Sunscald On Trees – Guide to Symptoms, Causes, & Treatment

When planting young trees in your landscape one should take care to prevent Sunscald. Damaging to thin bark of young trees or new growth, it occurs from extreme temperatures and can be damaging or even fatal in large patches. This is because it weakens the tree and makes it more susceptible to insect or disease.

What is Sunscald?

In trees, sunscald is the death of cambium bark tissue due to extreme temperatures. Primarily occurring on lower sections of South or Western facing trunks that are unshaded in Winter, it is caused by abrupt freeze/thaw cycles. Protection of young trees and not over-pruning of older trees prevent the condition. [1]

Sunscald on trees is also sometimes referred to as sunburn, if the damage is primarily caused by higher temperatures in the Summer.

In vegetables, sunscald is characterized by white spots surrounded by a yellow halo caused by a combination of heat and solar radiation. It is more likely to occur on detached fruits of a green color. Even in the same solar conditions, attached fruits were of lower temperature than detached fruits (tomato, pepper, cucumber). [2]

Sunscald symptoms

The early or initial symptoms of Sunscald are not always obvious. You may notice some simple discoloration in spots on the trunk, or small cankers. Later stages will be more obvious, as there will be a vertical wound running lengthwise on an exposed South or Western face of the trunk.

The wounds may ‘bleed’ early in Spring as the damage works to heal itself. Eventually the bark around the wounds will fall off, leaving a vertical scar and showing the wound closure. You should not ‘help’ exfoliate the bark, as if you inadvertently remove healthy bark you may open a pathway for disease to enter the tree.

The causes of sunscald

Sunscald in Summer

Summer sunscald (sunburn) is caused by extreme heat on thin bark of young trees that don’t have a fully developed canopy. This can also be caused by over-pruning when too much foliage is removed. And, if a sapling is grown in shady conditions , but then transplanted to a full sun environment, sunscald may occur from extreme heat/solar radiation.

Another contributing factor so sunscald in Summer on recently transplanted trees is that the roots don’t have time to establish, which reduces the amount of moisture a tree is able to absorb/transport throughout it’s trunk and leaves. This moisture can help cool the tree, and it is therefore more important to either transplant trees in the Fall (dormant season), or water regularly if transplanted in the Spring/Summer.

Sunscald in Winter

Winter Sunscald (also known as frost cracking) is caused by rapid freeze/thaw cycles and occurs on exposed thin-bark trunks and new growth stems of younger trees. When these layers of thin bark are warmed by the sun, they can experience rapid freezing when the sun goes behind a cloud or the sun sets, inflicting the damage. [3]

For instance, in Minnesota the coldest days of Winter are often clear and sunny. Research carried out found that rapid freeze/thaw cycles occur quite frequently on days when the sun frequently dips behind clouds, only to reappear. Amazingly, it was observed that should the sun go behind a cloud on cold and windy days the temperature of the cambium bark layer can fall to that of the air temperature “within 2 or 3 minutes”. [3] It was further observed that “every cloud obscuring the sun will cause the bark to freeze in a few minutes if the air temperature is sufficiently low”. Thus frequent and rapid freeze/thaw cycles can be quite damaging to tender bark of young trees.

It has been observed via thermal couple on thin copper wire inserted into bark temperature differences of 25C between the sunny and shady sides of a trunk. Furthermore, if the sun dips behind a cloud the temperature change can be 5-10C in as little as 10 minutes. This rapid change in temperature, at certain conditions can cause a rapid freeze/thaw cycle that can cause physical damage to the bark tissue.

Other contributing factors

Older trees, or older growth of trees are not as susceptible to sunscald as their bark is thicker. Thicker bark has higher heat capacity and ability to insulate itself. Furthermore, with a more developed canopy, the tree would naturally shade itself in the wild. But – if a forest is partially cleared, trunks or mature trees may be exposed to sunlight. Depending on the species of tree, sunscald may occur as the trunks were not used to direct sunlight.

In addition to young trees, over pruning can expose too much of the tree to direct sunlight, which can be particularly damaging to young growth. As younger branches will still have thin bark.

What trees are susceptible to sunscald?

Evergreen trees are more resistant to sunscald, as their year-round foliage can shade the thin bark on young trees. Deciduous trees, losing their leaves in the Winter are more prone to damage. Examples of deciduous trees known to frequently have sunscald damage include Maple, Ash, Birch, Honeylocust, Walnut, Apple, Crabapple, Cherry, Tuliptree, and Willow.

Long term effects

The minimum effects of sunscald will be cosmetic damage to the tree. In more severe cases the inner tissue can become exposed, possibly leading to a weak point as there is no bark (bark is very strong in tension). While in the more extreme cases the trunk can be weakened leading to breaking of the tree in storms, or death of the tree.

The prescence of sunscald damage also makes the tree more suceptible to fungal diseases, that can eventually kill the tree. Furthermore, exposure of inner tissue of many trees can attract disease carrying insects that are attracted to the odor.[4] These insects are frequently vectors of disease. Examples would include Dutch Elm disease and Oak Wilt.

Treatment of sunscald

There isn’t much in the way of treatment for already damaged/dead bark tissue. Hopefully the tree is able to seal it’s own wound over the coming years.

But should you notice sunscald damage, you should peel away any dead or loose bark (taking care to avoid removing healthy bark). Doing so will eliminate hiding places for insects as well as improve the appearance of the wound. Do not apply any pruning sealer, as the effects may be worse as it can trap spores/diesease.

In addition to the above actions, you should take care to keep the tree as healthy as possible. Apply a top-dressing of compost around the dripline/under canopy in Fall/Spring. And make sure the tree meets all of it’s watering needs. Healthy trees do not succumb to disease/damage as easily as stressed trees.

Prevention

The best way to deal with sunscald is to prevent it. To prevent sunscald, there are several strategies.

• Applying wraps to trunks
• Painting trunks
• Planting trees in Fall
• Mulch/Moisture

Wrapping trunks to prevent sunscald

For young saplings being planted in an open area, one should wrap the trunk in kraft paper or some other light-colored coating. This will help insulate and reflect direct sunlight, aiding the bark in maintaining a lower temperature or temperature fluctuation. In Winter, this helps prevent rapid freeze/thaw cycles, while in Summer, a loose fitting wrap can prevent extreme temperatures.

Once the canopy of the tree is developed enough to shade the trunk, the wrap is no longer needed.

One important note – make sure the ‘wrap’ during the summer months is loose-fitting and allows airflow. Also, periodically inspect the wrap to make sure that there is no infestation of insects under the wrap.

Do not wrap the trunk in dark colored materials, as they will absorb more sunlight. This could lead to more severe sunscald, as the increased temperature of darker materials may be more damaging than no covering at all.

Painting trunks white

One can paint the lower part of tree trunks white to help regulate the temperature. Eggert found that when painted white, the lower section of tree trunks in fruit orchards stayed within 10F of the air temperature. [5] This has been done in fruit orchards for decades. Although it isn’t always practical as many people wouldn’t want trees with white trunks in their yard.

Planting during dormant season

Autumn is the best time to plant both trees and perennials for many reasons, but one of the best is to avoid heat stress. When planted in cooler temperatures, the roots of tree saplings have time to adjust and grow without significant heat loads. Then come Spring, the roots will be established and the moisture uptake of the tree will be greatly improved.

Mulch/Moisture

Furthermore, ensuring that young trees are adequately watered the first year after transplanting may be a key factor to helping prevent, or lessen the severity of sunscald on certain ornamental trees. [6] Research found that a significantly lower proportion of adequately watered trees during their first growing season developed sunscald, in comparison to non-watered transplants.

Also, applying a coarse mulch around the base of the tree (keep a 2″ gap around the trunk) can help the soil retain moisture.

Read more about Native Trees here