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.