Invasion of the Tree Roots: How to Prevent It

Contrary to popular belief, tree roots do not cause cracks in water or sewer pipes. They will exploit existing cracks, though. Just plan ahead before planting and eliminate that possibility.

Today I’m addressing a common myth — that tree roots invade water and sewer pipes. Yes, it’s a myth. Despite all the anecdotal stories going around, tree roots do not cause or initiate cracks into a water or sewer line. They may exploit an existing crack, but not cause one.

Here’s how it usually goes.

Scenario #1

The soil surrounding a water line is clay or clay loam. This type of soil expands and shrinks during wet and dry weather. If the water or sewer line is not properly constructed or a fill material such as gravel or sand is not properly placed around the line, then the line will crack over time. When these minute cracks appear, tree roots can exploit the cracks.

Some species are better adapted to exploiting cracks and better adapted at surviving low oxygen/high water environments like lines. These species include the willows, poplars, ashes and elms. Upland species such as oaks, pines and hickories cannot tolerate low oxygen/high water environments and are less likely to invade the cracks.

Scenario #2

Trees have large structural roots that extend 6 to 10 feet from their trunk. Over the course of time, these increase in diameter. If a pipe were located next to these roots, the roots would exert pressure over time against the pipe, deforming or cracking it. Again, tree species adapted to exploiting these cracks will take advantage of the situation and insert themselves into the pipe.

How to minimize scenarios 1 and 2

First, in areas with clay or clay loam soil, ensure all lines are installed according to accepted standards. To increase confidence in the integrity of the system, install 4 to 6 inches of aggregate, sand or expanded shale around the water and sewer lines.

Second, avoid planting a large shade tree closer than 5 feet to a water line and certainly avoid species such as willows, poplars and ashes. Water lines are generally found in a straight line from the water meter to the house.

With extra effort and planning, a homeowner can minimize the probability of leaks and therefore root intrusion. Remember that roots cannot incite cracks in the lines, but on occasion they can exploit them. Plan ahead and eliminate that possibility.

Picture of Mark Peterson
Mark Peterson
Mark A. Peterson was a conservation project coordinator for San Antonio Water System before retiring. With over 30 years of experience as an urban forester and arborist, Mark is probably the only person you know who actually prunes trees for fun. When not expounding on the benefits of trees and limited lawns, you're likely to find him hiking San Antonio's wilderness parks or expounding on the virtues of geography and history to his friends.
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