Answer: You can't repair it; the best thing to do is prune it back to its main source, probably the trunk. To make a proper pruning cut, do not cut the branch off flush with the trunk. Cut just beyond the branch collar, a slightly raised area where the branch and trunk meet. At this point, the tree's own meristematic tissue will seal the pruning wound. It's the tree's natural healing system.
The proper way to prune a large branch is to make three separate cuts. The first two are to eliminate the weight of the branch and prevent it from tearing bark down the trunk of the tree, which is unsightly and also allows diseases and pests an entry. 1. Start with an undercut (cutting from the underside of the branch upwards) about 8 to 10 inches from the trunk, but cut only about halfway through the branch. 2. About 2 to 4 inches further out on the limb, cut the branch off from the top to bottom. The undercut prevents the bark from tearing as the limb falls. 3. Now that the weight of the heavy branch is off, make a clean, smooth cut just past the branch collar.
Don't apply any sealants to pruning cuts. Research shows that sealants will not prevent decay and actually interfere with the tree's healing ability. There are a few exceptions to this rule when certain diseases or insects are a high risk; for example, rose borers and oak wilt.
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