Answer: In most cases where there is a severe erosion problem, two things need to be done. One is to control and direct the surface runoff better, the other is to plant the slope or otherwise stabilize it. In some cases, a mixture of plants is suggested because this will provide a variety of types and depths of plant roots and this is better for holding the soil in place. In other cases, a mass planting of just one plant is adequate. I would strongly suggest you consult with a professional who has training in drainage and grading to determine the best way to handle the water to reduce the erosion before you plant. If the problem is severe enough, it will preclude the establishment of plants, by washing them away and/or leaving an impoverished soil or an impossibly steep situation.
Peaceful Valley Farm Supply offers erosion control seed mixes, and as I suspected, they feature crown vetch and other legumes (clovers) as a foundation to erosion control. Their mixes also contain grasses and some feature wildflowers! You can usually find these seeds at farm supply stores, but if you want to check out their catalog, here's Peaceful Valley's address: PO Box 2209, Grass Valley, CA 95945; ph# 916/272-4769. Good luck!
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