The Myth of Root-Filled Stormwater Drain Pipes

Disturbance to pipes and sidewalks caused by roots is one of the most commonly cited complaints about trees. One of the many questions I get asked about is whether they penetrate and plug up drain pipes frequently. The answer – perhaps surprisingly – is no.

Professor Bill Hunt, PhD, PE and his colleague Ryan Winston, PE, both at North Carolina State University Extension, have created hundreds of experimental rain gardens to research drainage behavior, growing media, capacity, suitable plants, and more. Many of these rain gardens are only planted with herbaceous perennials, but a great many of them have been intensively planted with trees and shrubs. For monitoring purposes 100% of these experimental rain gardens have drain pipes underneath.

During the course of these multi-year experiments, pipe cameras are sent down annually. EVERY drainage pipe is checked for roots, breakages, and other disturbances. Dr. Hunt and Ryan have told  me that over this decade-long course of experiments, they have never found roots in the pipes. Ever! The sample size is in the hundreds – a large and robust population. There are no current peer reviewed journal articles on this topic that I’m aware of, nor probably will there ever be.

Tree roots have been getting into wastewater sewer pipes for 100+ years, so it seems reasonable to assume that tree roots will create the same problems for stormwater drain pipes . But they do not. None of us know exactly what is keeping the stormwater drain pipes root-free in frequently saturated conditions. But, since it’s not a problem, no one that I know of has pursued research funding to figure out the reason why.

I have an untested theory that plant roots do not get into these pipes because drain pipe flow runs intermittently and contains no nutrients (these are pirated by microorganisms from the water that is passing through the soil column). Roots that might otherwise colonize these pipes would find themselves alternately in completely saturated or completely dry conditions, and always without nutrients, therefore they stay away. Sewer pipes however, have perennial flow, year round, in any weather or any season, and they are filled with nutrient-rich waters. They are a very hospitable place for tree roots, and this may be why root disturbances are common in sewer pipes. This last part is based on my experience, not experimentation.

L. Peter MacDonagh, ASLA, is the Director of Science + Design at the Kestrel Design Group.

10 comments

  1. I have also heard that when roots are found in pipes, it is often because the pipe was cracked from age or some other factor. The roots found their way in after the cracks occurred but did not cause the cracks.

    • Yes, that’s exactly right. I once heard San Francisco’s Urban Forester, Carla Short, speak about this. She pointed out that roots grow in to cracks in drain pipes the same way a branch will grow in to a hole in someone’s roof – but no one would blame a branch for causing the damage to the roof in the first place!

      • Problem is, once a small root makes its way through, it begins to expand as it gets more nutrients and grows. This adds more stress on the cracks and expedites their depreciation.

  2. Saying that tree roots do not cause cracks in pipes is very short sided. When tree roots grow and increase in size, they put pressure on the surrounding soil and thus the pipes buried in it. They probably don’t poke a hole in a pipe just to find the water within it, but the growth of the root near the pipe can cause stress on the pipe causing the pipe to crack and creating a place for water to escape. Once that water escapes, the roots will find a way in.

  3. I have had problems with roots clogging my sewage drain pipes. Naturally, it shocks me to find out that stormwater drains don’t get root damage. I think that your theory about them running intermittently and without as many nutrients is probably a good theory. I also wouldn’t be surprised if these drain pipes are stronger than those used for my wastewater drainage.

  4. Charles J Hardebeck
    Reply

    My prob;em instead of roots entering the storm drain was that they grew into the small gap between the underground drain and the house, eventually causing the former to buckle

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