Learn about the different factors that can lead to winter injury on woody plants.
The frequency and severity of winter damage to trees and shrubs is determined by a number of factors, including the plant species or cultivar involved, the location and conditions under which the plant is grown, and the exact timing of weather extremes during the dormant period.
Contrary to popular belief, plant damage is not generally caused by an unusually cold winter. Low-temperature injury is more often associated with extreme temperature fluctuation than with prolonged cold weather.
Some plant damage that is referred to as “winter injury” is not caused by extreme cold temperatures, but by temperature fluctuations that may occur between autumn and spring.
Plants acclimate to the cold of winter through exposure to gradually falling temperatures and changes in day length that occur in autumn. Plants that are entering dormancy, but are not fully acclimated, can be stressed or injured by a sudden, hard freeze. Rapid or extensive drops in temperature following mild autumn weather can cause injury to woody plants. Extended periods of mild winter weather can de-acclimate plants, again making them vulnerable to injury from rapid temperature drops.
Erratic temperature fluctuations in spring after plants come out of dormancy can damage to delicate, new tissue. Once spring growth has begun, a late spring frost can cause damage to de-acclimated woody stems, blossoms, and new shoots. Frozen, succulent, new tissue turns flaccid, appears watersoaked, and withers within a short time. Though symptoms resemble blight diseases, injury appears suddenly after a hard frost or freeze, while diseases such as bacterial fire blight, juniper blight, and pine tip blight are progressive over time.
While temperature fluctuations have always occurred in northeastern Illinois, they have become more frequent in recent years. Protection measures put in place to prevent winter injury should be kept in place until temperatures stabilize. Some protective measures may need to be removed as temperatures rise, and put back in place as they fall again.
Every species and cultivar of trees and shrubs has a limit as to what minimum temperature it can tolerate. Each woody plant is considered winter hardy to that minimum temperature. When temperatures drop lower than that minimum, plants will be damaged.
Plants most likely to suffer winter injury are those that are marginally hardy for the area or those already weakened by previous stress. Species such as rhododendron, holly, and some magnolias may survive several mild winters in the Chicago region before a more typical winter causes injury.
Flower buds are often the most susceptible.
If plants with marginal hardiness are used, they should be planted in protected sites, such as courtyards or sheltered areas.
Woody plants considered winter hardy in the Chicago region may still suffer damage if they are in poor health or under serious stress as they go into winter.
In general, low temperatures are much less damaging than rapid and extensive variations in temperature.
Frost cracks appear as shallow to deep, longitudinal cracks in the trunk of trees. Frost cracks often, but not always, occur on the south or southwest sides of trees because this area experiences the greatest temperature fluctuations between day and night.
On a bitterly cold day, with full sun, the trunk of a tree is warmed by the sun and expands. A sudden drop in temperature, due to development of cloud cover or the setting of the sun, causes the outer layer of wood to contract more rapidly than the inner layer, which results in a long vertical crack at weak points in the trunk.
Once a frost crack occurs on a tree, it is likely to appear annually.
Young trees with thinner bark are more susceptible to this. Trees most susceptible to frost cracks include London plane, maple species, crabapples, fruit trees, lindens, poplars, birches, and willows.
Sunscald can occur under the same conditions that cause frost cracks. Frost cracks and sunscald can occur together or separately.
Sunscald often appears as an elongated canker or damaged areas on the trunks of thin-barked trees. Sunscald often develops on the south or southwest side of trees following a sudden exposure to direct sun.
In winter, the temperatures on the sun side of the trunk may exceed air temperatures by as much as 20°F. This is thought to trigger the de-acclimation of trunk tissue. The bark slowly darkens, turns reddish-brown, and becomes rough. After a time, the callus tissue eventually cracks and falls away. Sometimes only the outermost cambium layer is damaged and a sunken area appears on the trunk.
Affected trees often have sparse foliage, stem dieback, and stunted growth.
A browned or scorched leaf tip on evergreen foliage in late winter and early spring may be a sign of winter injury. Browning usually occurs from the needle tips downward.
Symptoms of winter burn can occur on both needled evergreens, such as hemlock, juniper, pine, and yew, and broad-leaved evergreens, such as boxwood and rhododendron.
Winterburn is usually attributed to desiccation, or loss of water, through leaf transpiration. Winter sun and winds dry needles out. Water in the stems and roots is frozen and unavailable to replenish the loss.
A rapid drop in temperature after a warm sunny day can cause further injury to the plant. Applying an anti-transpirant, also called antidesiccant, helps reduce transpiration and minimizes damage to the foliage. Two applications per season may be needed for full season protection. Follow all directions on the product label.
To reduce the incidence of winterburn, continue watering evergreens in fall for as long as the soil will accept water.
The soil provides a certain amount of insulation for the root system of a plant. Mulch, leaf litter, and snow cover add insulation as well, generally keeping soil temperatures from falling much below freezing. Roots, however, can be damaged by winter temperatures in certain situations.
Plants with roots near the soil surface are more at risk for root damage.
Plants that are overwintered in containers outdoors may also suffer root damage due to the small volume of soil in the container.
Plants with frozen roots may wilt and decline after growth resumes in the spring.
Heavy snow and ice from winter storms can cause damage by bending and breaking branches.
Multi-stemmed evergreens, such as yews, arborvitae, and junipers, are often the most prone to damage.
To protect these plants from limb breakage prior to winter, tie branches together loosely with strips of cloth or coated twine. Remove in early spring.
The branches of many hardwoods may be seriously damaged in winter storms. Improper removal of ice or snow from the tree or shrub can increase damage.
Heavy snow should be removed gently before it freezes to limbs and branches.
Removing ice that is encasing branches can cause additional damage and should not be attempted. Instead, allow ice to melt off naturally.
There are many products used for deicing pavements, and some can cause damage to trees, shrubs, and other landscape planting.
Sodium chloride, known as rock salt, is an effective ice melter, but can be quite damaging to landscape plants.
Magnesium chloride is less harmful to plants and is still an effective ice melter. It can cause damage to concrete.
Potassium chloride causes less damage than rock salt, but may not be as effective for melting ice at lower temperatures. It causes less damage to concrete than magnesium chloride.
Calcium chloride does less harm to landscape plants and works well at lower temperatures.
Calcium magnesium acetate is a safer option for use around landscape plants. It often comes with a higher cost and does not melt ice at very low temperatures.
With any of these products, it is important to avoid overapplication.
Symptoms of salt damage appear in spring and include browning of evergreens, leaf scorch, branch dieback, and dead areas in turf.
Branches and twigs can be damaged by salt spray caused by vehicles driving on treated roadways.
Physical barriers of burlap or tarp may reduce salt spray from roads. These materials should be used as barriers between the plants and the roadway. They should not be used to wrap the plants.
Roots can be damaged from salt remaining in the soil. Salt will leach through well-drained soils, but damage can be extensive in poorly drained soils. Thorough watering of the area may help leach the salt out.
Choose salt-tolerant species for sites where salt stress may be a problem.
Mice, rabbits, and other animals often damage young trees in the winter by feeding on the bark and girdling the trees.
Damage occurs most commonly when there is prolonged, heavy snow cover and food is scarce.
Rabbits feed on the bark above the snow, while mice feed near the ground level. Mouse damage is usually more severe when the trees are surrounded by heavy grass, weed cover, or heavy mulch, so it is helpful to pull mulch away from trunks and branches.
The most effective deterrent to girdling by mice or rabbits is to wrap the trunk and low branches of young trees with screen wire or hardware cloth from below the ground line to high enough above the possible snow line to prevent rabbits from reaching the trunk or branches.
To help control mouse damage, maintain an area free of grass or weeds for a 1- to 2-foot radius around the base of the tree.
Various chemicals are available to repel mice and rabbits, but are often not reliable in wet weather.
For more information on animal damage refer to Animal Damage.
Winter injury can be difficult to manage successfully, but several measures can help minimize the effects of winter injury: