Answer: In my experience, quick coverage is important in such a situation both to stop erosion and control weeds. Banks also tend to be dry. For quicker coverage you would be able to propagate a large number of viable plants faster by dividing and rooting tip cuttings. A creeping form of sedum would do well on the sunny side of the bank, as might creeping phlox, creeping thyme, spreading junipers or cotoneasters or even the old standby's vinca minor and English ivy. If it is shady you might look at the traditional groundcovers of ajuga or vinca or ivy or pachysandra. You might also consider a stoloniferous form of hosta or perhaps some ferns. To some extent your selection will depend on the type of soil used for your berm. Hemlocks do best in a humusy and moist soil, so if you can provide that for them then ferns and hostas should do well also. It may however be difficult to keep a berm moist without a supplemental watering system in place.
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