Sunscald on Trees – Prevention

It might surprise you to learn that trees do get sunscald or sunburn and the remedy is not an SPF 30 lotion. Sunscald on trees usually happens on the south or south-west exposed bark. The bark is damaged and in severe cases can result in the death of the tree.

Historically, sunscald has been prevented by wrapping or painting the trunk of the tree. Do these preventative methods work? Are they still recommended? Time to have a closer look at the problem.

Sunscald on Trees – What is it?

Sunscald has been studied for many years but the science is still not conclusive. To start with, there are really two types of sunscald; one happens in winter and the other happens in summer. The one that happens in summer is sometimes called sunburn or summer sunscald to distinguish it from winter damage.

Summer sunscald happens when the bark of trees gets too hot. This usually happens in warmer climates and it usually affects young trees or young branches of older trees. The tissue in the bark gets so hot that cells start to die. The comments in the rest of this post will not specifically deal with this kind of damage, but much of the discussion does also apply to summer sunscald.

Winter sunscald, which I will call sunscald, happens in colder climates during the winter. In colder climates the warm sun heats up the bark during the day, followed by a sudden drop to low temperatures at night. Rapid temperature changes cause damage to cells in the bark.

Both types of sunscald cause lesions or breaks in the bark. Over time these can open up into large damaged areas as shown above picture.

Most people talk about sunscald occurring on the lower trunk but it can also occur on the thin bark of one and two year old upper branches.

What Causes Sunscald?

It is clear that the sudden changes in temperature cause the damage but other factors also play an important role.

Certain species of trees including birch, maple, linden (basswood), boxelder, ash, balsam fir, Douglas fir, spruce, and eastern white pine are more susceptible to sunscald.

Young bark is more susceptible than older bark which is thicker and provides better insulation for the living inner cells.

The south side of trees have bark that is conditioned to strong sun. If these trees are then moved from the nursery to the landscape, and turned 180 degrees, they now have their sensitive north side facing south, making them sensitive to sunscald.

Factors Predisposing Trees to Sunscald

Bark thickness seems to be an important factor in determining if a tree will develop sunscald. The thicker bark found on certain species and on older stems and trunks are less likely to be damaged.

The other main contributing factor is water stress. Trees that are well hydrated during the winter months are less likely to get sunscald. This is one reason why it is critical to keep newly planted trees well watered.

Preventing Sunscald

A number of products have been used to prevent sunscald. They all work on the principle of keeping bark from experiencing extreme temperatures. Various wraps around the trunk, either paper or plastic, as well as white paint reflect sun light and therefore keep the bark from getting too hot. Wraps will also hold in some warmth at night but this will have a very limited effect on the low temperature reached.

Brown colored paper wraps actually absorb heat and can increase the temperature of the bark. Only white or silver products reduce temperature extremes.

Problems Caused by Tree Wraps

The above mentioned solutions do work to reduce sunscald, but they also cause some problems. When ISA members were asked if they had ever observed damage that they would attribute to the use of some type of trunk protective material, the vast majority said yes.

The following types of problems have been reported.

• cracks developing behind wraps do not heal as quickly
• excess moisture held behind wraps may foster fungal canker development and bacterial rot
• excess moisture may cause the bark to freeze easier causing damage
• wraps create a favorable environment for woodborers
• some latex paints have been reported to cause injury to certain maple species
• compression from the wraps may restrict water, nutrients and/or photosynthates from normal movement within the stem (ref 1)

It should be noted that many of these claims have limited scientific proof. In some cases they are anecdotal observations and in others cases the research only looked at one or two tree species. Nevertheless it should be a concern.

Physical Protection

One of the main benefits of tree guards is the physical protection from things like lawn mowers and weed trimmers. These can cause severe damage to trunks.

The best solution here is to use large mulched areas around the trees.

Tree guards also prevent damage from animals eating the bark or rubbing against it.

Should Tree Guards be Used?

Sunscald is a real problem and using the correct guards will reduce the risk of sunscald. However, it is unlikely that this benefit outweighs the potential problems it causes.

The International Society of Arboriculture (ISA) (ref 2) reports that “Studies using most common tree wraps have shown that they do not prevent extreme fluctuations in temperature on the bark which cause sunscald. In some cases, the temperature extremes are worse. And tree wraps have also proven quite ineffective in preventing insect entry. In fact, some insects like to burrow under it.”

The ISA does not recommend the use of tree wraps  to prevent sunscald.

Best Tree Guard Solution

One solution that probably works well for homeowners is to wrap the trunks with a cylinder of chicken wire which is then filled with leaves in the fall. The leaves should be removed in spring. This will slow down temperature fluctuations and keep animals away from the trunk. To my knowledge science has not studied this solution.

Painting trunks with white latex paint also seems to offer some protection without having the insect and fungal problems. Aesthetically it is not very pleasing.

Damage from machinery is best dealt with by keeping it away from the trunk. A large mulched area around the tree works well.

Plastic spiral wraps work well for homeowners reducing the chance of sunscald and preventing animal damage. These should be removed each spring to reduce the potential insect and fungal problems listed above. As the bark ages and gets harder the wrap can be left off.

All trees should receive adequate water in fall and winter. This may be the best way to reduce sunscald.

Mulch has been shown to reduce sunscald injury. All trees should be mulched with wood chips.

References:

1. Tree Stem Protection; http://www.myminnesotawoods.umn.edu/2009/12/tree-stem-protection/
2. Top 7 Myths of Tree Care Debunked; http://www.treesaregood.org/newsroom/resources/SevenMythsDebunked_Feb13.pdf
3. Mulch Affects Cold Injury; http://www.okstate.edu/ag/asnr/hortla/pdfs/smith/coldinjury.pdf