Cold and freeze protection of blueberries in Florida
Dr. Dorota Z. Haman
Agricultural and Biological Dept.
University of Florida
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.
•  A freeze involves an entire region, has significantly
lower temperatures, and may last for several days.
Definition of frost/freeze warnings issued by National Weather
Service.
Warning
Wind Speed
Air temperature
Frost
Below 10 MPH
Above 32oF
Frost/freeze
Below 10 MPH
Below 32oF
Freeze
Above 10 MPH
Below 32oF
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.
Irrigation systems
•  Surface
•  Sprinkler
•  Microirrigation
This stage can
tolerate 15 - 20 F
temperatures.
This stage can tolerate 25 to 28 F.
Heat transfer concepts
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.
Latent heat transfer
•  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.
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.
,2
Relative humidity (RH)
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.
Rate temperature fall depends on relative humidity.
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.
•  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.
Psychrometer
From R. L. Snyder
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.
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.
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
Radiation Frost
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
Inversion develops
Cold air in the low spots
White or black frost damage
Easier to protect
Difficult to protect
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)
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).
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
Uniformity
•  High system uniformity is necessary for effective frost
protection.
Sprinkler overlapping
•  The overlap is extremely important for frost/
freeze protection
•  Higher winds require closer spacing of
sprinklers
Ice forming indicator
The temperature of a
plant covered in ice
will drop below a dry
plant if the ice dries.
Ice forming indicator
Wind speed
greatly reduces
effectiveness of
sprinkler irrigation
for freeze
protection.
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.
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.
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)
Automatic frost monitoring and alarm systems
Provide early warning by:
text message to your mobile
activation of:
sirens,
strobe lights and
irrigation systems
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.
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.
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.
Thank you for your attention