Issue: July 1, 2000

• Can I prune a tree now?

Can I prune a tree now?

We just moved into this house a few months ago. A mulberry tree in our backyard has grown too close to the house. Standing on the roof, I trimmed one branch back. Before I continue trimming other encroaching branches, I wanted toknow what to use to paint the stump where the limb used to be? Laurence Mitchell

The Cooperative Extension Service in New Mexico does not recommend use of pruning paint for most pruning cuts. There is no true benefit to the tree, and there are some disadvantages. It is important that the pruning cut be made properly so that the inherent defense mechanisms of the tree may successfully repel insects and diseases.

In the case you described, prune the offending branch back to a side branch which is at least one-third the size of the branch being removed (one-third its diameter at the point of the cut). Try to make a diagonal cut starting a little above the branch to remain and ending behind the top of that branch. There should be some good pictures on pruning books in your library. Look for information that quotes Dr. Shigo and speaks of "branch collars" and "branch bark ridges" so that you know the book is including the most recent information.

Now is not a good time for major pruning of most trees. Summer pruning has a debilitating effect on trees. In some cases, as you have described, it is better to prune now to avoid damage to the roof or other parts of your home. Just don't get too aggressive at this time of the year. Save any large scale pruning that can wait for the dormant season when the effect of the pruning is not as debilitating to the tree.

Regarding the perceived need for the pruning paint, it was once thought that the paint prevented the entry of disease spores into the wood. In fact, the spores are present on the surface of the cut before you have finished cutting the branch, so the paint cannot prevent the fungi spores from reaching the fresh cut. I have seen pictures of mushrooms growing through the pruning sealer. In that case, the pruning paint helped create a moist, protected environment favoring the growth of the fungus. If the cut is properly made and left open to the air, disease should not have much of a chance of becoming established. The tree has mechanisms for protecting itself and for closing the wound. We want to favor the tree's mechanisms which is the purpose of the type of pruning I mentioned, especially in the case of cutting the branch back to another healthy branch (though smaller).

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.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!