Q. I planted three redbud trees (variety Merlot) in August 2022 when we moved into our new house. Now, one of the three appears to be dead, with the bark split in several places along the length of the trunk. A scratch test does not show any green underneath the bark of the branches. I plan to replace the plant, but what do you think is going on? — Steve Burch, Raleigh, North Carolina
A. The issue you describe suggests a classic case of “southwest injury,” also known as sun scald. Several thin-bark species, such as your redbud — as well as maple, ash, apple, crabapple and others — can be susceptible to this issue. The damage occurs during the winter, on the south and southwest side of the tree trunk, where it is exposed to direct sunlight during the course of the day. The temperature differences between the exposed and shaded sides of the trunk can be significant as the sun moves.
The higher temperatures activate dormant cells, which, being active, are then damaged by the cold when temperatures drop to near freezing at night. A rapid change in temperature causes a crack in the bark. The living cells just under the bark are irreparably damaged and killed. In some cases, depending on the extent of the damage, plants can compartmentalize the wound and survive. In other cases, the plant may continue a slow decline and eventually die.
Note that this issue is not associated with extreme cold but with the sudden drop in temperatures.
The vulnerability of some species more than others results from anatomical differences in stems. Thin-barked species lack the abundance of cork cells found in their thicker-barked counterparts; these cells insulate their vascular tissues (the phloem and cambium) from the outer bark, where the rapid temperatures change occurs. The cork and bark cells are non-living, whereas the phloem and cambium are alive, and are highly sensitive to cold. In thick-barked species, these sensitive tissues are further distanced from the damaging temperatures, with a greater lag in the freeze-thaw cycle.
The wounds disrupt the movement of stored reserves and the formation of new tissues at the site. Damage usually becomes evident in late winter to early spring.
While southwest injury is more problematic in colder regions of the country, it can occur in milder areas as well. Plant stressors such as drought, herbicide injury, insects and disease can predispose plants to bark cracking, and hinder their recovery.
Young, recently planted trees that have not yet developed a thick, corky insulating bark are more at risk. Fortunately, as they mature and their caliper measurement rises to more than 4 inches, the risk of southwest injury diminishes.
What, if anything, can be done to help protect trees from this issue?
- First and foremost, ensure good tree health to promote rapid establishment and development of newly planted trees: Healthy trees have the best chance of resisting and recovering from southwest injury. Good health begins with keeping trees properly hydrated.
- Watch for signs of disease and insect damage.
- Use white tree guards to deflect sunlight and prevent overheating of the trunk. Place these in late fall and remove after the last hard frost (do not leave them on year-round). Guards should fit loosely and have slits for good air movement. Be sure to check them periodically throughout the winter.
- Recognize that certain species are susceptible, and avoid using them in vulnerable situations. Or you can use them but take greater care — for example, by planting them on the north or east side of buildings or evergreen plants, which would shield lower trunks from the southwest sun. You could also add evergreens to the south and west of your newly planted trees.
And one more thing (or two)
We should be in the clear now with cold weather. So again, I’ll share the Virginia Cooperative Extension’s Home Garden Vegetable Planting Guide) for recommended garden planting and harvesting dates. It’s an automatic PDF download at bit.ly/3J21mpu. This will help you get that garden going.
After writing this column for almost eight years, I’ve decided that today’s will be my last. I’d like to thank all of you who have written in with your questions and experiences. And also, many thanks to my colleagues at the paper, whose behind-the-scenes efforts have made this column possible. Keep on growing, everyone.
Editors’ note: We will miss Ken dearly.
Reach Ken Spencer at wkspen@gmail.com
