Protect Your Trunks: Sunscald Kills

Southwest Yard & Garden

(Follow-Up to Last Week’s “Transplanting Plum Trees” Column)

by Dr. Marisa Thompson

Besides being girdled and slowly killed by hardscape at the base, this ash tree in Belen looks normal on the northeast side of the trunk (left) and severely wounded on the southwest side (right) due to sunscald (aka southwest injury). Temporary trunk protection, like a loose-fitting paper wrap, could have prevented this damage when the tree was younger and the bark was thinner. Photo credits M. Thompson.

Just after last week’s column on transplanting 8-year-old plum trees was published, City of Las Cruces Community Forester Jimmy Zabriskie contacted me about another important consideration: sunscald. Zabriskie pointed out that care should be taken to be sure transplants are oriented in the same direction in their new spot as they were when they were originally planted. The concern here is that the southwest side of the trunk may have already been hardened and is better able to withstand afternoon sun during winter months. If a tree is inadvertently rotated, there could be a higher risk of getting winter sunscald (aka southwest injury) on that tender side. Zabriskie also notes that orientation should be considered when transplanting other ornamental plants like shrubs, cacti, and agaves.

I’m glad Zabriskie brought this up because I’m concerned that winter sunscald is a much bigger problem for our trees than we realize, and not just for new transplants. What’s more, it’s preventable with a few simple steps. 

Have you ever noticed bark buckling off the tree trunk? Or blisters on the southwest side of the trunk while the other side looks fine? Go outside and take a look for yourself. Sometimes the differences are shocking.

Another prime example of a crabapple trunk—this time in Albuquerque—that’s normal on the shaded northeast side (left), but the southwest side of the same tree (right) has been severely damaged by winter sunscald (aka southwest injury). Notice this tree is experiencing additional stress from being placed in a parking lot and having its roots covered with plastic and rock. Landscapers are pulling up the plastic to allow better air and water flow to the tree roots. Photo credits M. Thompson.

What happens, in short, is that bark on the southwest side of the trunk is exposed to afternoon sun, and the sap gets warm enough that it starts moving in the tree. Normally this is fine, except that on especially cold nights in winter the cells carrying sap rupture, causing damage that’s irreparable. This is particularly a concern on trees with thin bark—often younger trees, or species like honey locust and apple. 

Winter sunscald is especially a problem in climates with intense sun exposure and extreme fluctuations between daytime and nighttime temperatures, like we have in New Mexico.  

Painting the trunks of trees with white latex paint (do not use oil-based) is an easy way to protect the trees from southwest injury because the white surface reflects more sunlight and the trunk stays noticeably cooler. Dilute the white latex paint to half strength with water. It goes on easy and can stay on year-round. As the tree grows and as the paint is exposed to the environment, it will naturally fall away. It is not needed during the summer, but you may need to reapply it next winter.

Low branches on the south and southwest side of the tree help by shading the trunk, so leave these branches intact as long as possible. Shrubs or other things shading the trunk can help too.

Another option, if you do not want bright white tree trunks in your landscape, is a light-colored trunk wrap that is loose enough to allow airflow and does not dig into the bark (white paper or even newspaper will do). A clear or dark-colored wrap is not recommended. To be safe, loosely wrap your trunks when the trees go dormant in December, but do not forget to remove the wraps each spring. 

To access another column with more examples of whitewashing tree trunks for sunscald prevention, go to https://nmsudesertblooms.blogspot.com/ and search “injury.” While you’re there, search “plum” to find the original column on transplanting fruit trees.

For more gardening information, including decades of archived Southwest Yard & Garden columns, visit the NMSU Extension Horticulture page (http://desertblooms.nmsu.edu/), follow us on social media (@NMDesertBlooms), or contact your County Extension office (https://aces.nmsu.edu/county).

Marisa Thompson, PhD, is the Extension Horticulture Specialist for New Mexico State University and is based at the Agricultural Science Center at Los Lunas.

Edible New Mexico Magazine article crosslink -- GROWING YOUR FUTURE FAVORITE TREE