Answer: Unfortunately, most perennials are not able to choke out weeds on their own. You will need to removed the existing weeds and prepare the soil for this planting just as you would any garden area. You will also need to mulch and weed in between the plants while waiting for them to fill in, and also water them while they become established. During the time you wait for them to fill in, you may also need to take steps to limit erosion if the bank is very steep.
For part sun you might consider Campanula rotundifolia, Vinca minor, or Ajuga reptans; for full sun you might look into Cerastium, low growing sedums, creeping Phlox (P.subulata) and low growing thymes. For a shadier site you might consider the various hostas, Vinca minor and even Pachysandra. To some extent your choice will be dictated by the amount of sun hitting the bank and at what time of day along with what type of soil is there and how relatively wet or dry it is. All of the above bloom although the pachysandra flowers are not really noticeable.
The vinca, ajuga, and pachysandra are usually planted in the spring as small starts. These are sold in quantity for the purpose of ground cover and are usually relatively inexpensive. They will also spread out and cover ground so you can plant them as far as a foot and a half apart or so if you want to reduce your costs. The other plants will vary in price but will spread and also can be divided to increase your stock over time.
Most perennials are relatively difficult to start from seed, but it can be done if you have some skill and patience -- they often take two years to reach a presentable size. Once you have identified some suitable plants you could research their propagation needs and see if you think it is feasible. Also, if you need a huge quantity of plants you may be able to arrange ahead of time to purchase them from a local supplier at a discount.
Your local professional nursery personnel should be able to help you analyze the growing conditions on the bank and identify plants that will thrive there (they may have different or more suggestions) then you would select your favorite from among those. Best of luck with your project!
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