AG0604.14
County Agent Notes
Steve Moore, County Extension Agent for Agriculture
WINTER INJURY ON TREES AND SHRUBS
Dr. Bill Fountain, UK Extension Horticulturist, has recently written a fairly complete
summary of the winter of 2013-14 and how it has affected our landscape trees and shrubs. I’ve
taken the liberty of shortening the article. If you’d like to have a copy of the entire piece, please
contact me at the Henry County Extension Office (845-2811).
There are some unusual circumstances regarding the weather and our plant material this
winter. The plant hardiness zone for most of Kentucky is zone 6. Looking at the weather service
maps for the entire country, we share zone 6 with some places that have dramatically different
environments (for example: Texas Panhandle, Coastal New England, and the Alaskan
Panhandle). And if you review weather service data, Kentucky was on average no colder than
we have historically experienced. However, the visible indication of dead plants and utility bills
indicated that something was different.
Temperatures associated with the Plant Hardiness Zones are based on the 30 year average
of the lowest winter temperature experienced in a region. This is not the lowest temperatures
experienced over the last 30 years, just the average. The winter of 2013-14 did not reach the
historical lows of approximately -20°F experienced in parts of Zone 6 Kentucky in recent
decades. The extensive damage now being observed in landscape plantings are the result of
factors not recorded in the PHZM. Our recent winter was different in many aspects.
Duration of Cold
Winter temperatures fell as would be expected in a normal winter. What was different
this year is that the temperatures stayed consistently low for extended periods. This allowed the
soil to freeze deeper and stems to remain frozen longer than in previous winters.
Wind, Humidity and Sun
The majority of the water lost by plants is from their leaves. Deciduous plants drop
foliage in the fall to reduce their need for water. Water loss continues through winter but at
significantly lower rates. The small amounts of water lost from dormant stems must be replaced
to prevent damage. Sometimes this is not possible if the soil or stems are frozen. Damage was
compounded when frozen stems were physically shaken or bent by ice, snow and wind.
What Has Happened?
Many evergreen plants turned brown in late winter and early spring. Many of these
broadleaf evergreens are marginally hardy in our climate. They are from milder climates where
they retain evergreen foliage all year long. Some of these species, such as Monkey Grass or
Southern Magnolia survived previous winters because of the milder than normal temperatures
experienced.
Other broadleaf evergreens may look green and healthy during the transition into spring.
But problems can accompany the first really hot days. Unseen is significant damage to the
plumbing system of the plant. The result is leaf and stem death as if it were much hotter and
dryer. Examples of species where this has been seen include boxwood and Laurel cherry.
Sunscald
In late May or June, well after the cold winter months have passed from our minds we
begin to see the development of another type of winter injury. This problem is called sunscald.
It is not only disfiguring but often leads to a long, slow mortality spiral. It usually appears only
on the south or southwest side of trunks and only on recently planted trees. The first indication
is a small vertical crack in the bark. These cracks often run from close to the soil line up to the
lower branches. As the crack opens the bark begins to peel back exposing the wood. This is
more than a disfiguring problem. It results in the loss of conductive tissues essential for growth
and development. At best, trees stressed by sunscald will reestablish more slowly and are more
susceptible to diseases and insects. While these trees may ultimately survive, replacement trees
outgrow severely damaged trees.
Sunscald is almost always limited to young, recently installed landscape trees. It is most
common on species with thin bark than trees with thick or exfoliating bark, such as Maple,
Linden, Pear, Crabapple, Cherry, Plum, and Willow, with the most common the Red Maple.
Sunscald can be prevented by shading the trunks of young, newly planted, thin-barked
trees. A double layer of plastic or fiberglass windowscreen is an easy and economical way to
accomplish this.
Cold Damage to Roots
The least hardy part of any plant is its root system. Roots grow in the ground where the
insulating effect of soil buffers the roots against extremes of heat and cold. When they are
subject to freezing temperatures for an extended period, they can be killed.
Winter injury to landscape plants appears with multiple visual symptoms. While these
injuries are associated with low temperatures, injury is usually the result of a combination of
different environmental and cultural conditions (low temperatures, duration of cold, lack of soil
moisture, low humidity, wind and sun).
Managing Winter Injury on Landscape Plants
Rule number one in diagnosing winter injury and making recommendations is don’t be
impatient. If the foliage or the tips have been damaged but the stems and buds are still green,
give the plant the opportunity to put out new growth. Sheering dead foliage will immediately
improve the appearance of the plant.
Spring fertilization is not recommended, especially for plants suffering winter injury.
The addition of nitrogen can encourage more growth than the damaged stems can supply with
water during the hot, dry summer months. The addition of water during dry periods is more
beneficial than the addition of fertilizer.
The best long-term approach is to match the plant to the site. This can involve using
hardy needled evergreens where evergreens are desired and deciduous species that originated in
our climatic zone.