Sunscald is a condition that can happen to a tree's bark in winter due to sudden changes in temperatures. On sunny winter days with good snow cover, the sun heats the bark of the tree, usually on the southwest side, and the light reflected off the snow intensifies the effect. The living cells in the tree – the cambium – break dormancy and begin to grow. Then when temperatures fall below freezing at night – or even during the day, with passing cloud cover – this living, growing tissue doesn't have enough time to harden off, and dies.
William Jacobi, Colorado State University, Bugwood.org
The injury usually occurs on the lower trunk, but in some cases can be higher up or on the larger branches. The bark splits open, and over time, dries, flakes and peels. The split is usually long and unsightly. With time, a tree can heal itself, forming a callus over the wound. This takes years, however, and in the meantime insects, fungal spores and diseases can enter the tree through the split and weaken and kill it. Sometimes the injury isn't apparent for months until the tree starts to grow.
It's also known as southwest winter injury and usually occurs in late winter and very early spring when temperatures fluctuate.
Trees that are susceptible to sunscald are those that enter the winter stressed by drought or heat, were planted during the hottest part of the summer, or have had either lower branches or surrounding trees and structures removed that were shading the trunk. Moving a tree from a shaded or protected site to an exposed, sunny position can also cause sunscald.
G.J. Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org
Young, deciduous trees or those with dark or thin bark are most at risk. Susceptible species include dogwood, honey locust, maple, walnut, linden, aspen, birch, crabapple, ornamental cherry, tuliptree and fruit trees. The dense, needled branches of evergreens usually protect them, but even they can get sunscald, especially if surrounding trees are thinned or the lower branches of a tree are removed.
How to prevent sunscald:
The best way to prevent sunscald is to provide good care to young trees. Keep them well watered. Newly planted trees need about one inch of water per week, either from rainfall or through irrigation. The water should percolate to a depth of one to two feet.
Mulch the entire root zone of the tree with mulch to a depth of two inches, but keep the mulch at least six inches away from the trunk.
If thinning the lower branches of trees, don't remove them all at once; spread the pruning out over a few years.
An easy, temporary solution is to simply prop a wide, light-colored board over the southwest side of the tree, the area most likely to sunburn. This will shade the trunk.
Joseph O’Brien, USDA Forest Service, Bugwood.org
Wrap the trunks of susceptible trees with white tree wrap in late fall. This will reflect the heat and keep the cambium from breaking dormancy. Take the tree wrap off right after the last frost, because it can invite insects to burrow beneath, and if it is left on too long, it can girdle the growing tree. Plastic tubing also works if the diameter of the tree is small enough.
People used to paint the trunks of susceptible trees with white latex paint to reflect the light and keep the bark cool. This works well, but makes the tree pretty unattractive, especially if it's a specimen tree in a home setting. But it can be a practical idea for fruit trees, if appearances don't matter.
Treating Sunscald:
Removing loose or dead bark that creates hiding places for insects can help a tree with sunscald damage. Experts no longer recommend putting any kind of paint, tar or sealant over the wound. Studies have found that it does not help. Keep the tree well watered and do not fertilize unless a soil test indicates it is necessary.
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