Killing unwanted trees or preventing stumps from sprouting can be a real challenge. No matter how often the sprouts are removed from the offending stump, they always seem to re-appear within a matter of weeks. If they are not removed, what was once a single tree trunk can quickly become a jungle of sprouts that are destined to become trees.

Although the most successful option is to simply remove the entire stump when the tree is cut, this is not always possible due to the inaccessibility of the majority of most home gardens to large equipment. Mechanical stump grinders do a good job of pulverizing tree stumps, but again, space may limit their use.

An option that works fairly well is to simply cover the offending stump with heavy weight black plastic, securing the edges to prevent any light from reaching newly formed sprouts. Sprouts that develop beneath the plastic will grow for awhile, but eventually whither and die due to the lack of light. Although this process may take a couple of years to completely kill the unwanted stump, it is both an easy and effective way to get the job done.

A third option is to use an herbicide registered for use as a brush killer. Most of these products contain a non-selective herbicide, like triclopyr, that is absorbed into the foliage of the sprout and translocated into the tree’s root system. Using this option means considering the consequences of the herbicide you select. Many of the herbicides that control undesirable woody plants vary in environmental stability, leachability, flashback potential and handling requirements. Control methods can damage surrounding vegetation and neighboring trees and prevent desirable vegetation from being established for several years.

Sometimes the roots of different plants share vascular tissue through grafting. Root grafting occurs primarily within the same species, but may occur between plants within the same genus. This phenomenon can be of great importance. An herbicide can move (translocate) from a treated tree to an untreated desirable tree, killing or injuring it.

Desirable trees and shrubs may also fall victim to another phenomenon known as “flashback.” This happens when the herbicide that is applied to kill the tree sprouts is translocated into the roots and once the roots die, the herbicide is absorbed by adjacent trees and shrubs.

Question: We need to dig a trench along side some of the trees in our yard. We are worried about causing damage to the roots. Our neighbor has told us that as long as we dig our trench outside of the “drip-line” we should be okay. Is this true?

Answer: There is a common misconception that the root systems of large trees consist of deep taproots, plus some side roots that extend out to the drip line. Research however, has shown that tree root systems are more likely to be shallow and wide spreading.

It appears that well-developed taproots are rare in landscape trees. Over 90 percent of the absorbing “feeder roots” are likely to be in the top 12 inches of soil. This is often the result of deeper soil being more compacted and less fertile. Roots always grow best where the soil environment is most conducive to their development.

In most cases tree roots spread far beyond the drip line. It is now thought that roots are likely to spread three times the distance from the trunk to the drip line. One study found that 60 percent of the absorbing roots of Colorado spruce trees were growing outside the drip line.

Cutting through feeder roots may lead to severe damage or tree death. Therefore, consider tunneling under the roots instead of cutting a trench through them if underground pipes or cables must be placed near trees. Remember to avoid grade changes around trees. Adding soil can suffocate roots, and lowering the grade can destroy many of the absorbing roots. Destruction of large buttress roots could cause the tree to fall.

EDITOR’S NOTE: For answers to local gardening questions go to www.pnwmg.org or call 360-249-4125, the WSU Extension office in Elma, or call Rachel Gana at 642-8723 or acornwp@yahoo.com.