Blue rug junipers,cotoneaster repens, daylillies, purple cone flowers, gailardia, coranation gold yarrow, evening primrose, blackeye susans, iris', coreopsis, dianthus, lavendars, just to name a few

Daylillies especially are economical and take zero care to get started. Mine were free hand me downs, and I literally took each corm, stuck it two inches in unamended clay in December, and forgot about them. I didn't even water them. They put out green foliage right away, and followed by attractive blooms in summer. Many will rebloom as well, especially the compact stella d'oro (slightly overused, but also easy to find pass alongs for free from neighbors). They are deciduous, but the foliage appears much sooner than many other plants. Mine went dormant in November, and began pushing up new growth in February, so the "no green" period is much shorter than other plants. And they will spread and fill in well. There's a whole forum here about daylilies that you can find a lot more info in.

There's also a groundcover forum for more info. It sounds like a nice opportunity to make a sloping garden to me! Why not mix up many of these plants for greater interest and textural differentiation? Good luck!

Here is a link that might be useful: Groundcover Forum

You definitely have a problem. The only thing that will completely stabilize your slope is clump forming grasses - they have roots that go at least 15 feet down, virtually everything else has surface roots. And many clumping grasses will tolerate clay soil and full sun. But they don't give a groomed garden look. They can look neat and tidy but that involves a lot of work on your part, and slopes are hard to work on.

If this was my problem I would aim for a wildflower meadow look with plenty of tall grasses to try to keep things stable and I would add in the daylilies, black eyed susans and such to keep something blooming all summer long.

I have seen slopes covered in Verbena so some people make that work. It always dies out after a few years in my yard. Some forms of it are amazingly winter hardy and pretty much bloom year round.

I have seen slopes covered in blue rug junipers also. I love the easy maintenance but you have to remove every speck of grass or weeds because they can overpower the shrubs and weeding on a steep slope is difficult. Also, any shrub is going to need a lot of watering to get it established and if this summer is a dry one, that will be hard to do.

If the clay is holding up and not washing downhill you may not need to worry about erosion control at first. It all depends on just how steep your slope is.

If you live in a neighborhood where a wildflower meadow out front won't go over well you could choose one type of plant and cover the entire slope with it. That mass of uniformity will look managed and planned and hopefully solve your problem and depending on what you grow could be virtually maintenance free.

Try cotoneasters. They are very drought resistant once established, are evergreen except for maybe in very early spring when they can drop all their leaves as the new ones come out, and have very attractive berries.

I agree that daylilies are great, the only downside to them is that if you are looking for a plant it and leave it alone type plant, they are not ideal as they will need to periodically be dug and divided or their flowering will decrease over time. Not a bad thing since you can share them with friends or trade them for other plants.

If you go for an evergreen, try blue pacific juniper versus blue rug since blue rug lays essentially flat and blue pacific is more upright. I have blue pacific junipers in various areas of the yard as groundcovers and they are also very drought resistant once established.

Here is blue pacific juniper under the red buckeye in the backyard.
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I have full sun, south. My cascading rosemary still has blooms and is obviously evergreen. It's 3 years old now and probably about 4 feet across and 2+feet high. It grows so fast, smells so good and the tiny blue flowers in cold weather is pretty nice.

I like gardening on a slope. We went through the same decision -- an expensive wall, or try planting it. The planting has turned out to be great. Mine is a big perennial garden, so you may not want that much garden. In my full sun, south facing garden, I have agastache, salvia, stachys, ice plant, grasses, irises, buddleia, coneflowers, monarda -- and much more.

You can look through my gardening blog to see a lot of examples. My outer gardens also have to be deer resistant, if that's a concern.

Cameron

Here is a link that might be useful: gardening blog

Thank you to everyone for your great and varied suggestions. They give me a lot to think about. I am still not sure what to do yet, but at least now I have more ideas than when I started!

I have a similar situation and covered my slope with hardy iceplant in the fall of 2007. I had a lot of iceplants already in my yard, so just broke off branches from them and planted on my slope. It roots very easily, and by spring of 2008 the iceplants were growing well. The iceplants had covered about half of my slope by the end of growing season, starting from cuttings planted about every 2 feet.

Iceplant has a lot of things going for it. It blooms heavily in spring (purple flowers) and then sporadically until frost. It is very drought resistant. It roots easily, grows fast and stays low to the ground. No need to ever fertilize or water it.

The only potential downsides are that it needs plenty of sunlight and good drainage. If you are planting on a slope, drainage should not be an issue. It will grow in the shade, but won't flower well.

I also have a south facing steep slope and planted Homestead Purple Verbena on parts of it. I've only had it there for a year, but it has done really well and grown very quickly. It flowered for a long time last summer and already has blooms on it this year. Didn't seem to skip a beat in my clay soil either.