The Role of Vegetation

To comprehend the benefits and contributions of how vegetation influences soil erosion and slope stability, you may think of its role as either hydrological or mechanical in nature. The mechanical contributions arise from the physical interactions of either the foliage or root system of the plant with the slope. The hydrological mechanisms are those processes of water use and movement in the slope when living plant materials exist in the soil. The general roles that vegetation plays in slope maintenance and reinforcement are summarized in Table 2 (below). The net effect of vegetation is usually beneficial to slope stability.

The protection of the slope against shallow seated landsliding is a key benefit of a revegetation or existing vegetation maintenance program. The function that mixed vegetation provides by increasing the apparent cohesion of the surface soil structure of a slope is illustrated in Figure 7 below. The different types of root systems that plants provide can strengthen potential shallow- seated failure planes on your slope by both fiber reinforcement of the near surface soil and binding soil structure together into a larger unit through tap or lateral root networks.

Table 2. Vegetation and Slope Stability

Legend: (+) Beneficial to stability    (-) Adverse to stability
Hydrologic Mechanisms
Foliage intercepts rainfall, causing absorptive and evaporative losses that reduce rainfall available for infiltration. +
Roots and stems increase the roughness of the ground surface and the permeability of the soil, leading to increased infiltration capacity. -
Roots extract moisture from the soil which is lost to the atmosphere via transpiration, leading to a lower pore-water pressure. +
Depletion of soil moisture may accentuate desiccation cracking in the soil resulting in higher infiltration capacity (uncommon around Puget Sound). -
Mechanical Mechanisms
Roots reinforce the soil, increasing soil shear strength. +
Tree roots may anchor into firm strata, providing support to the upslope soil mainly through buttressing and arching. +
Weight of trees surcharges the slope, increasing normal and downhill force components. (Tree weight in some situations is beneficial to slope stability. Trees should not be arbitrarily cut to "unweight" slopes). +/-
Vegetation exposed to the wind which transmits forces into the slope. (Degree of diverse effect is dependent upon exposure and health of vegetation. Typically a minor consideration for Puget Sound Inland Waterways). -
Roots bind soil particles at the ground surface, reducing their susceptibility to erosion. +

(Source: Modified from Greenway, 1987)

An Ounce of Prevention

No amount of slope disturbance followed by replanting should replace rational site planning when it comes to avoiding slope disturbances. Should you have the option, maintain all the native vegetation you can and potentially accept the natural retreat of the slope crest. Accordingly, you should plan the location of your residence carefully. Maintaining a greenbelt along slope crests is good practice. Do not assume cutting trees to "unweight" your slope is benefical to slope stability - often it is not. Also, remember as a general rule, do not introduce water onto or into your slope.

Figure 7. Root Reinforcement of a Slope

Planning and Installation Goals

Since you have now spent time observing your slope features and have begun to understand what is happening around your slope, you should be able to establish some basic planning and planting goals which address the problems you have noted. You should divide the slope into different areas if this makes sense based on your observations. This may allow you to accomplish different goals in those areas. Now is the time for you to consider any drainage improvements to your slope (see For More Information). These improvements should be compatible with your planting program.

Typically, the slope can be separated into at least three areas: the crest, face, and toe. Planting objectives are usually slightly different in these zones. At the slope toe, you may be concerned with providing plants which resist down-slope soil movements, are tolerant to wet soils or occasional saltwater spray, and can handle changes to coastal marine deposition or erosion processes. On the slope face you may decide to select plants and planting techniques which tolerate a range of soil and light conditions, can handle some soil movements, can resist shallow seated landsliding, manage surface runoff, and are compatible with other slope uses. Along the slope crest you may want to revegetate a buffer area to strengthen soils and reduce erosion. Pulling these objectives together is an important milestone in your planting effort. Obtain assistance if you find the planning process is complicated for your situation.

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Slope Preparation for Planting

The first task of your planting program will be to prepare the planting area just prior to planting, avoiding and protecting against wet weather conditions. This task requires that soil areas be prepared keeping disturbance to a minimum according to the requirements of each planting technique. Once the slope has been prepared, you should mark the positions or alignments of your planned plantings. Excavations should be performed concurrently with plant installations. This will allow installations to proceed effectively and reduce your total time on the slope.

Plant Species Selection

An effective method to properly select plant species involves coordinating your site conditions and planting objectives with the recommendations of local nursery publications and qualified native plant professionals. Plant material can be both purchased or gathered depending upon your own situation or plant availabilities. Your local native plant nurseries and state agencies involved in revegetation programs should be able to assist you in locating appropriate planting materials. The Hortus Northwest Journal to native plant sources is a valuable resource or you can contact DNR and SCS for potential plant sources. You should use Table 3 (Plant Selection Guide) as a starting point for your plant selections. There are still many other species from which to choose. It is generally not desirable to select ivy, Scot's broom or other ornamental exotic plants. Selecting native plants will usually increase the success of the planting program and reduce your long-term maintenance requirements.

Seeding and planting of vegetation should be done carefully. In many cases, grass and legume seed mixtures will have to be seeded by hand scattering along the face of the slope. The seed should then be covered with an appropriate mulch material. For large scale planting on hard-to-reach areas, machines called hydroseeders which spray mixtures of seed, water, and mulch materials are recommended. These applications are commonly used by the Soil Conservation Service and other agencies.

Where broadcast seedings are made, time of seeding for grasses and legumes is very important. Seeding should be avoided in July, August, and September wherever possible as extensive drought periods can occur. Legume-based mixtures should be seeded as early as possible but no later than mid-June. Grass-based mixtures can be seeded before and after July through September. It should be realized that healthy, herbaceous ground covers may require an initial fertilizer application to achieve higher successes. Select native species and use a slow-release formulation (do not over fertilize).

Woody plant materials should come from plant stock which is dormant and should be planted immediately. Materials can be installed up to 48 hours after cutting if they are kept cool and moist by covering cuttings with moist mulch.

Willow, cottonwood, and dog-wood can be planted as cuttings or saplings and are particularly good for seep zones and other wet areas of slope faces. However, avoid planting willows near artificial drains because their roots seek water and may eventually clog or disrupt the drains. When planting other shrubs and trees on slope areas, consult qualified nursery people to determine appropriate species for your conditions.

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Mulching of seeded or planted areas is of particular importance to slope plantings. Mulch protects against rain and wind while seeds are germinating and plants propogating. It also reduces loss of soil moisture during extended dry periods. Because of the severe nature of most coastal slope areas, a mulch cover addition is necessary if vegetation is to be established from seed.

A wide variety of mulches can be used. These range from scattered straw to sprayed fiber. More common materials and methods may include: hay or straw (11/2- 2 tons/acre), jute netting, plastic netting (not recommended), manure or compost (not recommended), wood fiber, or fiber matting. Check with your local Soil Conservation Service office, nursery people, or garden and farm centers to get more information on local availability or suitability for your situation.

One other short-term anchoring method which has helped to stabilize slopes during vegetation establishment in some areas susceptible to shallow soil movements involves "nailing down" a slope face with 5 foot metal fence posts. The posts are driven perpendicularly into the slope face in a grid pattern with 10-15 feet spacings between them. The posts are cross-connected with heavy wire or cable which has the effect of tying the entire slope face together from top to bottom and side to side. The posts should be driven in almost all the way into the ground, wired, and then the slope should be planted and mulched.

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