January 17, 2009

1 - Minimize damage to tree roots when building footers.

Yard and Garden January 17, 2009


I was wondering if I made a mistake last summer. I dug a trench to pour a concrete footer to support a storage building. In digging the trench, I cut several large roots from a nearby cottonwood tree. Did I harm the tree? Was there any way to avoid the problem?

- Tony J.


There is a good chance that you did injure the tree. Large roots carry the water and nutrients accumulated by the smaller roots farther from the trunk. By cutting a large root, you remove a large portion of the water and nutrients that would have reached the tree. The extent of the injury depends on the size of the root that you cut, the overall health of the tree, and the status of remaining roots.

If the root was larger than your wrist, it is unlikely that it will be able to produce new growth to replace what you removed. If the tree is old and in declining health, it may not be able to produce new roots from even a smaller root. The last hope is that the other major roots on the tree (they are the ones you see near the surface) are able to supply enough water and nutrients to sustain the tree. Even so, the tree may lose branches. If branches begin dying back over a structure or in an area where falling branches can cause injury or damage, remove those branches as they begin dying back. If the whole tree begins to decline, it may become necessary to remove the tree completely.

There may have been another way to support the foundation of your storage building other than trenching for the footer. I have seen "bell-holes" dug to create concrete piers for very large structures in other parts of the country. These piers, if engineered properly, can support the foundation on which a structure is built. To minimize damage to roots, when making the bell-holes, begin excavation using pressurized water. This will allow you to find and avoid damage to roots in the upper soil. Once you are past the upper roots, you can use augers or other methods to excavate the rest of the hole for the pier. Then build the foundation supported by the piers rather than footers. This may not be an economical or practical way to support the foundation, but it does increase the chances for tree survival.

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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