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, 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

Several chemical changes 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

Sunken areas created by winter sunscald usually 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 hiding out in the fissures created by sunscald.

Winter sunscald can also damage leaves. If skies have been overcast for some time, followed by a bright sunny day, 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. Going from protective shade to direct sunlight can damage the outer skin enough to allow insects and diseases to enter. In other cases, the fruit exterior may look normal, but the underlying tissue becomes damaged and is usually inedible.

Winter sunscald is rarely fatal, except to one- or two-year-old trees. But 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. Other thin-barked trees, such as ash, beech, birch, maple, and willow, often develop winter scald if unprotected.

Preventing winter sunscald

There are several ways you can prevent winter sunscald from damaging your trees. First, irrigate your trees properly. A water-stressed tree is far more likely to develop winter sunscald. Also, avoid damaging the trunk with lawnmowers, car doors, and weedwackers. Damaged tree trunks have a hard time protecting themselves. You can prevent mechanical injury and stabilize soil temperatures with a thick layer of mulch around the tree. Just keep the mulch from touching the trunk, which can 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. Just don’t whitewash your trees with enamel paint. They will suffocate. Instead, combine equal parts water and white latex paint (preferably one without extra chemical additives) and use that to paint your trees.

Whitewashed nectarine tree (Kate Russell)

Traditionally, people wrapped their trees to prevent winter sunscald, but now we know that those wraps can harm trees. Tree wraps allow moisture to collect, setting the stage for rot, and they provide an excellent hiding place for pests and fungal diseases. Also, cracks that occur under the wraps do 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 the damage has occurred, you can speed up 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. Sealants trap moisture in the wound, increasing the chance of disease and decay.

Finally, you know that 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.