1 - Painting trunks of thin barked trees helps protect against southwest injury, a disorder due to our sunny days and cold nights.

Yard and Garden - November 19, 2011

Q.

Now that the cold weather has begun, I was thinking about painting the trunks of my trees with white paint. I hear that helps protect the trunks of trees during the winter. Can I paint the trunks of young trees also?

- Ron J.

Albuquerque

A.

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". Southwest injury results from the daily warming of the tree trunk and nightly freezing. The great variation in our day and night temperatures plays a major role in this disorder. It is called southwest injury because the damage often develops on the exposed southwestern side of the tree. This is where the greatest warming occurs on sunny days. This warmed area will often lose its winter hardiness early due to the warming of the trunk. The tree outer layers of the tree trunk (phloem and cambium layers) may then be injured or killed. This results in the bark splitting or separating from the trunk. The xylem layer is a little more protected since it is deeper inside the trunk, but it may also be injured. The phloem and xylem are vascular tissues responsible for carrying 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) 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, you reduce the warming of the trunk during the day. White is used because it is not harmful to the tree and 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. Some species, such as honey locust and apple trees, often have a thin bark. 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. Shrubs or other things shading the trunk can help. Young trees often have thin bark and will definitely benefit from white paint. 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.

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.

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