Recently I had a chat with an arborist who was convinced that when a street tree dies it is related to root barriers. Try as I might, nothing could dissuade him of this notion. And that got me to thinking that after all these years, the idea of how root barrier work and what they do is still a message worth explaining.
The idea for the root barrier is based off an article, “Root Growth Around Barriers,” published in 1967 by Brayton Wilson of Harvard University. In this article, Mr. Wilson explains that roots can be deflected downward using a 90 degree rib.
The name “Root Barrier” itself is a bit misleading. Our tree root barrier is a mechanical device that re-directs and guides the tree roots down and away from the hardscapes. It isn’t an actual barrier that prevents the tree from getting what it needs (more on that later). Once they’re grown to the bottom edge of the barrier the roots will grow horizontally, and at that point they are too far down to up heave the sidewalk or driveway. The root barrier’s job is done.
Understanding this concept should help planners when specifying the root barrier. Ideally, if the root barrier is being used in a highly compacted and urbanized area, it would be used in conjunction with the Silva Cell. This way you can ensure that the tree is getting the nourishment it needs to live a long life and that your hardscapes are being protected.
Trees don’t die because their roots have been moved along a 90 degree angle. You can see many examples of tree roots being re-directed by items that are not actual root barriers and the trees are doing just fine. Trees die because they don’t have adequate soil and nutrition, they don’t have proper drainage, they are infested or vandalized, or the nursery stock was not so great in the first place. Proper planning can help avoid some these pitfalls, and root barriers can be a part of this solution by extending the life of the hardscape and directing tree roots to resource-rich areas.