Prepping Fruit Trees for Winter: Part II

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.

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 winter sunscald (aka southwest injury). Painting the trunk white with a 1:1 mixture of white latex paint and water or a 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

The Problem

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, honey locusts, 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.

Diagnosis

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.

Group of Land & Water Summit attendees on a field trip in a South Albuquerque park in February 2018. Poor tree in the foreground has classic winter sunscald on the southwest side of the trunk. Photo credit Marisa Thompson

Treatment

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.

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.

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

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

Links:

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook page.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!