Kevin Schoessow, University of Wisconsin Extension Area Agriculture Development Educator for Burnett, Sawyer and Washburn Counties has been getting many questions about rodent damage. Here's his advice.
Question: Because the snow was so deep this year, rodents were able to chew the bark from the trunks of our fruit trees. Is there any way the trees can survive this amount of damage?
Response: Rest assured you are not the only person with this frustrating problem. If the girdling is more than 2/3 of the way around the tree trunk, there is little chance the tree will survive if nothing is done. This rodent damage removes the cambium tissue (new bark) layer which is responsible for moving nutrients and water from the roots to the buds. With this disruption in the trees ‘plumbing’ there is no way to keep buds and branches alive. Where there is partial girdling, there is a chance that part of the tree will remain alive, but the tree parts above girdled area will eventually die.
About the only hope to save or replace the missing bark and re-plumb the tree is to do a tree grafting technique called the bridge graft. This involves harvesting a one-two year old scion twig from the same tree or another apple tree and using this twig as a means to transfer nutrients and water. The twig must be long enough to bridge the gap from the base of the tree to above the girdled area. The twig is cut to a point on both ends and then inserted under the new bark at the base of the tree and above the wound. Several twigs are used, with a twig placed every inch and a half or so. This scion twig is then nailed into position with a small finish nail, then sealed with wax or pruning seal and perhaps some tape as well. The wounded area is left as is as there is no need to put a wound sealer over that. If the graft union is successful nutrients and water flow through the grafted scion twigs and feed the upper portions of the tree.
Here is an article with some good pictures showing this process.
Right now is the perfect time to do this! While twigs are still dormant.
Here are additional resources
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