Winter Injury to Trees and Shrubs
The severity of winter damage is determined by a number of factors, including the species and conditions under which the plant is grown and the timing of weather extremes. 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. TEMPERATURE FLUCTUATION Plants that are dormant but not fully acclimated can be stressed or injured by a sudden, hard freeze. Rapid or extensive drops in temperature following mild autumn weather 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.
LOW TEMPERATURES Some species or cultivars are injured if temperatures fall below a minimum tolerance level. Plants most likely to suffer winter injury are those that are marginally hardy for the area or already weakened by previous stress. In general, low temperatures are much less damaging than rapid and extensive variations in temperature.
FROST CRACKS Frost cracks, sometimes called radial shakes, appear as shallow to deep longitudinal cracks in the trunk of trees. They are most evident in winter at temperatures below 15°F. Frost cracks often occur on the south or southwest sides of trees because this area experiences the greatest temperature fluctuations. A sudden drop in temperature 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.
SUNSCALD 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 de-acclimation of trunk tissue. 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.
WINTERBURN A browning or scorched leaf tip on evergreen foliage is a form of winter injury. Browning usually occurs from the needle tips downward. Winterburn is usually attributed to desiccation or loss of water through leaf transpiration. Winter sun and winds dry needles. Water in the stems and roots is frozen and unavailable to replenish the loss. Applying antidesiccants, helps reduce transpiration and minimizes damage to the foliage.
ROOT DAMAGE Root tissues apparently do not acclimate to temperatures much below freezing and can be killed or severely injured by soil temperature below 15°F. This is especially true for shallow rooted plants. The presence of mulch, leaf litter or snow cover can insulate most soils sufficiently to prevent soil temperatures from falling much below freezing. Plants with frozen roots may wilt and decline after growth resumes in the spring.
BREAKAGE Heavy snow and ice storms cause damage by bending and breaking branches. Improper removal of ice or snow from the tree or shrub might increase damage. Removing ice encased on branches can cause additional damage and should not be attempted. Instead, allow ice to melt off naturally. Properly pruned trees are typically less prone to storm damage.
SALT DAMAGE Salts used for deicing pavements can cause damage to trees and shrubs. Symptoms of salt damage appear in spring and early summer and include browning of evergreens, leaf scorch, branch die back, and dead areas in turf. Branches and twigs can be killed from aerial deposits, and 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.