Without using more advanced stabilization techniques, the maximum slope to be considered for vegetative stabilization is 1.5 horizontal to 1 vertical (1.5H:1V). There are many good plants in the form of grasses, vines, shrubs, and minor trees that can be used for slope stabilization projects. Plant selection is dependent upon the goals of your erosion control program and site conditions. Typically, effective programs incorporate structural diversity in plant selections (trees/shrubs with ground covers) and use a mix of species.
The vegetation along the edge of the top of the slope serves as a protective buffer for the slope face. If possible, you should maintain or reestablish a greenbelt which would provide a buffer between the slope face and residential structures. This is particularly important in areas where slopes are too steep and too high for economically feasible stabilization methods. For example, some coastal high bluffs in residential areas along Seattle, Tacoma, Edmonds, and Port Townsend waterways may be too severe to attempt stabilization. In cases like these, you may choose to maintain a strip of dense natural vegetation along the bluff edge. The roots of the vegetation can strengthen a bluff's resistance to slumping or sliding. If the bluff edge is currently cleared, a strip may be left undisturbed and a revegetation program implemented.
Vegetation should be established on patchy and barren slope faces or terraces to reduce erosion. Planting practicality depends greatly on the character of the slope, and particularly on the slope angle. A slope of 1.5H:1V (33 degrees) should be considered the dividing line between a manageable slope and a slope steep enough that vegetation would be difficult or impossible to establish without employing other reinforcement techniques.
If room exists at the top of a slope, low slopes can be graded back to a gentler configuration. 3H:1V or flatter slopes are ideal because these slopes can be prepared and planted with wheeled vehicles. However, in most coastal areas, slopes are too steep or too high for mechanical planting techniques. Slope regrading may be neither economically feasible nor technically desirable for the individual property owner.
Various species and mixtures of species can be planted on slope faces and expected to succeed in this rather severe environment. These include seed mixtures of grasses and legumes and a range of shrubs and minor trees.
Large trees should be used on the face of slopes sparingly and with caution. Should these trees collapse because of undermining of the root system by erosion or by windthrow, large volumes of earth can be disturbed by the tree roots when they pull from the slope. The resulting large, bare areas are opened to further erosion, which may endanger adjacent land and vegetation. New major trees should not generally be established on the face of coastal slopes. Existing major trees should be closely monitored for signs of undercutting and toppling. If the trees become unstable, they should be cut before they fall. Root systems should be left intact to bind the soil for a short period of time while new live, well-rooted vegetation establishes itself. Establishing new vegetation prior to felling a tree would be advantageous to the slope protection program.
In those situations where the bottom of your slope is susceptible to frequent or periodic wave attack, vegetation alone will not be adequate as an erosion control tool. In such cases a form of structural toe protection may also be required. If the toe is not subject to coastal marine erosive forces, trees and woody shrubs can be useful in resisting upland landsliding and tolerating the dynamic changes in the coastal shore system. Vegetation at the slope toe can sometimes help reduce marine erosion to manageable levels.
Late fall and early spring are usually the best times for slope installations. During these periods plants are semi-dormant, the slope soils are easiest to work, and vegetative cover is at a minimum. If slope moisture is an installation problem, fall usually provides the best opportunity to work with the slope.
Use the Ecology publication - Vegetation Management: A Guide for Puget Sound Bluff Property Owners to plan for a range maintenance considerations in your erosion control program. Most programs do not have significant long-term maintenance requirements.
Copyright © Washington State Department of Ecology. See http://www.ecy.wa.gov/copyright.htm