Killing tree roots after cutting tree

Question:

I had my Siberian elms cut recently because I am allergic to them. However, the people I hired to cut them did nothing about destroying the roots. I am planning to borrow a drill and do it myself. However, I've forgotten what products to use. What do you suggest?
-Marilyn C.
Raton

Answer:

People often drill into the trunk and apply herbicides or other substances to kill the root, but this is often done improperly. When they do this, they drill the holes in the center of the wood, and they often treat several weeks after the tree has been cut. The results are often unacceptable. The treatment should be applied immediately after cutting the tree, and the chemicals should be applied to the outer portions of the trunk, just inside the bark. An even more effective method is to apply the chemical before cutting the tree.

The object of such treatments is to have the chemical move downward to the roots and kill the roots. The layer of wood just inside the bark is the tissue (called the phloem) that carries material from the leaves to the roots. Deeper inside the wood is the xylem that transports water and minerals from the roots to the leaves. It is important to apply the chemicals in the phloem of the tree. Once the tree has been cut for a few days, the downward movement of materials stops and the chemical doesn't work effectively. If you delayed too long, it would be more effective to apply chemicals on regrowth next year rather than to try now.

The chemicals to use are a non-specific herbicide glyphosate herbicide (example: Roundup) or a broadleaf herbicide (example: brush and stump killer) that is made for the purpose of killing trees. In either case, the chemical must be labeled for this purpose and used according to the label directions.

If the roots are still alive and growth resumes in the spring, dilute the herbicide according to the directions and spray it on the plant after growth resumes in the spring. It will be absorbed through the leaves and will translocate downward to kill some of the roots. If you try it now (in the fall), the instructions should tell you to use it full strength as you apply it directly to the phloem tissue (in the holes at the outer edge of the wood). You can also apply it to "frills" or notches cut through the bark of larger stems after the tree sprouts have grown. Make these frills by cutting downward v-shaped notches through the bark and applying the herbicide with a paint brush or other object that allows very careful application into the notches. Be careful not to spray the herbicide onto plants that you want to preserve.

If the tree doesn't develop sprouts in the spring, you won't need to apply herbicide at all. Some trees do not resprout, but others will resprout.

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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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