I get a lot of calls about using the our Geomembranes (aka water barrier, aka bamboo barrier) as a substitute for the Tree Root Guides. In these tough economic times, it makes sense that everyone is trying to find ways to save a few dollars here and there. However, the geomembrane does not have the same features as the tree root guide and should only be used with trees in certain situations.
The reason geomembranes cannot be used the same way a root barrier is used is because it lacks one critical feature: the vertical rib. The vertical rib that is present on the tree root guides (root barrier) is what re-directs the tree roots down and away from the hardscape above. See my previous entry about how root barrier came to be to understand the importance of this rib.
But don’t feel sorry for the Geomembrane because it lacks this feature and appears to be the little sister to the Tree Root Guides. Because of its smooth surface, the Geomembrane can make a great root block — but only if the trees are 8’ (2.4384 meters) or further away from the barrier.
A perfect example of this type of use is Brooklyn Botanic Garden Case Study. Here the Geomembrane is being used to block the roots from the September 11th Liberty Memorial oaks from intruding into a rose garden.* In this instance the 36/30 size was used to block the roots. Every site has it’s own specific issues and requirements. By the way, if you are ever in the area you can visit these trees yourself — they are along the along the eastern and western edges of the Cherry Esplanade.
If the Geomembrane is used closer than 8’ to the tree, stability issues can arise. The tree roots may move horizontally along the barrier without ever establishing adequate anchoring. It should also not be used when there are hardscapes on all four sides of the tree. This is because the smooth sides of the geomembrane basically “pots” the tree in the pit. The roots will circle around and around much like if they were in an actual pot. Girdled tree roots make for a sad, dying, or dead tree.
So, in sum: the Geomembrane (remember, aka water barrier, aka bamboo barrier) can be an effective block if used in the appropriate situation. Remember the rule of 8’ or more and the geomembrane can become a hero.
*If you are further interested there are other Living Memorials to honor and memorialize the tragic events of September 11th.