Do you know the percent grade of the slope? If greater than 50%, you will need to use more structural measures such as terracing, retaining walls or barriers. Slopes between 30 and 50% grade, can be stabilized with a combination of jute mats/straw wattles and vegetation. If less than 30%, vegetation and mulch alone should be able to stabilize the slope. Of course, depending on soil type and other conditions, no amount of established vegetation will hold a steep slope if other forces are present that would contribute to a landslide.

For vegetative options, grasses or ground covers provide the best coverage. This list of Pacific Northwest Plants for Erosion Control will provide some options, as will this list of trees and shrubs for erosion control. It is generally best to use a variety of plants with various rooting depths, so consider a few trees and shrubs along with some groundcovers and grasses. Plants that spread by rhizomes, or underground stems, are fast to establish and provide good coverage for slope stabilization. Native plants will require less water, and therefore less maintenance in the long term. Consider evergreen shrubs such as creeping Oregon grape, salal, red flowering currant, or Nootka rose. For groundcovers, kinnikinnick, coastal, wild, or wood strawberry. For grasses, Roemer’s fescue is a good option. You will need to consider sun exposure and water requirements when selecting the plants. All of these plants will need water in the first year or two to become established.

You might want to consult a landscape professional who can visit the site and look at the situation. They will be able to provide ideas for stabilizing the slope either by structural methods such as terracing or straw wattles, or by installing plants with deep and widespread roots. They can also advise on whether some type of French drain or other measure will be needed to channel the runoff.