Why do white trunks protect trees in Idaho?

PRESTON — Many young trees, and even some older ones, are susceptible to winter sun injury. This is referred to as southwest winter injury or simply sun scald. It is caused by repeated freezing and thawing of the bark on the southwest side of the trunk. The sun warms the bark during the day, causing sap to flow. As temperatures drop below freezing at night, sap inside the tree freezes, causing cells to rupture.

The bark and vascular tissue just under the bark becomes damaged and portions die. This damaged section of the tree becomes attractive to wood-boring insects and creates an opening for disease and decay to enter the tree.

To avoid this, the trunks of thin-barked trees can be protected by wrapping them with white tree wrap or by simply painting them white with a water-based paint. Interior latex paint mixed at a 1:1 ratio with water is most often recommended. Tree wrap is available at many nurseries, garden centers and farm stores. Both help reflect the winter sun from the trunk surface, allowing the trunk tissue to stay dormant. Wrap or paint the trunk from the base of the tree to the lowest branches. Tree wrap should be removed in the spring to avoid retaining moisture against the trunk and harboring insects.

My method of preference is to paint trunks — mostly because I’m cheap and lazy. I don’t want to wrap and unwrap my trees each fall and spring. Homeowners are often hesitant to paint trunks on ornamental trees for aesthetic reasons. I suggest a white trunk on a young tree is a petite price to pay for a healthy, happy tree in the future. As trees grow and bark thickens, the need for reflective protection decreases, and the white paint degrades and fades away.

In recent years, benefits beyond sun scald prevention have been seen with trunk painting. Research has shown that a painted trunk deters wood-boring insects. This benefit was increased with the addition of sheetrock joint compound (1/3 each of latex paint, water and joint compound).

Another side effect has been a reduction of rodent chewing damage. Apparently rabbits, mice, and voles prefer plain bark to that coated in paint. An additional hack is adding sand or another grit to the paint as added deterrent to gnawing rodents.

Whatever your personal preferences, sun protection is a good move for young and thin-barked trees. If your tree has already been damaged through southwest winter injury, removing the dead bark and providing protection now will help heal the wound.