Winter Injury                                      GD 20

Ann Hazelrigg, Plant Diagnostic Clinic Coordinator

The term "winter injury" is used to describe several types of plant damage caused by environmental conditions during
late fall, winter, or spring. Damage ranges from a marginal scorching of leaves to complete killing of plants. It is often
difficult to differentiate winter injury from disease, insect, or chemical injury. Winter-injured plants will often leaf out
normally in the spring only to collapse after stored food reserves have been totally used up by the plant. Occasionally,
damage does not become apparent until 1 or 2 years after the injury occurred. Vermont winters can adversely affect
trees and shrubs several ways: frost injury, low temperature injury, winter desiccation, sunscald, frost cracks, and
snow and ice breakage.

Frost Injury

Late spring and early autumn frosts can injure active tissues that are insufficiently "hardened" to withstand the cold
temperatures. This type of injury may occur on native or exotic plants although the latter are usually more vulnerable. A
result of late spring frosts can be the death of expanding flower buds on species such as magnolia or lilac, or the death
of young, succulent, actively growing shoots. In some areas of Vermont, frost injury can occur every month of the
year.

Management
Clip and destroy dead tissues in the spring. The plant will not suffer any long-term effects.

Low Temperature Injury

Plants frequently injured directly by low winter temperatures are those planted in areas north of their appropriate
hardiness zone. Such species cannot harden off at an appropriate rate or to an extent sufficient to withstand prevailing
winter temperatures. However, even hardy plants can be injured during unusually cold periods or when temperatures
drop rapidly or change frequently. Excessive application of nitrogen fertilizer, or pruning in August or September may
promote new growth that will not mature and may be damaged by freezing. Flower buds, vegetative buds, branches,
stem, crowns, bark, roots, or even whole plants may be injured. Containerized plantings are particularly vulnerable to
low winter temperatures since their roots are not protected by being below ground. Cold temperature injury that
occurs during winter may not be evident until injured tissues fail to grow the following spring.

Management
Avoid planting exotic species north of their plant hardiness zones. Containerized plants should be placed in
protected areas, sunk into the ground, grouped together, or heavily mulched to avoid low temperature injury to
roots. To allow proper hardening of plant tissues, avoid heavy applications of nitrogenous fertilizer in late
summer. Mulch around the bases of root-tender plants to help protect their crowns and roots from freezing
temperatures. Even with good management, injury to young growth or insufficiently hardened tissues may still
occur as a result of unusual weather patterns. Little can be done to prevent injury in these instances.

Injured and dead tissues should be pruned and discarded or destroyed to discourage invasion by disease
organisms. Replace plants with species adapted for the appropriate plant hardiness zone.

Winter Desiccation Injury

This type of injury, commonly called "winter drying" or "winter burn," is usually observed in late winter or very early
spring on evergreen plants. Broadleaved evergreens, such as rhododendron, exhibit browning or even total necrosis
(death) of their leaf margins (leaf scorch) depending on the extent of injury. Narrowleaved evergreens, such as white
pine, exhibit browning of needle tips when injury is slight. Extensive injury may result in browning and premature drop
of entire needles. The injury occurs during sunny and/or windy winter weather when plants lose water from their leaves
through transpiration faster than it can be replaced by roots that are frozen in soil.

Management
Plants that are properly watered during dry periods in late autumn are better equipped to withstand this type of
injury. Mulching around the root zones of susceptible evergreens will also help to minimize the damage. Placing
a protective barrier of burlap over or around plants to protect them from winter winds and sun will help to
reduce the incidence of this injury. Antidesiccant sprays applied once in late autumn and again in mid-winter
may also prove helpful.

Sunscald

This type of injury occurs when the sun heats tree bark during the day and then the bark rapidly cools after sunset.
These abrupt fluctuations are most common on south or southwest sides of trunks and branches, and they may kill the
inner bark in those areas. Young and/or thin-barked trees are most susceptible to winter sunscald.

Management
Wrapping trunks of susceptible trees with protective "tree wrap" is the most effective way to minimize this type
of winter injury.

Frost Cracks

Frost cracks are splits in bark and wood of a tree that occur when winter sun causes a differential expansion of wood
beneath the bark. The initial crack is often accompanied by a loud snap. In winter, the crack may become wider and
narrower during colder or warmer periods. Such frost cracks often close and callus over during the summer, only to
open again in subsequent winters. This callusing and recracking may lead to the formation of large "frost ribs" on the
side of affected trees.

Management
In mid-autumn, wrap the trunks of young trees with commercial tree wrap paper or burlap to protect against
frost cracks. Large frost ribs can be braced to prevent reopening during the winter, thus, enhancing callusing
and healing. Frost cracks in trees are ideal sites for the entrance of wood decay organisms. Affected trees
should be checked regularly to insure they are free from serious decay and, therefore, not a hazard to
surrounding buildings and living things.

Snow and Ice Breakage

Heavy snow or ice on weak limbs or on limbs with foliage (as in the case of evergreens) can result in breakage.

Management
Prune trees and shrubs to reduce the amount of snow or ice they will collect and/or eliminate those branches
which will be inherently weak. Branches with a wide angle to the main stem are generally stronger and can
support more snow and ice than can those with a narrow or acute angle. Plant trees and shrubs away from
places where snowmelt from roofs will drip on them. Wooden barriers may be built over small shrubs to allow
snow and ice to slide off rather than accumulate.

Also refer to factsheet OH 3: "Preparing the Garden for Winter."

Before using any pesticide, read the label and follow all precautions!

Based on material developed in 1992.