Winter in the garden can be a very beautiful sight indeed, but it certainly can have many downsides, at least from a plant’s perspective. One of these down-sides is sun scald injury to deciduous tree trunks (commonly known as Southwest injury). What is Southwest Injury? Which trees are most susceptible? What can be done to prevent Southwest injury?

What is Southwest Injury?       

Southwest injury also known as sun scald injury is a condition where the sun or strongly reflected sunlight, warms the area just beneath the outer bark of a tree (the phloem and cambial area). As this area warms the living cells within become active, but when the temperature of this area suddenly drops (as when the sun goes behind clouds or sets for the day) the sharp temperature drop can kill these newly activated cells. The resulting damage appears as discolouration and vertical cracks. The damage is usually located on the south or southwestern side of the trunk. As the crack enlarges insects and disease may move in to worsen the problem. If the damage is not too severe the tree may callous over the wound on its own. Care should be taken at this point to prevent further injury.

Which trees are most susceptible?

• Young deciduous trees are highly susceptible as their bark is very thin and tender. These young trees are usually grown in a sheltered position at a nursery and need time to acclimatize to their new home and to develop mature bark that will serve to protect them from the winter sun.
• Thin barked deciduous trees like maples, cherries, mountain-ash, apple, peach, tulip trees, ash, willows, birch and lindens are highly susceptible to sun scald. They may require wrapping for several years until their bark begins to thicken.
• Cherry, peach and plum trees with their thin dark bark are some of the most sensitive trees. They may need to be wrapped their entire life span.

What can be done to prevent Southwest injury?

For city gardeners wrapping the tree trunk with a commercial tree wrap (available at most nurseries) is probably the most effective way to protect your sensitive trees from sunscald damage. The wrap is easy to apply, affordable, and if applied correctly, is quite effective. Basically, how the wrap works is by reflecting the light away from the trunk of the tree, thus preventing the sun from warming the tissues to the point where they can become damaged, when the temperatures suddenly drop. Tree wrap is usually applied just before hard freezes, approximately the beginning of Nov. and are removed when the chance of hard freezes has past, approximately the end of April. Removal is important as the wrap can hold in moisture and created a haven for insects and disease. Newly planted trees should be wrapped for the first couple of years until they have a chance to develop thicker bark. Similarly thin barked trees should be wrapped for several years. To apply start at the base of the trunk and spiral the wrap up the trunk to the first main branch or just beyond. To fasten the ends, use a piece of electrical tape (or other waterproof tape), or pin into place (pinning wrap to wrap, not pinning to the tree. This can be a bit tough on the fingers but a thimble can be quite helpful).

Other tips for preventing Southwest injury include:

• Not planting highly susceptible trees in unprotected southwestern exposures.
• Plant susceptible trees on the eastern or northern side of buildings and tall ever greens. These structures will help to shade the tree’s bark and prevent large temperature fluctuations.
• Make sure your trees are well watered prior to freeze up. Drought stressed trees tend to be more susceptible to Southwest injury.
• Do not locate susceptible trees near large light coloured, reflective surfaces. Although sun scald typically happens on the Southwest side of a tree it can happen on other sides if the light being reflected towards the tree trunk is intense enough, as with a south or west facing white fence or building structure.
• Sunscald can also occur during the summer months. When this occurs, it is usually the result of a large branch being pruned away or broken off during a windstorm. The sudden removal of this branch exposes bark and neighbouring branches to intense sunlight for which it is not accustomed to.

Photo Credits: all photos have been taken by the author.

Resources:

‘Sunscald Injury or Southwest Winter Injury on Deciduous Trees” by Katie Wagner, Extension Assistant Professor and Michael Kuhns, Extension Forestry Specialist: http://extension.usu.edu/files/publications/factsheet/NR_FF_021pr.pdf

C.E. Swift, W.R. Jacobi, M. Schomaker and D.A. Leatherman*Revised by E. Hammond** Colorado State University Extension, Fact Sheet 2.932, ‘Environmental Disorders of Woody Plants’, https://extension.colostate.edu/docs/pubs/garden/02932.pdf

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