Winter Injury

Winter injury may occur under various weather conditions and could be widespread even in winters that are considered mild. Conditions that cause winter injury include: early and late frosts, low temperatures, fluctuating temperatures, lack of snow cover, sun and wind. Damage caused by winter injury might not be visible until growth starts in spring or until well after the initial flush of spring growth. Understanding the types of winter injury may help to diagnose winter injury problems this spring and help prevent injury in the future.

Frost Injury
Frost injury occurs when a plant is not appropriately acclimated to freezing temperatures when a frost occurs. The damage occurs either in the early fall or spring. Recently, the Northeast has had several unseasonably warm early springs which have increased the risk of spring frost injury. The early warm weather causes plants to break dormancy and produce susceptible growth earlier than normal. This growth is vulnerable to frost conditions that are still likely to occur. Frost injury damages buds, flower buds, and shoots. Frost damage is usually immediately visible after thaw and is the result of water freezing in the plant tissues.

Plants that are exotic or borderline hardy are the most susceptible to frost injury. Plant specifics should be appropriately selected for your USDA Hardiness Zone (go to www.planthardiness.ars.usda.gov). Avoid planting susceptible species in low lying areas, known as frost pockets. Frost pockets experience earlier and later than average frost conditions, increasing risk of damage. Management practices that result in late season growth, excessive nitrogen fertilization or pruning in mid/late-summer, may increase the risk of fall frost injury. The risk of frost injury can be avoided; however, it is impossible to eliminate, as erratic weather can not be prevented.

Low Temperature
Low temperature injury occurs when the temperature goes below a plant's physiological temperature threshold or the plant is not appropriately acclimated. Common weather conditions that may induce low temperature injury include drastically fluctuating temperatures, extreme lows, or rapid declines in temperature. Low temperature injury may occur at any time from late fall to spring. Low temperature has the potential to damage any plant tissue including roots, buds, flower buds, or vascular tissues. The damage occurs when intracellular liquids freeze, damaging the cells. Damage caused by low temperatures is not always immediately noticeable in the spring. In some instances, plants damaged by low temperatures have enough reserves to push out spring growth but collapse after the reserves are used up. Typical low temperature injury includes shoot and limb dieback, death of buds, cambial and vascular damage, and plant death.

Choosing plants of the correct USDA Hardiness Zone is crucial to avoiding low temperature damage. Plant cultivars can range drastically in their low temperature thresholds. Choose cultivars with proven performance or hardiness in your area. Container plants are extremely vulnerable to low temperature injury of the root system. This can lead to reduced vigor or death. More information about overwintering container grown plants can be found here.

Fertilizing and pruning late in the season was once believed to increase the risk of low temperature winter injury. Recent research has debunked this theory and fall fertilization has become a standard practice for woody ornamentals. Fertilizing nutrient deficient plants in the fall may actually reduce the risk of low temperature injury to those plants. Timing of these practices should not be overlooked. If these practices are implemented too early, the result may lead to late season growth. Late season growth is more susceptible to frost and low temperature injury.

Desiccation
Desiccation injury occurs when plant tissues lose water, particularly on sunny and windy days when water is lost not replaced. This type of injury causes marginal necrosis of leaves and needs or complete necrosis of leaf/needle tissues. Occasionally, dieback of young twigs may also occur. If extensive injury occurs, leaves and needles may prematurely abscise.

Desiccation injury can be easily avoided or prevented. When plating susceptible species and broadleaf evergreens, avoid exposures that are windy. Susceptible plants may also be wrapped with burlap or sprayed with anti-transpirants. Avoid transplanting broadleaf evergreens in the fall and make sure needled evergreens are transplanted with sufficient time to allow some new root growth. If fall conditions are excessively dry, plants should be watered to insure adequate moisture in the root zone as winter approaches. During winters that lack significant precipitation, early spring water may also be necessary.

Frost Cracks & Sunscald
Frost cracks and sunscald are similar types of winter injury, both resulting from the rapid freezing of water in plant tissues. Frost cracks occur when internal defects or bark injury results in tissues that expand and contrast at different paces, causing bark to split. Sunscald is the result of bark being warmed by the sun and then rapidly freezing when the sun sets. The rapid freezing damages vascular and cambial cells. This type of injury is often called southwest injury because tissues with this exposure are most susceptible.

Frost cracks can be avoided by eliminating wounds to the bark of young trees. Even though the tissues may callus over, this will be a site for repeated cracks. Eventually, the cracks may form a large frost rib that offers a site for decay organisms to enter the wood. Sunscald is best avoided by appropriate site selection. Avoid planting thin barked species, such as Prunus species, in sites with southwest exposures.

Frost Heaving
Frost heaving damage occurs when plants are heaved out of the ground. Plants are heaved when weather conditions promote freezing and thawing of the ground. Recently transplanted plants are the most likely to be heaved. Once a plant is heaved, the roots are exposed to low temperatures , sun, and wind. Roots are extremely susceptible to these conditions and damage is likely. A plant may be entirely killed if there is extensive root damage.

Established plants rarely have issues with heaving; however, transplants may not have enough time to root into the surrounding soil. To avoid heaving of transplants , mulch after the ground has frozen to buffer the effects of freeze and thaw cycles. If plants become heaved, replant immediately and mulch. Heaving is more common in years with poor snow cover.