Landscape
And
Nursery
Dialog
Mary Ann Rose
Commercial Landscape
& Nursery Specialist
The Ohio State UniversityDecember, 1996
Tree Roots and the Hardscape: Researchers' Perspectives.
Keywords: landscape, tree care, plant selection, roots, landscape maintenance
T ree root systems were the topic of a workshop held at the American Society for Horticultural Science meetings in October. An interesting program given by Dennis Pittenger, University of California Cooperative Extension, and the discussion that followed summarized the current research and problems encountered with tree roots and hardscapes.
The soil beneath asphalt or concrete may not be as inhospitable an environment to roots as one might imagine. When roots grow under pavement, they tend to grow just beneath the surface where oxygen supply is greatest. Once roots successfully navigate their way under a sidewalk to open ground, the roots on the other side can exploit new moisture and fertility resources. Subsequently, roots thicken up and can exert great pressure on the pavement.
Preventing roots from damaging pavement begins with species selection and good design. Certain trees, like silver maple, have a reputation for surface roots, but according to my colleague, Dr. Davis Sydnor, just about any tree of ten-inch diameter can cause surface-root problems. Choose a small tree species for small spaces. A four-by-four-foot sidewalk cut-out is considered the bare-minimum space for a small tree; a six or eight-by-eight cut-out would be preferable for medium or large-size species.
Root pruning or trenching to correct a problem is costly and often severely affects the tree. Researchers that have attempted to determine how much of a tree root system may be safely removed have met with mixed results, - probably due to differential sensitivities among tree species. However, without a doubt, major root removal can cause tree decline and often mortality.
Preventing root growth by installing some type of barrier has been successful in some cases. Most rigid root barriers are manufactured from plastic and can take the form of solid, molded forms that are placed into planting holes, panels that are fastened together, or continuous sheets of material. The latter two types can be fitted to a planting hole or be used in a linear fashion along one side of a planting. Most materials are used to form a barrier between 12 and 24 inches deep. Some of the products have vertical ribs on the tree side that are supposed to direct roots downward and prevent circling. Some examples of these barriers are products from the Deep Root, Vespro, and Bumble Bee Manufacturers. Researchers that have worked with these materials have found that they do in fact direct root growth downward and protect hardscapes; however, there are drawbacks:
- Root escapes were the biggest problem with the barriers. All rigid plastic barriers must have a lip that remains at least an inch above the soil line. Settling, mulching, or adding soil to cover the barrier will 'encourage' roots to grow over it and defeat the purpose of the barrier.
- Mowing and foot traffic can damage or crack the portion of the barrier that is above ground. Not all plastics hold up equally well; polyethylene is usually considered superior in durability and resilience. Connections that fasten panels together also can give way. Once a barrier is compromised in any way, roots can grow over or through it.
- Since barriers entrain tree roots to grow downward before they can grow outward, trees within barriers may be destabilized and more subject to windthrow.
- In cold climates, freeze and thaw cycles may reduce the useful life of the rigid barriers and heave them. Not surprisingly, most of the research with the rigid barriers took place in California.
Flexible fabric barriers impregnated with a chemical toxic to roots are an alternative to rigid barriers. The BiobarrierTM product is an example of this type. Flexible root barriers won't heave in climates like Ohio, but are somewhat more difficult to install. The biggest problem with fabric barriers is that it is more difficult to prevent roots from growing over the top of them. Excess material must be allowed to remain above the surface. ItŐs even more important not to mulch over fabric barriers than the rigid types. I don't see why the fabric could not be looped around a plastic or metal collar to keep it above the soil surface, but apparently no one had tried this at the workshop. Circling roots also may be more of a problem than with the rigid types with vertical ribs.
Most researchers at the October workshop agreed that at best, properly installed barriers can buy some time before pavement is affected by tree roots. Roots that are entrained to grow downwards by barriers will grow towards the surface once the barrier is cleared. Thus pavement disruption eventually occurs, only later. Is the time that is 'bought' worth the considerable price of the installing the barrier? Participants in the root workshop did not offer a cost/benefit analysis. Only future observations in the real world will tell us that story.
