Winter Injury – Part 2: October 2019

Frost Cracks
Frost cracks, often called radial shakes, show up as shallow to deep up and down cracks in the trunk of trees. They mostly appear in winter at temps below 15°F. Frost cracks often occur on the south or southwest sides of trees because this part of the tree experiences the greatest temperature changes between day and night. A sudden drop in temperature causes the outer layer of wood to shrink quicker than the inner layer, which causes a long up and down the crack at weak points in the trunk. Once a frost crack occurs on a tree, it is likely to appear annually.

A stretched out canker found on the trunk of thin-skinned trees and is often referred to as “sunscald”. Sunscald often shows up 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 deacclimation 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.

Seasonal Burn On Evergreens
A browning leaf tip on evergreens in late winter and early spring is a form of winter injury. Browning occurs from the needle tips towards the rest of the needle. Symptoms of winter burn are present on many narrow-leafed evergreens, such as hemlock, juniper, pine, and yew, and broad-leaved evergreens, such as boxwood and rhododendron. Winter burn 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. A rapid drop in temperature after a warm sunny day can also cause further injury to the plant. Applying an antitranspirant, also called anti-desiccant, helps reduce transpiration and minimizes damage to the foliage. At least two applications per season, one in December and another in February are usually necessary to provide protection all winter.

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