February 6, 2016

1 - Painting a tree trunk white may help reduce the incidence of southwest injury causing splitting of the bark.

Yard and Garden February 6, 2016

Q.

I need some advice on splitting bark on my Pecan trees. Trunks are estimated 14" diameter, some are splitting 4 and 5 feet in vertical length right up the side of the tree. I can see the "tree" underneath which is drying in spots. I was told this is sunburn and should be painted white. I was also told that pecan trees are tough and not to worry about it.

Gary G.

Roswell, NM

A.

The bark of any tree may split as a result of what is often called "southwest injury". Trees with thin bark are much more likely to suffer this injury. It is called southwest injury because the injury that results in splitting of the bark occurs most often on the southwest side of the tree trunk. This injury is due to the warming of the trunk during the day (the greatest warming occurs on the southwest side of the trunk) and then refreezing during the night. The daytime warming causes the loss of cold hardiness in the living tissues of the tree trunk. These tissues are then more subject to damage by freezing at night. By painting the trunk white as you were advised can help keep the trunk cooler during the daytime. This helps reduce the physiological changes that cause the southwest injury. You can use white wash, or latex (water-based) white paint, to accomplish this.

Another strategy that may help is to prune the tree in such a manner that branches above the south and southwest portions of the trunk provide shade on the trunk during the winter months. Even though the leaves have fallen from these branches, the branches and twigs themselves can provide enough shade to prevent excessive warming of the trunk and loss of cold hardiness by the critical tree tissues under the bark.

Proper irrigation during dry winters may also help, but because of the precipitation this winter, lack of soil moisture should not be a consideration.

The bark on the north side of a tree may also split. This is often due to freezing of water under the bark. As water on the north side freezes additional water is drawn to that location. Ice continues to accumulate on the north side of the tree until the mass of ice is enough to split the bark. This is most likely to happen on the north side and in geographic locations where temperatures are very low and remain low for prolonged periods of time. When the bark splits under these conditions, it may sound like a rifle shot. However, such spitting on the north side of a tree should not be common in most parts of New Mexico.

You should contact your local NMSU County Extension agent, Sandra Barraza, to confirm that the problem is actually southwest injury. If there are other factors involved, she can contact Dr. Richard Heerema, NMSU's Extension Pecan specialist who will provide more information.

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.

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!