How to Fight Winter Sunscald in Trees

By Erin Marissa Russell

Do you know how to protect your trees from winter sunscald, sometimes called “southwest injury”? Without treatment, sunscald damage can be severe enough to result in serious damage and even death of the tree.

Luckily, there are things you can do to reduce the likelihood of damage to your thin-barked trees and even to reverse damage when it has already occurred. But first, you have to be able to identify winter sunscald, so you’ll need to know what to look for.

We’ll tell you how to diagnose winter sunscald and how to tell it apart from the other types of damage your trees may sustain. Then we’ll guide you through the ways you can defend your trees against sunscald and methods for repairing sunscald damage where it exists.

Identifying Winter Sunscald in Trees

Sunscald happens in the beginning of spring and is a result of the temperature veering widely between highs and lows. In early spring, the temperature during the day can be hot enough to warm the tree’s bark, but then it may drop to freezing overnight. When this happens, sensitive cells in the tree called cambial cells may not be able to withstand the freeze and damage can occur.

There are various signs and symptoms of sunscald damage, which are listed below.

Sunscald damage usually appears on the south or west side of a tree trunk. (That’s why it is sometimes called “southwest injury”.)

Hardwood trees with thin bark are most at risk for sunscald damage.

Sunscald can cause vertical cracks in tree bark, which allow pathogens to enter the tree and also begin to decay, weakening the tree’s defenses and ultimately causing further damage.

When sunscald damage is mild, the bark of affected trees can become discolored and appear red or brown.

More severely damaged trees can exhibit symptoms such as sunken areas or peeling bark that reveals the wood inside the trunk, making it susceptible to other types of damage.

Trees that have sustained damage from winter sunscald may also be infested with the larvae of wood-boring beetles.

Young trees are more at risk of sunscald damage than more mature trees because their bark is thinner. To evaluate whether a tree has sunscald damage, compare it to healthy specimens that are the same type of tree and, if possible, close to the same age.

Different types of cracks

Not all of the cracks that develop in tree bark are due to sun damage. If the crack shows up after a cold spell, and especially if it reappears whenever temperatures are low, it may be a frost crack. Frost cracks will close once the weather warms up and are most common on the south and west sides of trees.

But growth cracks are also possible, and these types of cracks do not indicate any kind of damage. Growth cracks are most likely to show up when the tree has been growing a lot and there has been plenty of rain. They are also temporary and will be covered with bark as the tree continues its growth. Growth cracks do not ooze, will not lead to decay, and do not expose the heartwood at the center of the tree.

Preventing Winter Sunscald in Trees

It is always better to take preventative measures to prevent a problem from happening to begin with, than it is to treat a problem once it occurs.

Use the following tips to prevent occurrences of winter sunscald in trees.

Whenever possible, remove reflective surfaces that can exacerbate sunscald from the environment. These can include light-colored fences, buildings, or walls near the tree that can reflect light and heat back toward the tree’s bark.

If you’ve purchased a new tree from the nursery, take extra care to protect it from sunscald as it adjusts to the new environment. One of the best ways to do this is by hardening off the tree so a gradual introduction can ease the transition to its new conditions. You can learn about this process in our article Hardening Off Plants: Common Reader Questions and Answers.

Stress due to lack of sufficient water can make it more likely that a tree will be damaged by sunscald. To reduce the risk of sunscald damage in your trees, make sure that they get adequate hydration, whether it comes from you as the gardener or is provided by nature as rainfall.

If you know the tree you are planting is susceptible to sunscald, consider choosing a spot for it on the north or east side of buildings or evergreen trees on your property. This way the bottom part of your new tree will get some shade in the winter. You may consider adding evergreens or a shade structure near susceptible trees that are already established in your yard as well.

Sometimes trees develop frost cracks in locations that were previously damaged by a mechanical injury or other wounds to the tree bark. Gardeners may be able to reduce a tree’s risk of developing frost cracks by being careful not to injure trees while they work in the garden.

Under natural conditions, trees are less likely to be damaged by sunscald than when they have been cared for by a gardener. This happens because, in the wild, the lower branches of a tree are left undisturbed and will help shade the tree’s bark from the sun. Often, gardeners prune to remove these lower branches from the trees in their gardens for aesthetic reasons.

Let your trees keep their natural defenses against sunscald damage. Don’t remove the lower branches when you prune young trees, and look for saplings that still have their low branches when you purchase new trees. For the first few years after adding a tree to your garden, be careful not to over-prune so that your trees can keep the foliage they grow to shade the lower portion of their trunks. On species that are susceptible to sunscald, the best approach is to prune them gradually over a period of years if dramatic visual effects are desired.

Apply a layer of mulch to the ground where trees are growing. Spread four to six inches of coarse organic mulch, but make sure to leave a space around each individual tree so that their bark does not touch the mulch. Adding mulch to the area around your trees will put a stop to the damage from light and heat reflected off the ground, and mulching also helps keep water in the soil where it’s accessible to the trees when they need it.

Surround trees with smaller companion plants that can offer some shade to the parts of the tree the canopy does not protect from the sun. Shrubs or small trees should especially be used to block the sun from the south side of trees you wish to protect.

Treating Winter Sunscald Damage in Trees

Preventative measures are not always completely effective when it comes to winter sunscald. Luckily, there are a few ways in which winter sunscald can be treated if it is affecting your trees. The following methods have been successful at treating winter sunscald in trees.

The tree wraps that are sometimes recommended to protect trees can provide a hiding place for insects. Whenever possible, choose an alternative way to shade trees from the sun. Tree wraps are best used on young trees with thin bark for the first two to three years after planting in your garden. Apply tree wraps in November. They should be taken off in the spring to prevent fungal problems due to trapped moisture.

Use white paint designed to prevent sun damage on vulnerable trees to protect their trunks and branches. You may have heard suggestions to use interior latex paint diluted to half strength, but this paint is no longer recommended because of new paint formulas that do not allow sufficient air exchange to be healthy for the tree.

Look for a paint designed specifically to protect trees against sunscald. These options will be marked with language like “tree paint,” “plant guard,” or “for whitewashing trees.”

However, make sure to choose one of these and not “tree marking paint,” which is not designed to protect trees from sun damage. “Tree marking paint” is aerosol paint (spray paint) for people who need to make marks on trees that are easy to see and will last a long time. Paint that is not designed for use on trees can be harmful either due to restricting the tree’s air intake or because it contains chemicals that have negative effects on trees.

You can add a shade structure like a fence or small wall to shade the south or west side of susceptible trees in your garden. Use light-colored boards when building shade structures, and make sure that fences for this purpose are opaque.

Winter sunscald occurs when the bark of a tree experiences extreme temperature fluctuations, which damages the bark of trees. The bark heats up in the direct sunlight during a warm winter day and then, when the sun goes down and the temperatures quickly drop for an overnight freeze, the bark of the tree is injured from how quickly the environment changed due to the temperature shift. Unlike summer sunscald, winter sunscald occurs in many climates, not just in warm climate environments. Luckily, there has been plenty of research dedicated to preventing and treating winter sunscald, so there are multiple ways to effectively handle the condition whenever it arises.