IDAHO - Honorable Mention

The Idaho Roadside Program

Revegetation of Steep and Highly Erodible Slopes in Idaho

By Gene Ross, Roadside Programs Coordinator, Idaho Transportation Department

Surface erosion, sedimentation, and shallow-seated slope failures in Idaho present a significant challenge to road construction and maintenance. Soil losses from disturbed areas, particularly steep slopes, deplete an important resource, cause excessive ditch and culvert maintenance, and may cause adverse effects on the quality of receiving streams, rivers and lakes.

Note the challenge of this site in need of erosion control. More than steep!

Previous Idaho Transportation Department (ITD) practices to revegetate disturbed slopes and sterile subsoils using commercial fertilizers, straw or wood fiber mulch, or erosion blanket have been mostly unsuccessful. These disturbed slopes where topsoil was unavailable or could otherwise not be used, could not grow or sustain viable vegetation and were therefore subject to continuous erosion. Under these circumstances, the seeds germinate successfully in the spring (after a fall dormant seeding), but no nutrients or beneficial soil microorganisms were present in the soil to provide for a healthy, vigorous-growing plant. The seedlings would not survive the first growing season.

Diversified, long-lived perennial vegetation capable of growing in sterile subsoils is the key component to long-term erosion control and slope stabilization. In order to establish a successful revegetation program, two very critical factors need to take place: 1) rapid initial germination/growth, and 2) long-term sustainable growth of the seeded species. Initial rapid germination/growth is necessary in order for new vegetation to survive during the first year of growth, especially during the hot dry summer months in arid or semi-arid regions of Idaho. Long-term sustainable growth cannot take place unless rapid initial germination/growth has satisfactorily occurred.

Controlling roadside erosion and sedimentation, slope stabilization and revegetation efforts are dependent on each other. To achieve maximum revegetative results, this dependence requires combining, in one application, several very important erosion control practices.

Using compost (dry applied) or organic-based soil amendments and soil biological stimulants (hydro applied) have shown that vegetation can be grown on steep, highly erodible slopes. Seeding applications, primarily performed on decomposed granite slopes, show that initial rapid germination/growth and long-term sustainable growth is possible. The soil amendment ingredients, referred to as a mulch mixture, are preferred because they can be hydro-applied, allowing the application to take place from the road shoulder. Because the slopes were steep and long, it would have been very difficult to treat the slopes without using water as a carrier. Compost, at this time, can only be applied dry, and problems associated with hauling, handling, application, and keeping the product in place after application have not been addressed to ITD's satisfaction.

The soil amendment and soil biological stimulant were chosen based on previous experience and familiarity with the products, as well as successful germination and establishment of healthy vegetation where the mulch mixture was used. Combining these products with bonding fibers and a suitable tackifier to form a bonded fiber matrix represents a new, potentially cost-effective application to erosion control and soil stabilization technology.

Seed species selection is based on whether the species will grow in particular site conditions, in this case the adverse or unfavorable conditions of steep slopes and sterile subsoils. A combination of grasses, forbs and shrubs represented in the mix provide for optimum root depth diversity. ITD is currently working with the University of Idaho to improve and expand the selection process of plant species (preferably native) that will grow under these adverse conditions.

The ability to mix a combination of products together, all performing beneficial yet independent functions in revegetation, has real merit. Finding the right combination of products (plant nutrients, soil building ingredients and erosion control) that perform on very steep slopes and sterile subsoils is a very important and cost-effective erosion control practice that can be utilized in almost all instances in roadside reclamation of construction and maintenance projects.