Tree roots…Where do they go?

Published 9:30 pm Monday, November 10, 2008

When there is a discussion at our garden club about trees in the landscape, the topic of “roots” almost always comes up. Can you give us your opinion and thoughts related to “roots”?

This is a subject that is discussed just as much by the so-called “experts” as it is by local garden clubs. Because tree roots can’t be seen (hopefully), they tend to be a mystery for many people. While most people do not give roots much thought, information about them may be more important than anything else about the tree or trees in question. More times than not, I get questions about tree roots after the fact and usually too late to do anything about the problem, good or bad.

Trees 101 says that tree roots have four major functions: they anchor trees, store energy in the form of starch, absorb water and nutrients, and conduct water and nutrients to the rest of the plant.

The large woody roots are similar in structure to the branches of the tree. These roots serve as anchors for the tree and also conduct water and nutrients. Almost all of the woody roots are found in the top 2-3 feet of soil, and they can extend in diameter up to two times the height of the tree.

The small, fibrous tissues located at the ends of the woody roots are called the absorbing roots. They have modified tissues called root hairs that take up water and nutrients. These absorbing roots (sometimes called feeder roots) grow where water and oxygen are found. This is normally in the top 12 inches of soil, where they have to compete with roots of grasses, shrubs and even ground cover. Trees do have sinker roots that grow down from lateral, woody roots. Sinker roots help with anchorage and are usually found within a few feet of the trunk. The downward growing tap root you so often hear about and see in tree drawings is normally only seen when the tree is very young and in the seedling stage. Some taproots are cut off by expanding lateral roots, while others are diverted from normal downward growth due to compacted soils. This results in a root system that looks more like a pancake than a carrot. You hardly ever see an uprooted tree with any root system that goes beyond 24 inches.

Tree root damage to sidewalks, driveways and foundations is the topic of calls we get quite often. If large trees like oaks are planted too close to these areas, their root systems will grow into these hard structure and cause problems. For this reason, the recommendation is to plant large growing trees at least 20 feet from structures and 10 feet away from walks or driveways. Although tree roots get a bad name as the reason for concrete structure failures, the truth is that roots are not typically the initial cause. Initial damage is almost always started by a crack in the concrete. If roots are growing under the cracked concrete, they can then cause the concrete to lift up as the roots grow larger. As for as damage to house foundations and walks, I am no building expert, but the supposed tree damage to homes is most often caused by foundation settling and not tree roots.

Another question asked quite often is what to do with surface roots that impede mowing. My answer to this is to either mulch the area and don’t try to grow grass there, or add light topsoil over time (not more than 2-3 inches per season). Never cut these surface roots, as doing so will likely damage the tree.

I hope this information clears up many tree root questions at your future garden club meetings.