GROUNDS MAINTENANCE/Texas DOT tests geotextiles for roadside weed control

Guardrails are important for both preventing cars from leaving the road and maintaining easements, but weeds and grasses frequently grow under them. This is not only unsightly, it also creates a hazard, since tall weeds and grass can obstruct a driver’s view of approaching vehicles. During dry weather, they can also be a fire hazard.

State transportation departments frequently deal with the problem by having employees mechanically trim the weeds or spray them with herbicides.

But the Texas Department of Transportation (TDOT) realized that these solutions might not be feasible when it prepared to tackle a roadside project near Austin in 1993. The project site was a large, multi-level, high-speed interchange at U.S. Highway 183 and Loop 1 which encompasses approximately 100 acres including road surface.

A worker operating a hand-held weed trimmer in such a location risks being struck by a passing vehicle, and the weed trimmer could also throw gravel through open windows of passing vehicles, leading to injuries and lawsuits.

TDOT decided against spraying herbicides because it exposes workers to traffic, can harm adjacent foliage, has a short life and requires repeated applications. Spraying is also becoming increasingly unpopular with the public.

Other options considered for weed control included paving with asphalt or concrete. Unfortunately, asphalt cracks as it weathers, offering new opportunities for grass and weeds to germinate. Concrete is less likely to crack, but weeds eventually grow in joints against existing curbs or guardrail posts. The high cost of concrete or asphalt repair in the event of guardrail damage is another drawback.

The Texas Transportation Institute (TTI), which oversaw the project for TDOT, decided that prevention of grasses and weeds was the solution and pilot-tested a geotextile fabric product, Biobarrier II by Reemay, Old Hickory, Tenn. The product is studded with small nodules impregnated with the herbicide trifluralin, which is nearly non-toxic.

The herbicide works its way out of the nodules and biodegrades over a period of one to six months. The chemical is released from the nodules as a vapor and adheres to soil particles, working to prevent root-tip cell division for as long as 10 years.

TTI, a transportation research institution affiliated with Texas A&M University in College Station, established six study plots, each about 100 feet long. Two soil types were used: crushed limestone road base and the native soil, which is black, cohesive clay. The material was installed underneath selected guardrails at depths of two and four inches, after which the plots were compacted with a plate vibrator. The plots were not seeded.

The test plots had unequal beginning points — different amounts and types of grass were growing near them when they were installed. The results showed that at the two-inch depth under limestone gravel, the plot was very clean with virtually no grass growing. Isolated individual weeds were present at some guardrail posts. The two-inch native clay soil plot had some herbaceous weeds present but no grasses.

The four-inch deep limestone gravel plot had no grass rooted in it, although there were some grass runners extending over it from the edges. There were no herbaceous weeds.

TTI officials are continuing to monitor them to determine if there are any differences in how each responds to encroaching vegetation.

TTI is preparing preliminary results of the study for TDOT. Jim Schutt, TTI landscape architect, says he found that the geotextile product should not be buried deeper than four inches, and limestone aggregate is the most effective material to put on top of it. “I think it’s going to work pretty well,” says Schutt. “I anticipate that most of its use will be in urban areas along high-speed roads.”

The Texas Parks and Wildlife Department, the Austin Parks Department, local environmental interests and TTI landscape architects all participated in the project.