Repairing damaged tree bark

Our dog ate most of the bark off the base of our apple tree the other day.� Will this kill the tree?� Can I save it?

Answer:

When a tree has been damaged by removing a ring of bark, the tree may die depending on how completely it was girdled. Removal of even a vertical strip of bark less than one-fourth the circumference of the tree will harm the tree, but not kill the tree. When the patch of bark is one-half or greater, the chances of tree death increase. Complete girdling (the bark removed from a band completely encircling the tree) will certainly kill the tree.

The reason for damage due to girdling is that the phloem layer of tissue just below the bark is responsible for carrying food produced in the leaves by photosynthesis to the roots. Without this food, the roots ultimately die and cease sending water and minerals to the leaves. Then the leaves die. As you can see from this process, there is a delay period before the roots and top dies. There are some stored foods in the roots and lower trunk that allow the roots to continue functioning for a little while. This delay gives you time in which you can try "repair grafting". Don't wait any longer because the roots will soon run out of food.

Repair grafting, also known as bridge grafting, provides a bridge across the damaged area. This will partially restore some transport of foods to the roots. If this bridge can carry enough food across the wound, the roots will survive and continue sending water and minerals through deeper tissues to the leaves. The leaves will then manufacture food that permits the tree to develop new tissues to close over the wound and restore normal plant processes.

To bridge the graft, first clean the wound by removing sharp edges and any bark that is pulled loose from the trunk. Then remove some healthy branches or twigs from the same tree. These should be about thumb size in diameter (or smaller if the tree is small) and one to three inches longer than the width of the wound on the trunk. Trim one side of each end to flatten it so it will lie flat against the trunk of the tree. Cut the other side of each end to form a wedge shape. Then cut flaps into the bark on the trunk by making two parallel cuts through the bark, starting from the wound. Make this cut a little longer than the bridges you have prepared. Do not cut the between these two parallel cuts (leave the flap attached at the end away from the wound). Carefully lift the flap and insert the bridge under the flap. The bark on the bridge should extend slightly under this flap (no cleaned wood exposed). At the edges of the trimmed bark of the bridge and under the flap of the trunk are thin layers of phloem and cambium. If these layers of the trunk and the bridge successfully fuse together, creating the graft union, the flow of food to the roots will be reestablished and the tree may survive.

An important point to understand is that the flow of material in the phloem is only downward. That means, don't put the bridge piece into the graft upside down. Mark the top part of the bridge before you cut it from the tree so that you will not become confused when you form the graft.

This is not the best time of year to do this, but to delay will greatly reduce the tree's chances of survival. This technique also works for vehicle damage to trees and damage by rabbits. When rabbits eat the bark in the winter, you can wait until early spring to perform the repair graft.

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.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

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