West Virginia Highway 47 Cut ~ Choosing the Right Mulch
Zwilling elected to use Flexterra FGM, a flexible growth medium that immediately bonds to the soil. Flexterra combines "thermally refined" wood fibers with crimped manmade fibers and other additives to produce an interlocking matrix with water-absorbing cavities that enhance germination and reduce soil loss.
"It maintains its strength even in wet conditions,"Zwilling says. "It will stick to a vertical slope—it locks on. You spray it on, it sets up and cures, and soil particles don’t move. Flexterra retains up to 1,500% of its weight in moisture and is the only hydraulically applied product that doesn’t require an extensive curing time. If it rains, or something blows in, Flexterra will stay in place. Most other products would just wash away."
Flexterra, which comes in 50-pound bags, was applied at a rate of 4,000 pounds per acre on these challenging slopes. "We were essentially painting on a blanket over the rock and soil," says Zwilling. "But we applied the hydroseeding mixture in stages, in case of bad weather." Although this product resists washing off better than others, in a significant rain event, some of the product will ultimately wash away. He stresses that Flexterra is not designed for areas of concentrated water flow, as in channels or roadside ditches. Its primary use is for slope stabilization.
He adds that paddle-agitation hydroseeding machines are needed in order to use spray-on blanket products to keep the highly viscous slurries in suspension. A jet agitation unit is unable to handle thick slurry, he explains, and Flexterra is a very thick mixture. He also prefers bigger hydroseeding machines on large projects, because "they have more power, more room for the mix, can cover a larger area, and are simply more efficient. You don’t need to reload the tank as much."
The initial proposal for the Highway 47 project was submitted in April 2006, but because of funding and timing issues, work didn’t begin until May 2007. From the start, everyone involved understood that there were some inherent limitations. "Nothing will grow on bare rock, so the area will never be fully vegetated," Zwilling says. But today he is proud to be able to claim, "Where there’s soil, there’s now vegetation. Nothing had ever grown here before. Rockslides cannot be totally prevented, but the risk of a dangerous rockslide has now been minimized." Theisen adds, "It doesn’t look like a golf course—we knew that it wouldn’t—but the amount of cover is significantly greater than before, and it is expanding, and the soil is stabilizing."