Protecting tree roots

I have an old, large cottonwood tree in my backyard. Its trunk is about 7 feet across. I plan to build a storage shed in the back yard and wonder how close I can come to the tree when I dig to pour a concrete foundation. I love the shade from the tree and don't want to harm the tree.

Any excavation necessary to build the storage shed has the potential to do considerable harm to the tree. Cottonwood trees often have very shallow root systems (on the surface or only a few inches below the surface). Since this is a large, probably old tree (by cottonwood standards), it will be especially sensitive to root disturbance. If you cut large roots, you remove all the smaller roots attached to the large root. This may mean that you will remove up to one-third of the tree water and nutrient gathering capacity. An old tree may not be able to recover from an injury such as this.

So, how closely can you dig near the tree? There is a concept called the "critical root radius" in which the diameter of the trunk at 4-1/2 feet above ground is multiplied by a coefficient (either 1.0 or 1.5) to determine the critical root radius in feet. A 7-foot diameter tree such as yours has roots that should not be disturbed closer than 84 to 126 feet from the trunk of the tree. (7 feet diameter X 12 inches/foot X 1.0 or 1.5 = 84 or 126 feet) This means that you should stay at least 84 feet to 126 feet from the trunk when excavating. For an older tree, the greater distance is much safer for the tree but perhaps unrealistic in your application.

There may be some means to avoid the problem. Is a concrete foundation essential? Must it sit directly on the soil? It may be possible to auger some holes into the ground to create piers upon which the foundation can be constructed. The piers (if carefully located) can be sited to avoid damaging the tree roots, then the foundation can be supported on the piers above the ground or just at ground level. Remember that if the foundation rests on the ground and the tree remains healthy, the roots can crack the foundation as they enlarge.

If it is not practical to make the foundation in this manner, you may just have to excavate and see if the tree survives. The tree may indeed survive but exhibit some die back in the branches. Removal of dead branches may leave a satisfactory tree that still casts sufficient shade.

Damage to large roots of older trees often allows entry of disease organisms that begin to rot the root. This results in slow decline of the tree. In this case, annual removal of dead and dying branches may become necessary. In a few years there may not be sufficient shade to warrant retention of the tree. In that case, tree removal will be a good idea because continued dying of branches creates a hazardous situation for people and structures below the tree.

It may be preferable to remove the tree early in the process and replace it with a new tree. You can replace it with another cottonwood if the site is appropriate and that is the tree you desire. However, you can choose another type of tree if you wish. You can select a tree that will not be as large as the cottonwood (if that is best for the site) or one that provides spring flowers or fall color. You have options - replacing the tree now or later or construction to avoid damage.

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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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