Answer: Well, with that slope I bet you want something fairly low maintenance! Is erosion a problem? (It usually is on such a steep slope...) Crown vetch is a common, hardy ground cover that grows to a height of about 2 feet, and is covered with pink or lavender flowers from early summer to frost. It is very good at preventing erosion and extremely low-maintenance; however, it is fairly "wild" looking.
Another possibility is daylilies. They are also hardy and make an attractive, low-maintenance cover. If the hillside is moist (perhaps leading down to a stream...) then various types of small willows might do well there.
There are so many options, it might be helpful for you to try to narrow down your ideas. For example, do you want a mix of plants, or all one type? Are flowers important? Is erosion a problem? (Will new transplants simply wash down the hill? If so, you'll want something that takes hold quickly.) What height of plant do you want--a very low-growing ground cover, 10-foot shrubs? How is the soil? Is it rocky/poor? Then a very durable plant is called for. If it is rich and loamy, then you have more choices. How much are you willing to spend? To cover the slope quickly, you would need to purchase quite a few plants. You could plant wildflower seed--but this does best on a prepared site that has been tilled and the weeds removed. (It would be all but impossible to till that slope, I bet!)
Hopefully, this will give you some food for thought...Good luck!
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