Answer: The most common plant for covering a bank is crownvetch (Coronilla varia). This fast-growing plant will quickly cover sunny banks, preventing erosion and choking out weeds. It thrives in most any soil. Crownvetch is covered with lavender-pink flowers all summer. Bird's foot trefoil (Lotus corniculatus) is another option--it forms deep roots to help stabilize banks. Both of these, however, are a little "wild" looking--like a meadow. If you were hoping for something more formal looking, you could try creeping juniper (though this plant generally prefers a sandier soil), daylilies, wooly yarrow (Achillea tomentosa) or Virginia creeper (Parthenocissus quinquefolia--very vigorous, can be invasive.)
The plants you choose should depend on how significant the erosion threat is (how steep the bank is), how quickly you need coverage, whether you want continuous flowering, how formal a look you want, and how much maintenance you will do. Once you've decided on a groundcover, you can add a few trees. Hope this helps!
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