Can I shave the top of tree roots?

I'm hoping you can help me, as I have not been able to find anyone who really knows the answer.

I have a mimosa tree in my front yard that I planted about 6 years ago. The roots are now breaking the surface and are quite big. They are growing over to the next yard and disrupting the neighbor's brick planter and also disrupting our brick planter. The roots are heading for the sidewalk - I can see their journey very clearly in the grass.

Can some of these roots be removed without damaging the tree? For instance, can we shave off some of the big root that is above the lawn? Gilda

I'm not sure anyone can really "know" the answer. (Will it kill the tree?) There is a concept called the critical root radius (CRR) that gives us some guidelines in dealing with how closely to a tree we can cut roots. According to this guideline, we measure the trunk of the tree at about 4.5 feet above the ground, multiply the diameter by 1.5. This gives us the CRR in feet. That is the minimum distance we want to stay from the tree trunk when cutting roots. Staying even farther from the trunk is better. So, the recommendation is that you cut the roots at a distance at least equal to the CRR from the tree to stop damage to planters and the sidewalk. It is not good to shave the top of the roots. They may survive it initially, but shaving the top will allow entry of disease organisms that will eventually result in the death of the whole root. Even if you stay outside this CRR distance, there is still a chance that the tree will be injured. There is also a chance that if you cut closer to the tree, the tree will not die. I would suggest that you not cut more than one or two roots (outside the CRR) in a single year. Cutting roots is not good but is sometimes necessary. If the distances involved are such that you can't cut the roots outside the CRR, then you may want to consider replacing the tree. The mimosa is often a deeply rooted tree if the soil allows development of deep roots. Even so, the most important roots will develop in the top 6 inches of soil. When roots are on the surface of the soil, it indicates that the soil is too wet, forcing the roots to the surface for oxygen, or that the soil is so compacted that the roots cannot grow through the soil and must grow over the surface. If you plant a new tree, choose a tree appropriate to the root space available. Smaller trees will have less extensive root systems. Also, prepare the soil over the rooting area for the tree by deeply digging or rototilling to break the compaction of the soil. The larger the area in which the soil is decompacted, the better. The roots can extend to a distance of four or more times the height of the tree. Loosening the soil allows roots to grow under the surface and allows penetration of the water and oxygen needed by roots. If the area in which you are growing the tree is small enough, you may want to consider planting a large shrub that may be pruned into tree form. A shrub will also benefit from soil that is loosened over a fairly large area before planting. Replacing a tree is often an avoided option, but it may be the best way to avoid long-term problems. If the replacement plant is chosen carefully, it doesn't take too long to have a tree or shrub that looks good in the landscape.

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.

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