Issue: July 12, 2003

Tree roots and construction

Question:

I have a huge maple tree in the back yard, close to the house. I want to build an addition on to the house as well as a brick patio. I need to remove roots to do this. How much of the root structure can be removed before causing damage to the tree? Jeff H.

Answer:

It is difficult to say how much of a tree's root system can be removed. I prefer to talk about how close to the tree you can cut roots. I teach a concept called "Critical Root Radius (CRR)" which recommends not cutting roots inside the CRR. This Critical Root Radius (radius, not diameter, in feet) is calculated by measuring the diameter (not radius or circumference) of the trunk (in inches, not feet) at four and one-half feet from the ground and multiplying by 1.5. A tree with a five-inch diameter trunk (at the four and one-half foot level) will have a CRR of 7.5 feet. The CRR provides a general guideline. On some tree roots, you can cut the roots more closely to the trunk; on others, you should stay farther away. This concept is based on the manner in which roots grow and their function. Inside the CRR you will find the roots branch little and do relatively little absorption of water and minerals. Within the CRR, the roots serve as pipes carrying the water and minerals harvested by smaller roots further from the trunk. Cutting the large root removes all branch roots and the absorbing roots associated with them. The larger roots have lost the ability to regenerate new roots, so lost ability to absorb water and minerals is permanent. Smaller roots (outside the CRR) can more readily regenerate new roots to replace those that were cut. If you must cut closer to the trunk, consider the tree's loss of stability in wind and under snow and ice loads. The roots provide a wide base for the tree, helping to keep it upright. Removing a portion of this base reduces the stability of the tree and may allow it to fall on structures or people. If the absorbing roots are cut, it may cause dieback in the top of the tree. Dead branches can then become hazardous. They should be pruned from the tree before they fall and cause injury. If the dieback is too severe, tree removal may be the best and safest choice. If you must cut the roots inside the CRR, watch the tree carefully to avoid creation of hazards to property and health. Your local Cooperative Extension Service can help you determine the hazard potential of the tree.

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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, office: 505-865-7340, ext. 113.