Ask Mr. Smarty Plants

We recommend grasses for controlling erosion because of their extensive fibrous root systems that serve to hold the soil in place.  However, I don't think just throwing grass seeds over the side of your bank is going to work very well.  The seeds need moisture to germinate.  If the moisture comes in the form of rain, it is likely to wash the seeds down the bank into the river before that have a chance to germinate and take root.  There are two possible solutions—an erosion control blanket or pneumatic compost/seed application.  The erosion-control fabric works by slowing the runoff water and allowing sediments to fall out rather than be washed away. Seeds are sown under the erosion-control material and grow up through the matting when they germinate. You can also insert plants into the soil by cutting through the matting. The roots of the plants that are growing through the erosion-control material anchor the soil to stop the erosion. If you use erosion-control blankets made of biodegrable material, they will eventually disappear leaving the plants to control the problem.  Many nurseries carry this erosion control fabric. 

The compost/seed application may be a bit more complicated and expensive than you had in mind since it does require a pneumatic blower, or some mechanical means, to spread the compost/seed mix. The US Composting Council offers information about suppliers of compost and compost technology, but we don't really know if this could be a do-it-yourself project.  You might check with a landscaping or environmental consulting company in your area who might have the machinery to do this to learn about the feasibility and expense of applying the compost/seed mixture this way. You can find the names of Landscape Professionals and Environmental Consultants in your area that specialize in native plants by searching in our National Suppliers Directory.

You can go to our Native Plant Database, and do a Combination Search for Oregon, with "grasses" for the Habit, and put in the amount of sun you have on your hillside. You will need to read the soil requirements to be sure the grasses you select will thrive in clay. We are going to choose some samples, but since we don't know what amount of sunlight your bank has, we are not going to choose on a Light Requirement. Follow the plant links to the webpage on each plant, and learn heights, soil types, sun, water needs, etc. on each plant. All those we have selected are native to Oregon and tolerate clay soils. Another resource that is closer to home and probably frequently deals with the same problem you are having is the Oregon State University Extension Service for Douglas County-Horticulture.

Grasses for erosion control in clay soil in Oregon

Aristida purpurea (purple threeawn)

Bouteloua curtipendula (sideoats grama)

Elymus canadensis (Canada wildrye)

Juncus torreyi (Torrey's rush)

Aristida purpurea

Bouteloua curtipendula

Juncus torreyi

Juncus torreyi