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Embankments are difficult to mow and present a safety hazard to maintenance crews. These roadside areas benefit from both native plantings and the use of geotextiles, helping control erosion and input costs. Photos: DuPont Land Management

The time-honored solution to roadside erosion is to evaluate underlying reasons for erosion, then address them with a healthy dose of engineering, concrete, and steel. But that solution is facing challenges as state DOTs are being asked to do more with fewer resources. At the same time, many transportation departments are seeking new environmentally conscious options, which has opened the door to technologically advanced and economical solutions.

One proven idea is an integrated approach to roadside management. Vegetation managers are realizing that native plants and flexible engineering techniques, such as geosynthetics, can provide solid erosion control solutions that enhance roadside stability and aesthetics.

Along many roadways, erosion is the result of nonnative, invasive weeds — from Palmer amaranth to leafy spurge — monopolizing the environment. Crews are being forced to spend increasing amounts of time and money combatting these invasive plants. Another growing cost involves maintaining safety for crews as they handle repairs along busy roads.

Rand Swanigan, roadside management specialist with the Missouri DOT, agrees. “If we can control broadleaf weeds, we typically reduce mowing, which not only saves us money but protects the safety of our operators.”

Without mowing or herbicides, invasive weeds like leafy spurge, pigweeds, buckhorn plantain, Russian thistle, and others can quickly take over large areas and transform them into monocultures. They crowd out native grasses, leading to areas of bare ground that are prone to erosion. Herbicides, however, control invasive weeds and many can now control weeds while leaving native grass species intact.

“Grasses serve as an important biofilter along roadsides,” adds Bill Nantt, landscape specialist for the Bay Area district of CalTrans (California DOT). “This means we often work in situations where we are trying to control noxious broadleaf weed outbreaks, but not leave bare ground.”

Native plants — the first line of defense

A 2010 University of Rhode Island study concluded that a mixture of native grasses provide year-round, deep and shallow erosion control. Deep-rooted grass species can anchor slopes. Rapidly spreading shallow grass species can slow runoff, prevent erosion, and provide attractive green roadsides.

Several state DOT handbooks for roadside vegetation management also suggest designing roadsides with native plants, because they are best suited to adapt to local growing conditions. Native plants also show better salt, drought, and heat tolerance than nonnative vegetation. Each region has its own unique challenges and objectives, but native grasses have been touted as a way to reduce watering and mowing and withstand extreme conditions in both winter and summer.

Local or regional studies can help determine which native species are most relevant for specific geographies. Contact your local agricultural university for a list of what's best for your area.

To help native plantings thrive, choose an herbicide program that targets specific invasive weed species while allowing native species to thrive. This allows a dual outcome — natural, long-lasting roadside erosion control and road surface integrity.

For more on using herbicides for roadsides, see the Roadside/Grounds Maintenance section on page 89.

Geosynthetics — an added layer of erosion control

In the search for alternatives to concrete and steel, using geosynthetics is truly an underlying approach to erosion control. While an herbicide program provides erosion control at ground level and in upper soil layers, geosynthetics can be used to stabilize embankments from below ground.

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State DOTs are seeking new and unique ways to approach roadside erosion including a mixture of below- and above-the-surface control techniques with geotextiles and roadside vegetation management.

These generally polymeric products are easy to install and cost-effective. They provide a stable, water-pervious base beneath the surface of slopes or roadsides, allowing watering to penetrate — but not move — soil layers.

“DOTs have been using geosynthetics for 35 years and they have gained a general acceptance for many uses,” says Sean Kiniry, president of Kintex, which supplies geosynthetic materials for erosion control. Those uses range from placement under roadways to earthworks stabilization.

Kiniry also adds that there's room for growth in the number of transportation departments using geosynthetics. “They're discovering that topsoil conservation is an important topic.”

Geosynthetics are key to conserving the underpinnings of topsoil to preserve and protect it in unison with a ground-level approach. “Water erodes soil. Geo-textiles are placed beneath or between two layers of dissimilar materials such as peat and stone, and anchored with one of the materials,” explains Kiniry.

Water can then flow through a pervious geosynthetic membrane and is displaced slowly over time. Just as a native plant carpet helps water disperse more effectively along roadsides at ground level, geosynthetics help effectively disperse water and control factors that cause erosion.

Native plants, given the opportunity to thrive by removing invasive species with a strategic herbicide program, and enhanced with stabilizing geosynthetics, will provide a value-centered alternative to steel and concrete in years to come.

— Dan Zapotok (daniel.j.zapotok@usa.dupont.com) is the product manager for Du-Pont Land Management, where he is responsible for products within vegetation management, railroad, and forestry markets.

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Strategic vegetation management
Establishing a custom herbicide program that targets specific nonnative or invasive species is critical to effective long-term vegetation management.

A long-term, effective herbicide plan for erosion control helps DOT crews and roadside managers achieve several goals. Strategic plans should include three primary steps.

1. Take out undesirable plants and brush. A broad-spectrum herbicide that targets nonnative invasive weeds can help provide protection for natural resources and help restore native perennial grasses.

2. Select an herbicide program that includes low-use rate products to cut costs and improve efficiency. A low-use-rate herbicide requires less storage space, is easier to transport, and allows crews to introduce less herbicide into the environment while still controlling difficult invasive weeds. Strategic herbicide application supports public and crew safety by eliminating brush species that are hard to mow or remove with hand-cutting methods, putting crews in harm's way near busy traffic lanes. Periodic control through herbicide applications can also remove unwanted plants that have acclimated to frequent mowing.

3. Remove undesirable species to allow moisture uptake by native plants and improve aesthetics. Erosion is most often caused by water that erodes sediment either above or beneath the soil surface. In areas where drainage is critical, brush can create specific problems for removal. Woody species and vines, like kudzu, can engulf an area and crowd out native plants. Vines can block sunlight and eliminate groundcover that traps sediment and water.

Selective herbicides are critical tools that add flexibility in an integrated vegetation management program. Management programs that include strategic herbicide use provide roadside managers with better expense management and long-term roadside erosion control. PW