I had an interesting call last week. The caller had large tree roots growing in his irrigation valve box and wondered what he could do to get rid of them. We discussed products known to work against roots.
His first problem was, why were the tree roots growing in the irrigation box in the first place. Roots grow where water is, which suggests that his valves or pipes were leaking and had been for some time since the roots were large. Secondly, there really aren’t products that make roots magically disappear. The solution is to cut the tree roots and then put a root barrier around and under the box. However, cutting roots could damage the parent tree, depending how far they are from the trunk.
Root barrier materials can be fabric, metal, plastic, concrete and other impermeable materials. I have heard of using the thickest roofing paper. Barriers are used to protect foundations, retaining walls, pipes and other structures from root penetration by guiding roots downward and away from the structure. They come in a variety of widths and thicknesses and withstand burial in soil for long periods of time. Ideally, they should be installed before the tree roots are there. Trenching, cutting roots and installing a barrier to prevent root reestablishment is a possibility, but is hazardous to the stability and health of a tree. It is generally suggested to install barriers to a minimum depth of 30 inches before trees are planted and extend the material above the soil surface to prevent roots from growing over the top. Remember, where the material overlaps is the weakest section of the barrier, so install carefully.
Some products do more than physically prevent root penetration. Some are plastic fabrics that contain time-release herbicides that prevent root tip development for 15 years or more depending on soil type. They do not negatively affect plant health. Roots hit the barrier with the chemical, and root tips die. Roots not near the barrier grow normally.
The first rule in preventing tree root damage is to select trees without invasive roots. Avoid poplars, aspen, silver maples, locust and elms for example. However, if trees are not watered deeply, many, even those without invasive roots, will develop shallow wandering roots that can become problems. The second rule is that trees should be watered separately from lawns to encourage deep rooting. The third rule is to plant the trees in the proper place.