifoliate
orange, acquire freeze hardiness at 70°F day
temperatures/50°F night temperatures whereas the
less freeze hardy rootstocks do not acquire freeze
hardiness until 50°F day/30°F night temperatures
occur. During the fall and winter, cooler
temperatures occur more frequently in northern than
in central or southern Florida, but even in north
Florida, unseasonably warm temperatures higher than
70°F day/50°F night cited above can reduce the
freeze hardiness of trifoliate orange rootstocks.
Trifoliate orange therefore may not be a consistently
freeze hardy rootstock.
Given acclimating temperatures as unbudded
seedlings, trifoliate orange rootstock is much more
freeze hardy than most commercial citrus rootstock
and scion cultivars. Trees on trifoliate orange
rootstock also tend to be more freeze hardy than trees
on other commercial rootstocks. Fully acclimated
two-year old trifoliate orange seedlings survived 3°F
temperatures in Georgia and Owari satsumas on
trifoliate orange rootstock survived 14°F
temperatures in Louisiana.
Interestingly, trees on Flying Dragon rootstock
have not been shown to be more freeze hardy than
trees on other trifoliate orange types. In one study
examining the effects of rootstocks on freeze
hardiness of navel oranges in California with
minimum temperatures of 18°F and a long
preceding period of freeze-hardening temperatures,
trees on Flying Dragon were similar to trees on
Pomeroy and Rubidoux trifoliate orange rootstocks.
Note also that when severe freeze damage does occur
to trees on trifoliate orange rootstock, they recover
more slowly than trees on more freeze sensitive but
more vigorous rootstocks like rough lemon.
3
Trifoliate orange is a satisfactory rootstock for
most sweet orange cultivars, especially navels but
fruit size for grapefruit scions on trifoliate orange
rootstocks may be relatively small. When trifoliate
orange is used as a rootstock for mandarins and
mandarin hybrids, bud union incompatibility has
been reported. Trees budded on trifoliate orange
produce excellent fruit with high soluble solids, good
juice color and a smooth thin peel. Fruit also holds
well on the tree. However, fruit, especially for the
fresh market, can be small because of heavy fruit set.
When used as a rootstock, trifoliate orange can
produce a standard tree size, about 15 feet tall, on
clay and loamy soils and on shallow soils but does not
develop a deep or widely ranging root system and
consequently is not drought hardy. On deep, sandy
Ridge soils, trees on trifoliate orange do not grow
rapidly, making it a good candidate for close
plantings with irrigation. It is poorly adapted to
saline or calcareous conditions but is useful under wet
conditions because of resistance to Phytophthora foot
rot. Tree stunting and rootstock bark scaling
associated with citrus exocortis and citrus blight, a
decline of mature trees, are perhaps the most serious
diseases affecting this rootstock. Some trifoliate
orange selections are tolerant to citrus nematode but
all selections are susceptible to burrowing nematode.
Performance of trees on Flying Dragon rootstock may
be similar to the above with tree size variability
discussed in following sections.
Polyembryony and Nucellar
Seedlings
The genetics of trifoliate orange rootstocks are
complex and may explain differences in the size of
trees grafted onto these rootstocks, especially Flying
Dragon. Citrus and related species are unique
because of the frequent occurrence of polyembryony
(one seed producing a number of embryos or
seedlings) and nucellar embryony (seedlings
genetically identical to the mother plant as opposed to
zygotic seedlings which inherit genetic characteristics
from both parent plants). Rootstocks that are strongly
nucellar produce a high percentage of nucellar
seedlings whereas weakly nucellar rootstocks can
produce a significant percentage of both nucellar and
zygotic seedlings. In one report, using seed from
Dwarfing and Freeze Hardiness Potential of Trifoliate Orange Rootstocks
Flying Dragon trees grown in four different locations,
the frequency of zygotic seedlings ranged from 0 to
75%. Researchers also found that the percentage of
Flying Dragon nucellar and zygotic seedlings
fluctuated greatly from year to year on the same tree.
When producing citrus nursery trees,
nurserymen select rootstock seedlings for uniformity
and discard any that are off-types that vary in
characteristics like internode length, leaf size, shape
and color, growth rate, foliage density, and
branching. Such non-uniform or off-type seedlings
probably developed from zygotic embryos (Figs. 3, 4,
5 and 6). If these off-types were used as rootstocks
and grafted with a scion cultivar like Valencia orange,
the resulting trees may not produce fruit of as
uniform a quantity and quality as that same fruiting
cultivar grafted onto comparable nucellar rootstock
seedlings. Rootstock disease susceptibility could also
vary. Although off-type rootstock seedlings should
generally be avoided, they may have potentially
interesting characteristics.
Figure 4. A closer image of plant A in Fig. 3.
Although the specific mechanism for dwarfing is
not clearly understood, when used as both a rootstock
and as an interstock, it has been suggested that Flying
Dragon tissue may bind or inactivate translocation of
a growth regulator produced by its roots. Further
research may focus on identifying and transferring
dwarfing factors to other rootstocks with more
desirable traits.
Figure 3. Flying Dragon seedlings from the same source.
Note variability in stem height and relative straightness of
stems and thorns.
The important point is that the occurrence of
polyembryony and nucellar seedlings varies in
trifoliate orange rootstocks. This variation can
explain why Flying Dragon as a rootstock seedling
and as a rootstock/scion combination can include
both nucellar and zygotic seedlings and can produce
both trees of standard size and trees of varying size,
including dwarf trees. Environmental factors like soil
conditions can also affect growth, tree size, and yield.
Selected research on tree size, tree density per acre
and yield of different scion cultivars on Flying
Dragon rootstock is presented in Table 1.
Figure 5. A closer image of plant B in Fig. 3.
4
Dwarfing and Freeze Hardiness Potential of Trifoliate Orange Rootstocks
Figure 6. A closer image of plant C in Fig. 3. This appears
to be a true-to-type genetic dwarf while the other plants
may be off-types.
5
Dwarfing and Freeze Hardiness Potential of Trifoliate Orange Rootstocks
6
Table 1. Selected research on growth and yield of scions on Flying Dragon rootstock (FD).
Experiment
Results
Country
(Publication date)
Mandarins, oranges and grapefruit on sour orange
rootstock with a FD interstock
11-year-old trees with an FD interstock were
1/3 smaller and had higher yield efficiency
than trees without the interstock.
Italy (1989)
Minneola tangelo on Cleopatra mandarin
rootstocks with four interstocks, including FD and
two budding methods
FD and other interstocks had no effect on
tree height as compared with the same scion
on the same rootstocks.
Florida (1992)
Mandarins and grapefruit on sour orange, Rangpur
lime, and other rootstocks with a FD interstock
planted deeply, assumably to induce interstock
rooting to control tree size after early vigorous
growth with the sour orange rootstock
6-year-old trees trees with a FD interstock
were 30-50% smaller. Trees planted at a 3 x
3
16 ft. spacing had a 1.6 ft. tree volume and
67 lbs/tree yield.
Israel (1992)
Minneola tangelo on nine rootstocks, including FD
3.5-year-old Minneola tangelo trees on FD
rootstock were 30% shorter (4.7 feet) than
Minneola tangelos on Cleopatra mandarin
rootstock (6.8 ft).
At 5 years old, trees could be planted at 333 583 trees/acre, 5X more than a conventional
orchard at 1.8X greater costs and 5.2 X
greater production.
Florida (1992)
Lemons on FD
12-year-old trees had 1/3 the canopy volume
of trees on sour orange rootstock with fruit
yield of 441 lbs/tree. Trees were 11.5 ft. tall.
Argentina (1996)
Sweet oranges, mandarins, and Clementines on
FD
Varying dwarfing capacity affected by scion
and soil characteristics. Irregular canopy
growth and poor production over extended
periods.
Italy (1999)
Oranges, lemons, and Clementines on FD
High density plantings on FD at 6.5 x 6.5 ft
spacings did not result in increased yields per
unit area compared with conventional
planting. FD did not restrict growth of
vigorous lemon scions.
Italy (1992)
Sweet orange on FD
9-year-old trees were 6 feet tall and @ 4.5 ft
in width.
California (1982)
Pineapple sweet orange and Ruby grapefruit
5-year old pineapple sweet orange and Ruby
grapefruit trees (15 x 20) on Flying Dragon
rootstock were 3.2 and 4.0 feet tall,
respectively, compared with the same scions
on Carrizo citrange which were 6.6 and 6.0
feet tall, respectively.
Florida (1980)
Valencia on FD
4-year-old trees (averaging 4 feet tall and
three feet wide) averaged 71 fruit per tree
compared with trees of the same age on a
vigorous rootstock, rough lemon (10 feet tall
and 7 feet wide) averaging 541 fruit per tree.
California (1986)
Valencia on FD
5-year-old trees (6 feet tall and 6 feet wide)
California (1979)
8 citrus cultivars on FD
New Caledonia
(1999) (South
Pacific off
Australia)
HS118
Recovering From Freeze Damage1
L.K. Jackson, D.P.H. Tucker and T.R. Fasulo2
Tree dormancy at the time of a freeze is probably
the most important factor which influences the
susceptibility of citrus to freezing temperatures.
Citrus trees are evergreen, never become fully
dormant and cannot withstand temperatures as low as
those tolerated by deciduous trees. Cold weather
preconditioning induces a degree of dormancy in
citrus if it comes gradually. Trees in active growth are
more severely injured by cold than those which are
somewhat dormant. One of the best ways to lessen
cold injury and to hasten recovery from cold damage
is to maintain healthy trees. Follow cultural practices
that tend to induce dormancy in the early winter and
strive to maintain this dormancy until all cold weather
has passed. Ability to recover from cold damage
depends largely on tree vigor. Weak trees showing
disease, insect damage, or nutrient deficiency
symptoms are usually the ones most severely
damaged and slowest to recover.
The leaves first become slightly flaccid as they thaw
and, if injury is not too severe, they gradually regain
normal turgidity and recover. Seriously frozen leaves,
however, gradually collapse and dry out, this is
accelerated by warm, sunny days immediately
following a freeze. The youngest leaves are most
easily injured, but all leaves may be killed in a severe
freeze. If twigs are seriously frozen, the leaves dry
out and remain attached for several weeks. However,
the damaged leaves are shed rather promptly if the
twigs and larger wood are not seriously injured. Cold
damage on trunks and larger branches may appear as
splitting or loosening of the bark. Certain areas,
especially at or near crotches, are particularly
sensitive. Localized damage to patches of bark on the
trunk and larger limbs (cold cankers) are often
mistaken for gummosis, since these may exude gum
at a later date.
CARE OF BEARING GROVES
SYMPTOMS OF COLD DAMAGE
Citrus leaves commonly assume a drooping or
wilted appearance during periods of low
temperatures, but remain firm and brittle if frozen.
Rules for the care of citrus trees that have been
injured by cold must be flexible and varied. Time of
year at which the cold occurs, condition of the trees at
time of injury, and weather conditions immediately
1. This document is Fact Sheet HS-118, a series of the Horticultural Sciences Department, Florida Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida. Publication date: June 1992. Revised: June 1994.
2. L. K. Jackson, professor, extension horticulturist, Department of Fruit Crops, Citrus Research and Education Center, Lake Alfred, Florida; D. P. H. Tucker,
professor, extension horticulturist, Department of Horticultural Sciences, Citrus Research and Education Center, Lake Alfred, Florida; T. R. Fasulo,
associate in entomology, entomologist, Department of Entomology and Nematology, Institute of Food and Agricultural Sciences, University of Florida,
Gainesville, Florida.
The use of trade names in this publication is solely for the purpose of providing specific information. It is not a guarantee or warranty of the products named,
and does not signify that they are approved to the exclusion of others of suitable composition.
The Institute of Food and Agricultural Sciences (IFAS) is an Equal Employment Opportunity - Affirmative Action Employer authorized to provide
research, educational information and other services only to individuals and institutions that function without regard to race, creed, color, religion,
age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For information on obtaining other extension
publications, contact your county Cooperative Extension Service office. Florida Cooperative Extension Service / Institute of Food and Agricultural
Sciences / University of Florida / Larry R. Arrington, Interim Dean
Recovering From Freeze Damage
leaves.
trunk.
larger limbs.
following injury will influence to a marked degree the
type of treatment to use. The natural reaction after a
freeze is to do something right away. Actually there is
very little that can be done at that time, as it is
impossible to determine the full extent of injury.
Twigs and branches may continue to die for a period
of several months to as long as two years following a
severe freeze due to latent damage to the bark and
wood which may not be apparent soon after a freeze.
Citrus trees on which twigs and branches have been
killed by cold should receive extra care during the
following season. Thus, a "wait and see" attitude is
best.
Fertilization
Fertilizer should be applied at the regular time,
but the amount of fertilizer should be reduced in
proportion to the amount of tree damage. A reduction
2
in the pounds of nutrients per acre may be in order
where twigs and limbs have been damaged, especially
if it is obvious that there will be a light crop. For
example, if 1/2 of the tree has been killed then 1/2 the
regular fertilizer should be applied. However, it is
best to treat these trees as young trees in that the total
amount of fertilizer should be applied in 3 to 4
applications. Trees which experience a top-kill will
also lose a portion of their root system. Therefore,
there is no point in over-fertilizing. Another factor to
consider is whether fruit will be produced in the year
following a freeze. For example, trees suffering
10%-15% wood loss should receive a regular
nutritional application as fruit will be produced that
year. Trees suffering 50%-60% wood loss will not
produce fruit that year and the nutritional program
should be adjusted accordingly. Research has
produced no definite guidelines in this area as yet, so
the production manager still must make his decision
based upon the local situation. Nutrient deficiency
symptoms may be intensified in freeze-damaged trees
due to the drain entailed by the large amount of
growth necessary to replace lost foliage. Thus, a
post-bloom nutritional spray of copper, zinc, and
manganese usually will be beneficial to new growth
and tree condition. After a freeze trees will continue
to lose some initial flush as die-back continues. It is
important not to apply nutritionals until a canopy has
been established to catch it. Apply potash and
nitrogen to increase leaf size because trees tend to
produce small leaves for a few years after suffering
total leaf loss.
Irrigation
Irrigation during warm periods in the winter
following cold injury should be avoided because it
may induce new growth which might be damaged by
later freezes. If freeze-damage occurs early in the
winter and soil moisture is adequate, it will be well to
delay irrigation in hopes of delaying tree growth until
the danger of additional freezes has passed. On the
other hand, trees that put forth new growth should not
be allowed to wilt for the lack of water. In this case,
growth has commenced and withholding water will
only retard subsequent growth.
Recovering From Freeze Damage
3
Pruning
CARE OF YOUNG TREES
Cutting dead wood from bearing trees which
have suffered heavy damage should not be done until
late spring or summer following the injury. This
delay is desirable since it is difficult to determine the
actual extent of injury until new growth commences.
It is well to remember when pruning that all cuts
should be made into living wood and, where possible,
at crotches, leaving no stubs or uneven surfaces. It is
advisable to remove heavy brush from the grove
immediately following the pruning operation.
Slight Damage
Weed Control
Weed control will be essential to rapid recovery
from freeze-damage, as weeds will compete heavily
with the trees for available moisture, nutrients, and, in
some cases, light. Weed populations should be at low
levels following a freeze which will provide growers
with a good opportunity to get many troublesome
species under control.
Disease Control
Greasy Spot Control After a Freeze
With loss of foliage, growers cannot afford to
lose any of the new flush. Because of the lost foliage,
the year following a freeze will probably have three to
four major flushes. Fungicide applications in July and
September may be needed.
Melanose Control Following a Freeze
Melanose control after a freeze is still a difficult
decision for the production manager to make. If fruit
is expected that year a fungicide application may
increase pack-out. Even on non-fruiting trees one or
two fungicide applications should probably be made
to help prevent infection of some new flush.
However, the percentage of leaves with melanose
will still be very high. Only weekly applications
would provide complete control and this is not
economically advisable. If you are applying a
nutritional spray, you should certainly include
copper.
If damage is slight and there are some leaves and
green twigs above the bank or wrap, no special steps
need to be taken. Trees in such instances have
sufficient living wood to develop new tops. Even so,
the banks or wraps should be removed from a few
trees and the trunks inspected to see that the bark is
firm. This process should be repeated at weekly
intervals as damaged trees often "sweat" in the bank
or wrap and may be attacked by a fungus that can
quickly damage or kill a young tree.
Heavy Damage
If the tree has been killed to or into the bank or
wrap, it may be a good practice to remove the entire
bank or wrap, or at least a portion of it, to expose live
tissue to sunlight and air. Otherwise, the bark beneath
the bank or wrap may soften and slough off, resulting
in complete loss of the tree. The extent of this
operation will depend upon the acreage involved and
the manpower available. When trees are unbanked or
unwrapped, it will be necessary to rebank or rewrap
them when freezing temperatures are forecast. Banks
or wraps should be again removed as soon as the
danger of cold has passed. This process may need to
be repeated several times before the winter is over.
Trees That Were Not Banked
Trees that were not banked at the time the cold
came should not be banked until the next forecast of
freezing temperatures. It is useless and often
detrimental to bank trees immediately following a
freeze, as they are likely to "sweat" in the bank and
be more severely damaged than if unbanked. Wait
until the forecast of additional freezing temperatures
to bank.
Cultural Practices
If tree damage is slight, exert every effort to keep
the young tree as dormant as possible. If heavy
damage has occurred, in addition to removing the
banks, it may be necessary (depending on conditions)
to water the trees. Pruning, fertilization, and spraying
should be delayed until the danger of subsequent
freezes has passed.
Recovering From Freeze Damage
4
FC36
Cold-Hardy Citrus for North Florida 1
Julian W. Sauls & Larry K. Jackson2
Introduction
The Florida citrus industry is about 400 years
old, having been started with the settlement at St.
Augustine and other areas along the St. Johns River.
The industry was centered in north-central Florida
until the big freezes of 1894-95 caused it to move
further south.
Today, there are over 340,000 hectares (850,000
acres) of citrus growing in peninsular Florida - all
south of Gainesville and most of it south of
Leesburg. Recurring severe freezes have limited
attempts to grow citrus outside the current citrus belt.
The home grower is at a disadvantage with
regard to climate in trying to grow citrus in north and
west Florida. It will freeze every year. In fact, there is
a 60-80% probability that -5°C (24°F) will occur
each year, a 40-60% probability of -7°C (20°F)
and a 20% probability of -9°C (16°F). Such
temperatures are not conducive to growing most
citrus, as citrus species are basically subtropical or
tropical crops.
The duration of freezing temperatures can be
more critical than the minimum temperature. For
example, serious damage may not occur during a brief
drop to -5°C (24°F), but could result after several
hours at -3°C (26°F). Moreover, previous
exposure to cold increases the plant's ability to
withstand cold. As the days shorten and nights get
cooler, plants slow active growth and attain
cold-hardiness. Satsuma may withstand -9°C
(15°F) in January when it is completely dormant
and hardy, but it may be seriously damaged at -3°C
(26°F) in mid-November.
Cold Protection
The home fruit grower cannot control the
climate, but there are certain other factors he can
change which will influence the chances of survival
of a citrus tree in north and west Florida. Moreover,
there are several steps he can take to modify the
immediate microclimate of a citrus tree and thus
enable it to withstand freezing temperatures.
The first consideration is the selection of the
proper variety. Kumquats and satsuma are the most
cold-hardy, edible forms of citrus available and both
will normally survive in north and west Florida.
Moreover, trifoliate orange is the most cold-hardy of
all citrus and an excellent rootstock for the others, as
it will convey its hardiness to the scion variety
budded on it. Thus, kumquat and satsuma budded on
trifoliate orange rootstock are the hardiest
1. This document is FC-36, one of a series of the Department of Horticultural Sciences, Florida Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida. Date revised : March 2000. Please visit the EDIS Web site at http://edis.ifas.ufl.edu.
2. Julian W. Sauls & Larry K. Jackson, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611.
The Institute of Food and Agricultural Sciences (IFAS) is an Equal Employment Opportunity - Affirmative Action Employer authorized to provide
research, educational information and other services only to individuals and institutions that function without regard to race, creed, color, religion,
age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For information on obtaining other extension
publications, contact your county Cooperative Extension Service office. Florida Cooperative Extension Service / Institute of Food and Agricultural
Sciences / University of Florida / Larry R. Arrington, Interim Dean
Cold-Hardy Citrus for North Florida
combinations available. These will be discussed in
more detail later.
The next consideration is planting site. Cold air
drains downhill, so higher elevations will be
somewhat warmer than sites at the bottom of a hill or
slope. Usually, the south and southeast sides of a lake
or other body of water will be warmer than the north
side or sites with no water nearby. Planting on a slope
or south of a body of water affords some cold
protection. The average residential lot does not
normally permit such consideration, however.
Planting on the south side of and at the correct
distance from the house is almost a must. Most
freezes and wind drift are from the north and
northwest, thus the house will act as a windbreak,
forcing the cold air up and over the house and citrus
trees, leaving the area near the south side somewhat
warmer. The house itself radiates considerable heat,
part of which will be absorbed by adjacent plants,
thus warming them.
Overhanging trees absorb heat radiated from
plants and the ground, reradiating some of it to the
ground. This is a temperature advantage, but growing
citrus beneath other trees is not entirely satisfactory
due to poorer growth and lower yields.
The soil under and around a citrus tree should be
completely free of weeds, grass or mulch during the
winter. Grass or mulch on the soil acts as an insulator,
hence solar heat is prevented from entering the soil
during the day and less heat is stored for release from
the soil at night. A clean packed surface allows
maximum heat absorption during the day and
maximum heat radiation at night. Moreover, moist
soil will absorb more heat than dry soil, so trees
should be thoroughly watered 2-3 days before a bad
freeze is predicted.
Finally, good tree health and nutrition will help
the tree withstand freezing temperatures. Follow
recommended cultural practices and fertilization to
maintain the trees in the best condition. Fertilization,
spraying and pruning should end in August or
September in order to allow the trees to harden off
completely before severe freezes are encountered.
2
Once the foregoing factors have been considered
and the citrus trees are planted, the dooryard fruit
grower must provide additional cold protection to the
trees during severe freezes. Young citrus trees should
be banked for the first 3-5 winters until the tree is
large enough to better withstand cold.
Banking is simply a technique in which clean,
surrounding soil is pulled up in a mound around the
tree to cover the bud union and lower trunk. Thus,
even if exposed parts of the tree are completely
killed, the bud union under the bank will still be alive
to grow back in the spring. Consult your local County
Extension Office for full details on banking.
Additionally, trees may be covered temporarily
with blankets, quilts, paper or other material as
further protection against hard freezes. However,
such materials should be removed each morning to
allow the trees to take full advantage of incoming
solar radiation.
Cold-Hardy Citrus
Trifoliate orange can withstand the lowest
temperature of all citrus when it is mature and fully
dormant, followed by kumquat, satsuma, calamondin,
sour orange, mandarin, sweet orange, grapefruit,
shaddock, lemon, lime and citron. Grapefruit,
shaddock, lemon, lime and citron are not
recommended outside the commercial citrus belt of
Florida, so they will not be considered further in this
fact sheet.
Some sweet oranges, some mandarins and
calamondin have sufficient cold-hardiness to be
grown north of the commercial citrus belt, but south
of a line drawn from the mouth of the Suwannee
River through Gainesville to Green Cove Springs and
then up the St. Johns River to the ocean (Figure 1).
They can be successfully grown in this area in most
years.
Sweet Oranges
Varieties of sweet oranges [Citrus sinensis (L.)
Osbeck ] which can be grown in this area are navel,
'Hamlin' and 'Parson Brown.' These oranges mature
in the early to late fall so that fruit would normally be
harvested before a severe freeze would be expected.
Cold-Hardy Citrus for North Florida
3
The fruit is small and oblate, with a flattened or
depressed blossom-end. The peel is yellow to
yellow-orange, very thin and smooth, easily separable
at maturity. There are 5-9 segments, the axis is small
and semi-hollow. The flesh is orange-colored, juicy
and acid. The seeds are few, with green cotyledons.
The fruit holds well on the tree.
Calamondin can be propagated by seed and by
cuttage, although seedlings may not fruit for several
years. It does not require a rootstock and is itself a
suitable rootstock for kumquats.
Figure 1.
These varieties are discussed in detail in another fact
sheet, so consult the local County Extension Office
for more information. The preferred rootstock is
trifoliate orange for maximum cold-hardiness, with
sour orange being second choice if trifoliate orange is
not available.
The most prominent citrus fruit which are
sufficiently cold-hardy to grow throughout north and
west Florida are satsuma and kumquats.
Mandarins
Satsuma
The mandarins (Citrus reticulata Blanco) and
mandarin hybrids for this area include 'Dancy'
tangerine, 'Orlando' tangelo, 'Robinson' tangerine
and 'Cleopatra' mandarin. All but 'Cleopatra' are
discussed in another fact sheet, so consult the local
County Extension Office for more information. The
preferred rootstock for cold-hardiness is trifoliate
orange, but sour orange or 'Cleopatra' mandarin will
be easier to find. 'Cleopatra' mandarin is used as a
rootstock for other citrus in the commercial industry;
thus it may be available only as seedling trees. Its
fruit are bright orange-red, typically mandarin, and
somewhat acid. Its principal value in dooryards is for
ornament, as it is attractive and bears fruit year round.
Calamondin
North and West Florida
The satsuma mandarin (Citrus unshiu
Marcovitch) apparently originated as a chance
seedling in southern Japan sometime before 1600
A.D. It is now grown in subtropical areas throughout
the world, but the most important industry is in Japan
where the satsuma accounts for the majority of the
citrus planted. Satsuma can be grown in the U.S.
from Florida to Texas along the Gulf Coast and in
Arizona and California.
Satsuma is the most cold-tolerant of commercial
citrus, with mature, dormant trees having survived
-9°C (15°F) without serious injury. Consequently,
it is adapted to regions that are too cold for other
citrus, as it has not proven commercially acceptable
in milder regions of the subtropics.
The calamondin (Citrus madurensis Loureiro)
can substitute for other acid citrus. It is widely grown
as an ornamental in Florida and California.
Calamondin is particularly attractive as a container
plant and is used extensively throughout the U.S. as a
house plant. It originated in China and is widely
distributed throughout the Orient.
There are several groups or varieties of satsuma,
although the majority are 'Owari' or varieties which
arose from 'Owari' by bud mutation. 'Silverhill' is a
nucellar seedling selection of 'Owari' which was
named in Florida about 1931 and it appears identical
to 'Frost Owari' in California. Satsumas propagated
in Florida may be called 'Owari', 'Silverhill,' or
simply satsuma.
Calamondin is mandarin-like in many respects,
but also resembles kumquats. The tree is dwarf and
bushy, being quite showy when laden with mature
fruit. It is nearly thornless, with small, broadly oval
leaves.
The satsuma tree is moderately vigorous,
medium-small, very productive and markedly
cold-resistant. Greatest cold-hardiness is attained on
Cold-Hardy Citrus for North Florida
trifoliate orange rootstock, which also causes some
dwarfing of the tree.
The fruit is medium-sized, medium-oblate to
subglobose, slightly necked and virtually seedless.
The peel is thin and leathery, easily separable from
the fruit and bright reddish-orange in color, although
good peel color may not develop until after fruit
maturity. There are 10-12 segments, which are
loosely separable, around a hollow axis. The flesh is
orange-colored, tender and juicy. The juice is quite
sweet, with moderate sugars and low acid content;
quality is excellent.
The fruit matures in October-November but
holds poorly on the tree, becoming puffy and losing
quality so that it must be picked promptly. The fruit
stores well after harvest.
The Kumquats
The kumquats (Fortunella spp.) are undoubtedly
of Chinese origin, the Nagami variety having been
introduced to Europe and the U.S. from China in the
middle of the last century. Other varieties were
introduced into the U.S. around the turn of the
century. Kumquats are most widely grown in China,
Japan, Taiwan and Malaysia.
Kumquats are primarily grown for ornament in
California and the Gulf Coast states and for use of the
fruit in the gift-package trade in Florida. The fruit
may be eaten fresh, peel included, but it is most
frequently preserved as marmalade or candied whole
fruit.
Kumquats exceed even satsuma in terms of
cold-hardiness, being able to sustain -12°C (10° F)
when fully dormant. Active growth occurs only at
relatively high temperatures, so the plants remain
semi-dormant during late fall, winter and early spring
in warm temperate climates. They normally bloom
long after citrus and cease active growth earlier in the
fall, which contributes to their cold-hardiness.
The plant is a shrubby evergreen tree, rarely 3 m
(10 feet) tall, densely branched with few or no thorns.
The leaves are small and simple, with hardly any
petiole wings. Trifoliate orange is the preferred
rootstock for kumquats grown in cold regions, which
further reduces tree size.
4
The fruit are generally small, globose, obovate or
oblong to oval. The fruit are very showy, being borne
in large numbers, yellow to bright reddish-orange in
color. The peel is medium-thick, fleshy, tightly
adherent, aromatic and spicy. The seeds are few, with
green cotyledons. The flesh is yellowish-orange,
moderately acid, and has little juice. The axis is small
and solid. The fruit matures in the fall and holds well
on the tree without appreciable loss in quality.
There are 4 varieties of kumquats grown in
Florida:
• Nagami (oval) kumquat [F. margarita (Lour.)
Swing.] is the most popular. Its fruit are oval, 3-4
cm (1 1/4-1 3/4 inches) long, about 2/3 as wide.
The fruit have 2-5 seeds and are pleasantly
flavored, with deep peel color.
• Meiwa (large round) (F. crassifolia Swing.) is a
large and round kumquat with a thick peel and
sweet taste. The fruit are commonly 2 1/2-3 1/2 cm
(1-l 1/2 inches) in diameter and nearly seedless.
• Marumi (small round) [F. japonica (Thumb.)
Swing.] is round, smaller than Meiwa, rarely
exceeding 2 1/2 cm (1 inch) in diameter, with 1-3
seeds. The peel is thinner and somewhat sweeter
than Nagami, but the flesh is somewhat acid. The
tree is usually more thorny than either Nagami or
Meiwa.
• Hongkong [F. hindsii (Champ.) Swing.] has the
smallest of all true citrus fruits, being hardly
more than 1 cm ( 1/2 inch) in diameter. Its fruit
are round with relatively large seeds. They are
virtually inedible, being quite tart. The plant is
very small and thorny.
Other Citrus
In addition to satsuma and kumquats, both of
which should be readily available in nurseries, there
are a couple of other types of citrus which have
excellent cold-hardiness.
'Changsha' tangerine is very much like the
satsuma which it resembles greatly. The fruit itself is
about the size of a satsuma, with bright orange peel.
The quality is not as good as satsuma and the fruit is
very seedy. The tree is a bit more upright-growing
Cold-Hardy Citrus for North Florida
than satsuma and is probably more cold-hardy than
satsuma and kumquat, having been observed growing
and fruiting as far north as Ft. Worth, Texas.
Little is known about 'Changsha' in Florida, as
it apparently is not propagated commercially. There
are a few specimen trees around the state and it seems
to do well in Gainesville. It comes true from seed and
the seedlings will produce within a few years.
'Thomasville' citrangequat is a hybrid between
trifoliate orange, sweet orange and kumquat which
was developed about the turn of the century in
attempts to combine the cold-hardiness of trifoliate
and kumquat with the quality of sweet orange. It is as
cold-hardy as kumquat, having fruited in Tuscaloosa,
Alabama.
The fruit is medium-small, globose to oval,
orange-yellow, seedy and somewhat acid until fully
mature. The trees are vigorous and thorny, with
mainly trifoliolate leaves.
It is doubtful that this variety is available as it
apparently is not being propagated at the current time.
5
HS 885
Your Florida Dooryard Citrus Guide - Selecting A Citrus
Tree for Your Climate1
James J. Ferguson2
Cold Tolerance
Except when cold fronts swoop down from the
Arctic, Florida has a mild subtropical climate. For this
reason the most important factor in selecting your
dooryard citrus tree is your geographic location,
especially in terms of cold tolerance.
Example: Coastal counties and areas near Lake
Okeechobee have traditionally suffered less freeze
damage than other parts of Florida. However, citrus
trees, even in these areas, occasionally suffer freeze
damage.
and, with some care, regrow damaged tree canopies
within several years to bear crops at pre-freeze levels.
Now, let me tell you about the horticultural and
environmental factors that affect cold tolerance.
Cold tolerance in citrus is influenced by these
factors:
• rootstock
• scion
• fruit load
• temperatures preceding a freeze.
Of course, other factors like tree size, intended
use (fresh fruit or juice), capacity of tree to store fruit
on the tree for an extended harvest period, drought
tolerance, and resistance to pests and diseases are
important and will be discussed.
Although no major freezes have occurred since
1989 (at this writing), six major freezes from 1981 to
1989 killed or damaged thousands of acres of citrus
trees. Chances are, severe freezes will occur again in
Florida. In spite of this, homeowners throughout
Florida, using current strategies for cold protection,
can bring their dooryard trees through most freezes
Most rootstocks can be placed in one of three
general groups according to their relative effect on
cold tolerance.
• Trees on rough lemon, Rangpur lime, Volkamer
lemon, Milam, Palestine sweet lime and Citrus
macrophylla are the least cold tolerant. But
because of their vigor, they recover rapidly if not
severely damaged or subjected to succeeding
freezes in one winter or over several years.
1. This document is HS 885, one of a series of the Horticultural Sciences Department, Florida Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida. Originally printed in hardcopy, 1995. Publication date: August, 2002. Please visit the EDIS Web site at
http://edis.ifas.ufl.edu.
2. James J. Ferguson, professor, Horticultural Sciences Department, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of
Florida, Gainesville, 32611.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational
information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin.
For information on obtaining other extension publications, contact your county Cooperative Extension Service office. Florida Cooperative
Extension Service/Institute of Food and Agricultural Sciences/University of Florida/Christine Taylor Waddill, Dean.
Your Florida Dooryard Citrus Guide - Selecting A Citrus Tree for Your Climate
• Trees on sweet orange and Carrizo citrange
induce intermediate cold tolerance.
• Trees on sour orange, Cleopatra mandarin,
trifoliate orange and Swingle citrumelo are the
most cold tolerant. However, trees on sour
orange rootstock are susceptible to strains of a
widely spread virus disease (tristeza) and are not
readily available.
Finally, a given rootstock's cold tolerance is
highly dependent on environmental conditioning. The
best example is trifoliate orange. As a seedling it is
very cold hardy and even sheds its leaves like a
deciduous tree. Trees budded on this stock develop
their superior cold hardiness only after being exposed
to temperatures that induce cold hardiness: 70° F
day/50° F night for about two weeks before a freeze.
The least cold-tolerant rootstocks don't become
cold tolerant until temperatures reach 45° F
day/26° F night. Since soil and air temperatures in
the warmest areas of Florida often do not reach the
70° F day/50° F night temperature range in the
winter, even the most cold tolerant rootstocks may
not be exposed to temperatures that induce cold
tolerance. In such locations normally cold tolerant
combinations like tangerines on Cleopatra mandarin
rootstock may be damaged as much as Valencia
orange on rough lemon rootstock.
A note on cold tolerance: If your citrus trees
develop cold tolerance after several weeks of cool
weather, extended, unseasonably warm fronts can
work in reverse, stimulating new tender growth and
canceling newly acquired cold tolerance.
The scion influences cold tolerance even more
than rootstock. If you plan to raise citrus, you should
know there are inherent differences in cold tolerance
among scion cultivars regardless of the rootstock.
Mandarins, as a group, are the most cold tolerant,
followed by sweet oranges and grapefruit. Lemons
and limes are very susceptible to cold.
Post-freeze observations in Florida, Texas, and
California have clearly shown that the scion influence
is greater than that of the rootstock during freezes
preceded by favorable cold-hardening conditions.
However, rootstocks also have a measurable effect on
cold tolerance.
2
Scion/Rootstock Selection
Scion/rootstock combinations, with their
advantages, disadvantages, and regional
recommendations are listed in Appendixes A, B and
C. You will find it worth your time to review this
information.
Pollination
Many citrus cultivars are self-fertile: they
produce fruit when self-pollinated. However, many
mandarin cultivars require a different pollenizer
cultivar to set a crop of fruit. Pollenizer cultivars
must have four qualities to be potentially successful:
• a bloom period that overlaps with the main
cultivar,
• consistent annual production of a good crop of
flowers,
• cold-hardiness the same as the main cultivar,
and
• the capacity to be self-fruitful.
Pollenizer cultivars should not require cultural
practices that differ widely from the main cultivar.
For example, Minneola tangelos and Temple oranges
or (pollenizer cultivars) are susceptible to scab, a
fungal disease controllable with copper sprays,
whereas Orlando tangelos and Sunburst tangerines
(main cultivars) are usually not susceptible. Sunburst
tangerines are also very susceptible to mite damage
whereas other commonly used pollenizers are not.
Commercially, pollenizers are planted no further
than the third tree row or approximately 60 to 90 feet
from the main cultivar, so use this as a guide if you
plan to plant any of the cultivars listed in Table 1.
Another alternative may be to graft pollenizer
cultivars onto the main cultivar to produce a “fruit
salad” tree, which bears fruit of different cultivars.
If you do this, you may have to prune more vigorous
scions, like lemons, more frequently to maintain
balanced growth of different scion cultivars on the
same tree. Also remember to remove all mature fruit
Your Florida Dooryard Citrus Guide - Selecting A Citrus Tree for Your Climate
from the pollenizer cultivar to promote a good bloom
the following year.
A note on flowering and fruit set: Citrus trees
flower and produce fruit in response to environmental
stress. In the tropics, drought during the dry season
provides the stress. In Florida, cold weather (day
temperatures between 50° to 64° F and night
temperatures between 46° to 55° F) usually
provide this stress.
Winter temperatures in Florida can hasten or
delay the bloom period that normally occurs in
March-April. The time (number of weeks) that citrus
trees are exposed to cool weather can also affect the
intensity of flowering. However, when late winter
freezes kill tender blossoms, few, if any, fruit may be
produced, except from erratic late blooms. In
temperate climate zones north of Florida, potted
citrus should also be exposed to cold - but not
freezing - temperatures before you bring these trees
inside for the winter.
3
Your Florida Dooryard Citrus Guide - Selecting A Citrus Tree for Your Climate
Table 1. Pollenizer cultivars for important self-incompatible citrus cultivars.
Main Cultivar
Pollenizer
Cultivar
Minneola
Nova
Orlando
Robinson
Sunburst
Minneola
U
U
U
U
U
Nova
?
U
S
S
S
Orlando
U
S
U
S
S
Robinson
U
U
S
U
U
Sunburst
1,2
3
S
S
S
S
U
2,4
S
S
S
S
S
1,2,5
U
U
U
U
U
Temple
Murcott
S=Satisfactory; U=Unsatisfactory; ?=Unknown
1
Tends to alternate bearing
Scab Susceptible
3
Produces too little pollen unless used as the main cultivar
4
Much more sensitive to freeze damage than the other cultivars
5
Bloom does not overlap any of the other cultivrars
2
Note: No sweet orange or grapefruit cultivar is considered a satisfactory
pollenizer, even though some seedy cultivars of oranges are slightly effective.
4
Examples of Instruments Used For Measuring
Atmospheric Conditions during Cold Weather*
* Mention of a trademark or proprietary product does not imply approval or the exclusion of
other products that may also be suitable.
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Fisherbrand* Traceable* Radio-Signal Hygrometer/Thermometer
Monitor humidity and temperature from up to 100 ft. away—even outdoors
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Temperature and humidity readings are transmitted to main unit from
remote sensor module
To use, place the main unit on desk or benchtop; the remote sensor in area
to be monitored
Remote sensor displays current temperature and humidity and also
transmits readings over a range of 65 to 100 ft. (20 to 30m)† to main unit
Jumbo display (11/8 in.) on main unit can be read up to 25 ft. (7.6m) away
Main unit can receive signals from up to three remote sensors on three
user-selectable channels
Readings are updated every 30 seconds and display temperatures in your
Remote Sensor Module
choice of °C or °F
(left) transmits readings to
Trend indicators show rising or falling humidity and temperature
main unit (right) from up
Minimum and maximum memory display allows monitoring of conditions
to 100 ft. (30m) away.
over a period of time
An alarm can be set (in increments of 1% RH and 1°C/°F) to sound when
humidity or temperature falls outside set parameters
Relative humidity range is 25 to 90% RH with a resolution of 1% over the entire range; accuracy is ±2% at
mid-range and ±4% at ends of range
Switchable thermometer range is -20° to +60°C and -4° to +140°F with 0.1° resolution and accuracy of
±1°C (1.8°F)
To assure accuracy, an individually numbered ISO 17025 certificate is supplied indicating instrument
traceability to standards provided by NIST
Manufactured from high-impact, chemical-resistant ABS plastic
Includes one main unit, one remote sensor module, flip-open stand, wall-mounting brackets, and Velcro*
strips for attaching sensor to almost any surface
Main unit measures 3/4L x 41/4W x 41/2 in.H (2 x 11 x 11cm) and weighs 6 oz. (170g)
Remote sensor module measures 3/4L x 21/3W x 31/2 in.H (2 x 6 x 9cm) and weighs 3 oz. (85g)
Unit operates continuously for eight months on two "AA" batteries and two "AAA" batteries (included)
Characteristics
Fisherbrand* Traceable* Radio-Signal
Remote Hygrometer/Thermometer, Main
Unit with Remote Sensor Module
Cat. No.
14-648-52
Qty.
Price
Each for $65.63
†Depending on location.
Accessories:
●
Fisherbrand* Remote Sensor Module for Traceable* Radio-Signal Remote Hygrometer/Thermometer
●
Fisherbrand Remote Sensor Module w/External Probe for Traceable Radio-Signal Remote Hygro/Thermo
●
Fisherbrand SS Thermometer Probe for Traceable Radio-Signal Remote Hygrometer/Thermometer
●
Fisherbrand Thermometer Probe in Bottle for Traceable Radio-Signal Remote Hygrometer/Thermometer
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Fisherbrand* Traceable* Pocket Hygrometer/Dew Point/Thermometer
· Fast response time
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· Minimum/maximum for all readings
· Humidity, dew point, temperature display
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Compact design includes an electronic capacitance polymer film sensor that is not
affected by condensation. Relative humidity range is 20.0 to 100.0% RH with
resolution of 0.1%. Accuracy is ±3% RH mid-range to ±5.5% RH elsewhere. Both
dew point and temperature ranges are -20.0° to 50.0°C and -4.0° to 122°F with
resolution of 0.1° and accuracy of ±1°C. Typical full-range response is a fast 30 to
60 seconds. ...more
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06-664-37
Characteristics
Cat. No.
Hygrometer/Dew Point/Thermometer,
Fisherbrand , Traceable , Pocket
Qty.
06-664-37
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Remote Alarm RH/Temperature Monitor
Fisher Science Education Catalog > Temperature Monitors >
Remote Alarm RH/Temperature Monitor > Details
Features remote ambient RH/temperature
sensor with a 7 ft. (2.1m) cord.
·Place sensor inside a desiccator, refrigerator,
drying, or other environmental chamber; view
RH or temperature with minimum/maximum
readings on the LCD monitor
·Selectable °C or °F, minimum/maximum
temperature, and RH alarms; audible alarm
will sound when parameters are exceeded
-
·20 to 99% relative humidity range with 1%
resolution and ±4% full scale accuracy
·-10° to +50°C (-14° to +122°F) temperature
range with 0.1°C/°F resolution and ±1°C (±2°
F) accuracy
·Dimensions: 3/4L x 23/4W x 41/8 in.H (2 x 7
x 10.5cm)
S68617
·3 oz. (85g) total weight
·Place on a benchtop or mount on wall
·Five-year manufacturer's warranty
Cat. No.
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S68617
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Fisherbrand* Traceable* Economy Digital Anemometer/Thermometer
Measures outside wind speed, air velocity in hoods, and gas movement in ducts
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Details
Unit will also display temperature of air being measured.
Temperature range of 32° to 140°F and 0° to 60°C with
resolution of 0.1°F or °C. Designed for heavy use and
easy, accurate operation; unit will provide years of
reliable readings. Instrument has four preset ranges:
0.4 to 30mps with a resolution of 1mps; 1.4 to 108kph
with a resolution of 0.1kph; 80 to 5910fpm with a
resolution of 10fpm; and 0.8 to 58.3 knots with a
resolution of 0.1 knots. Individually serial-numbered
Traceable Certificate is provided from an ISO 17025
calibration laboratory accredited by A2LA. It indicates
traceability to standards provided by NIST (National
Institute of Standards and Technology).
-
Accuracy is ensured by low-friction, precision ballbearing vane that rotates freely in low and high
velocities. Reading is updated instantaneously. A HOLD
switch "freezes" display to capture and record readings.
Rugged ABS case is chemical and shock resistant, and
has handy flip-open bench stand for easy monitoring.
Extra-large LCD display is 3/4 in. high (1.9cm); can be
read from 15 ft. (4.6m).
Ordering Information: Comes with low-friction ball01-241
bearing vane, nine-volt alkaline battery, carrying case,
NIST-traceable certificate, and instructions. Unit is 6L x 3W x 11/4 in.H (7.6 x 15.2 x 3.2cm). Handle length, 21/2
in. (6cm); wind-vane diameter, 23/4 in. (7cm). Cable length is 4 ft. (1.22m). Weight, 14 oz. (397g).
Cat. No.
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