Winter Sunscald

Summer sunburns hurt, but winter sunscalds are worse (for trees).


​You might not think sunscald, or sunburn, could occur in winter, but it can. [Ask any of your skier friends. They know.] Unlike summer sunburns, which are much like the sunburns we get, winter sunscald occurs when it gets warm enough in the daytime for plant cells just under the bark to break dormancy.


Breaking dormancy

​There are several chemical changes that occur as plants enter dormancy. These changes protect plants against the cold. Warm, sunny days can trick overeager cells into breaking dormancy and losing those protections. Then they die, leaving a sunken area below the bark.

Winter sunscald on ask bark (Rosser1954) Public Domain

Damage caused by winter sunscald

The sunken areas created by winter sunscald normally develop a protective callus, but not always. Once the tissue is damaged, it is more likely to develop cracks during the growing season. Slugs and other pests love to hide out in the fissures created by sunscald.


​Winter sunscald can also damage leaves. If skies have been overcast for some time and then a bright sunny day occurs, leaves may exhibit bronzing between the leaf veins. If conditions stay warm, the leaf will die. Losing one leaf doesn’t matter. Losing most or all of its leaves can severely weaken a tree.

Slugs seek shelter and food in a winter sunscald fissure (Rosser1954) Public Domain

If there are any fruits present in winter, they, too, can be affected by winter sunscald. This occurs when fruits that were protected by shade are exposed to direct sunlight. The outer skin of these fruits can become damaged enough to allow insects and disease to enter. In other cases, the fruit may look normal on the outside, but the underlying tissue becomes damaged and is usually inedible.


​Winter sunscald is rarely fatal, except to one- or two-year old trees. However, it can weaken a tree and make it susceptible to wood-boring pests and canker diseases. Nearly all fruit trees and all newly planted trees are vulnerable to winter sunscald. Thin barked trees, such as ash, beech, birch, honey locust, linden, maple, oak, white pine, and willow tend to develop winter scald, if left unprotected.


Preventing winter sunscald

There are several ways you can prevent winter sunscald from damaging your trees. First, keep your trees properly irrigated. A water stressed tree is far more likely to develop winter sunscald. Also, avoid damaging the trunk with lawnmowers, car doors, and weedwackers. Damaged trunks have a harder time protecting themselves. You can prevent mechanical injury and stabilize soil temperatures with a thick layer of mulch around the tree. Just be sure to keep the mulch from touching the trunk, which would set the stage for fungal disease.


If watering and mulching aren’t enough, whitewashing the trunk and exposed surfaces of major branches can help prevent winter sunscald. It will protect against summer sunburn, too. Some people find the paint unattractive. Personally, I like it. Just don’t whitewash your trees with enamel paint. They will suffocate. Instead, combine 1 part water with 1 part white latex paint (preferably one without extra chemical additives) and use that to paint your trees.

Whitewashed nectarine tree (Kate Russell)

Traditionally, tree wrapping was recommended as a protection against winter sunscald, but we now know that those wraps can cause more harm than they prevent. Wraps allow moisture to collect, setting the stage for rot, and they provide an excellent hiding place for pests and fungal disease. Also, cracks that occur under the wraps tend to not heal as well. Bottom line: don’t wrap your trees.


If your trees tend to get winter sunscald, you can block or shade the southwest side of your trees to provide some protection against winter sunscald.


Winter sunscald care

Once damage has occurred, you can speed the healing process by smoothing the edge of the wound with a sharp, sterilized knife. Do not remove more than 1/2” of the bark. Also, resist the urge to paint tar or sealant over the wound. This usually traps moisture against the wound, increasing the chance of disease and decay.


Finally, you know that old adage about getting lost in the woods and looking for moss on the north side of trees? Well, that piece of advice is useless. Moss grows on the side of the tree with the most moisture and sun protection. What you can do, however, is look for winter sunscald, which nearly always occurs on the south or southwest sides of trees in the northern hemisphere.


Now you know.