Assessing Winter Injury to Trees and Shrubs

Carole Frost
York County Horticulture Educator

The ideal time to assess winter injury to trees and shrubs is after new growth begins to emerge in the spring. Some injuries may not show symptoms until later in the growing season, which can make correct diagnosis difficult. Many plants have protective mechanisms that should not be confused with winter damage. For example, some plants will shed leaves (Nandina and Privet); some plants will roll their leaves downward or the margins inward (Rhododendron); some evergreens (Juniper, Arborvitae, Cryptomeria, and Boxwood) turn a red, purple or bronze color.

Winter injury can occur in many different forms, including low temperature/frost injury, winter desiccation, winter sunscald, frost cracks, snow & ice breakage and rodent damage. Damage symptoms include discolored needles or leaves, dead branches or branch tips, heaved root systems, broken branches or girdled stems.

Low temperature injury can occur during the winter season when unusually warm weather in autumn delays dormancy and is then followed by early frost or drastic temperature fluctuations. Injury can also occur in early spring when new growth emerges, followed by abnormally low temperatures. Symptoms of low temperature injury are foliar browning and dieback of buds, twigs and branches. Plants will often leaf out, then collapse due to damaged cell tissue in the vascular system.

Reducing the occurrence of winter injury can be accomplished by following some guidelines. Be sure to select hardy plants. Growing plants that are winter hardy or native will reduce the effects of low temperature/frost injury. Avoid fertilizing plants with high nitrogen late in the summer so new growth is not promoted. Injury to young growth or insufficiently hardened tissues may still occur as a result of unusual weather patterns. Injured and dead tissues should be pruned out to discourage invasion of the plants by diseases and/or insects.

Winter desiccation or 'winter burn' is usually observed in late winter or early spring on evergreen plants. Broadleaved evergreens, such as rhododendron, exhibit browning on their leaf margins. Narrow leaved evergreens can exhibit slight browning of needle tips to browning and premature abscission of entire needles, depending on the extent of the injury. Winter desiccation occurs more drastically on sunny and/or windy winter days when plants lose water from their leaves through transpiration faster than it can be replaced by the roots frozen in the soil.

To prevent or reduce the effects of winter desiccation, make sure plants are properly watered during dry periods in the autumn. Thoroughly hydrated plants are less susceptible. Placing a protective barrier of burlap around small evergreens will act as a windbreak and reduce the rate of desiccation. The barrier can also shade the plant, avoiding warming by the sun. Antidessicant sprays applied to evergreens may reduce water loss. Follow label directions for use. Research results are mixed on whether or not antidessicants are effective.

Winter sunscald usually occurs on the south or southwest sides of tree trunks and branches. Young and thin-barked trees are most susceptible. The bright winter sun warms the bark during the day. The bark cools rapidly after sunset causing injury and even death to the inner bark in those areas. Symptoms of winter sunscald are elongated, sunken dead areas in the bark.

Wrapping the trunks of susceptible trees with tree wrap is the most effective way to minimize this type of winter injury. If a tree wrap is used, it should be removed after one season to prevent insect or moisture damage. In commercial orchards, it is customary to paint the trunks of trees white to reflect the winter sun, reducing the buildup of heat during the day.

Frost cracks are splits in the bark and wood of a tree. They are caused by rapid drops in temperature that freeze the water within the trunk, forcing it to explode or split open. If not severe, the cracks can heal themselves by callusing over. However, many times the cracks reopen again the following winter. Frost cracks can be compounded by internal defects within the wood. Defective wood does not contract as readily as the outer layers of healthy wood when winter temperatures decrease rapidly.

Care should be taken to avoid trunk damage of trees, especially when young. Frost cracks in trees are ideal entrance sites for wood decaying organisms and insects.

Heavy snow or ice on weak branches with foliage can result in breakage. Evergreens are especially susceptible. Sometimes even strong branches from deciduous trees can be broken if the weight of ice or snow is extremely heavy.

Properly pruned trees and shrubs can reduce the accumulation of snow and ice collected on the branches. Removal of weak branches and those with acute or narrow angles can help reduce breakage. Avoid late-summer pruning that stimulates new growth.

Winter injury can be reduced by following the management tips provided. Injury can occur on a broad range of evergreen and deciduous plants and symptoms may vary. Be a careful observer and ask various questions to assist with diagnosing winter injuries to plants.

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