Protect trunks and branches from winter sunscald
Question: What do you recommend for winter care of fruit trees?
Bette A., Albuquerque
Answer: Last week, we addressed this question by learning about cold-hardiness and how to avoid drought stress in the dormant season by irrigating properly and using mulch. Another major way to support your fruit trees during winter is to protect the trunks and branches from winter sunscald (aka southwest injury). I’ve written about winter sunscald for this column before, but the issue is such a common problem in New Mexico, it deserves more attention — especially for newly transplanted and younger fruit trees.
During big shifts between daytime and nighttime sub-freezing temperatures, like we experience each winter all over New Mexico, the plant cells just under the bark warm up on the southwest side of the trunk during a warm, sunny afternoon and then at night, if temperatures drop below freezing rapidly, those same cells die, causing the death of the tree in that area of bark.
The cambium is a layer of dividing cells just inside the bark that produces new vascular tissues with each growing season. Injury or death of these tissues can seriously disrupt the growth and health of the tree.
Phloem and xylem are a tree’s vascular tissues; the xylem carries water and nutrients up from the roots to support leaf development and growth, while the phloem carries sugars and other material down to the roots for root growth and functioning.
The trees most susceptible to winter sunscald are those with thin bark, including young trees and species with darker bark, like cherries. Peaches, apples, maples, honeylocusts and ash trees are other common victims. In most species, as the bark becomes thicker and more cork-like over time, it provides insulation and protection.
Symptoms tend to be restricted to the southwest side of the trunk (and larger branches) and include bark discoloration, blistering or callusing, buckling and splitting.
If you suspect winter sunscald, here are two diagnostic tips: 1) Walk around the tree trunk to check if the symptoms are worse on that southwest side compared to the north or east sides of the trunk. If not, it could be a symptom of mechanical damage, like being hit by a car or material tied on the tree that has grown into the bark — and some trees just have naturally shaggy or bumpy bark all around the trunk. 2) Confirm that the tree received direct sun in late afternoons. Trees planted on the east side of a tall structure are shaded in winter afternoons, so sunscald doesn’t become an issue.
Once the cells die, nothing can be done to repair the damage. And while winter sunscald isn’t likely to kill a tree outright, it sure doesn’t help. The wounded area is more prone to infection and borers, but, again, no treatment is recommended, aside from prevention.
During the dormant season, keep the tree trunk and larger branches protected with a 1:1 mixture of white latex paint and water to reflect more sunlight and therefore reduce the warming of the trunk during the day. It is not necessary to remove the paint. As the tree grows and as the paint is exposed to the environment, it will naturally fade away. A fresh coat of diluted white latex paint may be needed next winter.
If you do not want bright white tree trunks in your landscape, you can dilute the paint further so that it’s thinner, but the light reflection will not be as strong as a bright white. Another option is a white trunk wrap that is loose enough that it allows airflow and does not dig into the bark. Some nurseries carry a tree wrap product made specifically for this purpose. Do not use a clear or dark-colored wrap, and do not forget to remove the wrap each spring.
Conifers with exposed bark can also be affected by winter sunscald. Whether it’s a deciduous or evergreen tree, low branches help by shading the trunk from the afternoon sun, so leave them intact as long as possible. Shrubs or other things shading that side of the trunk can help too. Protecting young and newly transplanted trees from winter sunscald is an important step toward successful establishment and longevity.
For more gardening information, visit the NMSU Extension Urban Horticulture page at http://desertblooms.nmsu.edu/ and the NMSU Horticulture Publications page at http://aces.nmsu.edu/pubs/_h/. Find your local Cooperative Extension Office at https://aces.nmsu.edu/county/.
Marisa Y. Thompson, Ph.D., is the Extension Urban Horticulture Specialist in the Department of Extension Plant Sciences and is based at the New Mexico State University Agricultural Science Center at Los Lunas.
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