I have a similar question that I posted on the hillside gardening forum but no replies yet, so I'm turning here:

Two years ago we had a large pond installed installed in our
sloped yard. The landscape company created an approx. 6' high hill on
one side (they miscalculated in design, but too late to do anything
about that now). We were concerned about erosion, and they claimed that
the mulch and a couple boulders they put on the hill would prevent that,
but it hasn't, and, of course, the 1-yr warranty has expired. Not only
is the hill eroding, along with plants on its edge, but buried wires are
being exposed (low voltage), and a small patio on one side is being
undermined.

The company that installed everything proposes a boulder outcrop
("bowling ball sized" + 2'-3' boulders) plus mulch and planting 10 flats
of pachysandra, at a cost of over $1800. They will re-level the patio
and add "boulder support" for "free" (I'm sure the cost is built in to
the rest).

The project was expensive to begin with, and I'm not happy that they
didn't address the hillside better initially. However, we have to deal
with what we've got. I think a retaining wall would hold the hill better
but probably would be way expensive, especially if it wasn't ugly.

Are we throwing good money after bad to accept their plan? Is
pachysandra a good choice? I've had it at several homes, and I like it
but it doesn't seem to grow very fast. Any better suggestions? The hill
is in a fairly shaded area, on the edge of woods. DC area--zone 6b or
7--and yes, we have deer.

I'd suggest either sweet fern (Comptonia peregrina) or wintercreeper (Euonymus fortunei 'Colorata') for the first poster. No showy flowers to speak of, but both are versatile, hardy plants. And groundcovers won't necessarily choke out or eliminate weeds but will certainly help to retard their development. The longer they are in place and the more complete the coverage, the better the weed control will be.

For the second poster, I'd suggest Liriope spicata: "There’s no groundcover known to humankind that’s as successful as
liriope at preventing erosion on even the steepest hillsides, at
surviving wet or bone dry conditions, and doing all that in full sun or
deep shade. You can yank it roughly out of the ground and cram it back
into the ground and it won’t complain.

Best of all for budget gardeners who need to cover ground, it’ll spread and fill out quite quickly.
Just divide the clumps you bought and they’ll soon be ready to be
divided again and before you know it — no bare ground will be showing,
and weeds don’t stand a chance in the thick mass of sheer liriope." From Sustainable Gardening

Thanks. I have a little liriope in my front yard. I haven't noticed that it's spread much in the few years I've lived here though. I will ask the landscape company about it. The one thing I liked about the pachysandra is that it stays green all year, but it, too, doesn't seem to spread much. We also have a patch of Houttuynia cordata in our yard that is pretty but seems invasive, so I considered that, as well as crown vetch, which grows all over the hillsides in PA and looks lovely, at least from a distance, but also has a bad rep.

There are two types of liriope. Liriope muscari is a clumper - Liriope spicata is a spreader. Using the first as a groundcover is much less efficient as it doesn't spread well enough to provide full coverage. And liriope is evergreen in your climate.

And I wouldn't recommend either the vetch or the Houttuynia. You'll live to regret it.

btw, I am also a landscape professional. I deal with different groundcovers for different situations on a daily basis.

Thanks Gardengal48. And helsharmar best of luck with your ground cover.

Does anyone know of any online nurseries? I'm looking for Wintercreeper ground cover and can't find it in my area. Guessing I'll have to order it online.

Online nursery, try http://www.millersgroundcover.com/prod_silver_dragon.htm
I have not ordered from them, but am going to order some of the silver dragon liriope (spreading). For zones 7-10.

Thanks for the info.!