Question: Why have some people painted their tree trunks white? – Shelly W., Las Cruces, N.M.
Answer: Have you ever noticed bark buckling off the tree trunk? I first noticed it in a mature mulberry tree in my yard in Las Cruces a few years ago. The bark on the west side of the trunk had buckled so much that huge pieces looked like they were about to fall off (see related photo). This phenomenon was likely caused when the tree was young by “southwest injury,” also known as “winter sun scald.”
Southwest injury is especially a problem in climates with intense sun exposure and extreme fluctuations between daytime and nighttime temperatures. In New Mexico, we are familiar with both. If my mulberry tree trunk had been painted in those early years when the bark was thinner and tenderer, the buckling southwest injury might not have happened.
Retired NMSU Extension Horticulture Specialist Dr. Curtis Smith explained southwest injury well in an archived column (Access the archives at http://aces.nmsu.edu/ces/yard/archives/): Painting the trunks of trees with a white latex (do not use oil-based paint) is a way to protect the trees from southwest injury. The phloem and xylem are vascular tissues responsible for carrying water and nutrients up from the roots (in the xylem) to support leaf development and growth and downward (in the phloem) to carry sugars and other material necessary for root growth and functioning.
The cambium is a layer of dividing cells (meristem cells) just inside the bark that produces new xylem and phloem. Injury or death of these tissues can seriously disrupt the growth and health of the tree. When you paint the tree trunk with white latex paint (diluted to half strength with water), you reduce the warming of the trunk during the day. White is used because it is not harmful to the tree and is effective at reflecting sunlight to moderate changes in the temperature of the trunk. Larger branches exposed to direct sunlight may also be painted on the sunward side to protect them. Smaller branches may more effectively radiate heat and often accumulate less heat from sunlight, reducing their susceptibility to this disorder.
The trees most susceptible to southwest injury are those with thin bark, including young trees and some species, such as honey locust and apple trees. In most species, as the bark becomes thicker and more cork-like, the trees develop their own protection.
Low branches on the south and southwest side of the tree help by shading the trunk. So leave them intact as long as possible. Shrubs or other things shading the trunk can help too. It is not necessary to remove the paint during the next growing season. 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.
Another option if you do not want bright white tree trunks in your landscape is a white trunk wrap that is loose enough that it allows air flow and does not dig into the bark. A clear or dark-colored wrap is not recommended. Do not forget to remove the wrap each spring.
Southwest injury did not kill my mulberry tree. It continues to provide shade in my old yard in Las Cruces, even with the badly buckling bark. But I know that the tree is not operating optimally and is more vulnerable to water stress, as well as insect and disease pressures, in comparison with healthy trees. For new tree plantings, consider all of the ways you can maximize performance from the start, including the selection of a more drought-tolerant species than mulberry.
Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at firstname.lastname@example.org, or at the Desert Blooms Facebook page (@NMDesertBlooms). Please copy your County Extension Agent and indicate your county of residence when you submit your question! For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page at http://aces.nmsu.edu/pubs/_h/