Tree roots shallow and threatening house

Question:

I have several large mature cottonwood trees in my yard that provide shade for the house. The roots of the trees are growing close to the surface and one is now buckling the sidewalk and moving towards the foundation of the house. If I cut the roots away from the house will the tree die? I would like to save the trees, but I do not want the roots to damage the foundation of the house.

Daniel F.

Answer:

It is not uncommon for tree roots to damage sidewalks and driveways. The cottonwood tree is one that is genetically programmed to produce shallow roots because it grows naturally in flood plains. In a flood plain, the area that dries first after a flood is that area nearest the surface. Since tree roots require oxygen, the area that dries first is the area that is best for tree roots. Hence, the cottonwood is programmed to produce shallow roots.

We like cottonwood trees because they grow rapidly if irrigated sufficiently and because they are native. We like the fact that they quickly produce shade for our homes. However, along with these desirable characteristics we get the shallow root system and the weak wood and the fact that the trees will often drop branches. There is no perfect tree, so we choose the trees that most closely fulfill our landscape needs. For many people, the cottonwood is that tree.

Your concern about cutting the roots is a valid concern. The roots are an important part of the tree and the large roots that damage paved areas are conduits for significant quantities of water to the trunk. Cutting large roots does indeed threaten the health and survival of the tree. In the situation concerning the sidewalk, it may be more desirable to replace the damaged area of concrete with new concrete, but this time allowing for continued growth of the roots. You can do this by building a "tunnel" over the root with a large concrete drainage tile and then pouring replacement concrete to make the sidewalk level or the grade over the root tunnel gradual enough not to be a problem. In other situations it may be appropriate to build a bridge as a landscape feature. A "moon bridge", as in Japanese gardens, may be appropriate in some landscapes. A wooden boardwalk replacing the sidewalk may also be appropriate if raised enough to leave room under it for continued growth of the root. These are just a few ideas of how to solve the problem without cutting the root.

You also mentioned that the root's approach to the foundation of the house was a concern. This may not be a great problem. As long as there is no water under the house from a leak or over-irrigation near the house, the root will probably not grow under the house. It is a biological fact that roots cannot grow in dry soil, they only grow in most soil. By assuring that the soil under the house is dry, you can keep the root from growing under the house. By creating a dry zone just outside the foundation (moving flower beds and other irrigated landscape features away from the base of the house) you will encourage the root to grow in other areas where there is moisture. This means that you must also direct roof runoff away from the foundation of the house. Gutters and water diversion into flower beds away from the house will avoid the problem of roots growing up to the foundation.

Cutting the roots may not be necessary. However, if you do choose to cut the roots, you must then watch for signs that the tree has been injured. Dying branches or leaning will indicate the problems. If the tree begins to lean, you may find it necessary to remove the tree to prevent it from falling on your house, car, or people.

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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