Cutting Off Roots from Trees is a Judgement Call

Q. When our landscapers installed new water lines for irrigation they cut through two major roots on a Chitalpa tree; one root was 4 inches in diameter and the other 1 ½ inches in diameter. These water lines could have been installed under the roots instead of cutting them. I am not sure why they did it this way. I am concerned about the future health of the tree.

A bit hard to see but a large Chitalpa root cut by landscaper.

A. If this is a problem for the tree, is a difficult question to answer. Will cutting the roots of a tree cause it to die? Cutting tree roots always damages the tree. Can the tree recover from this damage? The right answer is, it depends. From your description, the root removal done sounds ominous.

Tree roots grow where water is applied. If water is applied in shallow irrigations, it leads to shallow roots.

            You can typically remove about one third of the total roots with no problem. This is done sometimes when trenches are cut in the soil for burying irrigation lines. When roots are cut. But when roots are cut, about 1/3 of the top should be removed as well. This removal of part of the top puts the top and roots back in balance with each other. How much of the total roots were removed in your tree’s case? That is difficult to estimate.

Strangling roots should be removed as young as possible for obvious reasons.

            Look at the distribution of water applied for irrigation. This helps determine where the roots might be. Roots grow toward water because the soil is wetter in these locations. If there is a lawn close to the tree, then roots grow vigorously toward the lawn and less vigorously toward its own drip emitters. If other plants are growing near the Chitalpa, tree roots will likely grow toward the majority of drip emitters because there is more water there.

Trees growing in lawns oftentimes grow shallow roots because lawns are watered with shallow irrigations. These roots can be removed if they are far enough from the trunk and are not a major root of the tree used for water uptake, soil nutrients important to the tree or used for major support.

            At the very least, remove about 1/3 of the top. Remove entire limbs rather than giving it a “butch haircut” and removing the ends of lots of branches. Removing two or three major limbs is probably enough in your trees case.

This native mesquite near a river in Jerez, Mexico, sent its roots deep after underground water coming from the river.

            As a precaution, I would stake this tree. Major roots of trees are used for tree stability particularly during strong winds. The tree might need to be staked until the roots secure the tree in the ground. I would do it in case it is needed. Don’t wait until the tree starts to lean because of strong winds. Remove the stakes when the tree is stable. This might be one season of growth for smaller trees or up to three years for larger trees.