Pruning Trees in Winter

Q: Recently I noticed some crossed branches that are rubbing together in my green ash tree. I know this isn’t good for the health of my tree. I couldn’t see it last year when the tree was in leaf. Can I trim back these branches now?

A: Late winter pruning may be the best time to prune many deciduous trees. Dead, diseased and damaged wood can be removed anytime of the year as it’s needed. Pruning during the dormant period minimizes sap loss and subsequent stress to the tree. Also, by pruning now you’ll reduce the risk of fungul infection or insect infestation. And, as you noticed when the branches are bare it’s much easier to see where pruning is needed. The key is to prune the unwanted branch while protecting the trunk wood.

For your purpose thinning cuts target problems like yours and remove a branch back to the trunk or the larger ‘parent’ branch. A thinning cut will reduce the canopy density but will have little impact on the tree’s height. The purpose of a thinning cut is to reduce the likelihood of foliar disease and allow better light penetration into the canopy.

Ideally, all pruning cuts are made on branches less than 2 inches in diameter but for larger branches use the three-cut pruning method to prevent the bark from tearing.

Cut 1. The first cut will be on the underside of the branch, 12 to 15 inches from the branch crotch (or branch union). Make this undercut approximate one-third through the branch.

Cut 2. Moving out two inches more, make the second cut above and remove the outer portion of the branch.

Cut 3. Now you’re ready to safely make the cut at the correct pruning point just outside the branch collar.

There is no need to apply a wound dressing especially the tar-type dressings. They actually interfere with the natural wound-wood growth. The key to good wound closure is proper pruning. After pruning a callus tissue grows over pruning cuts. This callus tissue, called woundwood, is a protective element formed by the tree to prevent the spread of decay.

For more information about pruning trees go to the Colorado State University website at http://cmg.colostate.edu then click on Garden Notes.

Contributed by Eileen Tully, Certified Colorado Gardener. Illustrations and diagrams provided by the U.S. Forest Service.