The willow will suck moisture from plants so I suggest doing an all sedum, rock garden. Dragons blood will cover the wall. All of my sedums attract butterflies.
-M

Symantha,
Going through a blizzard here, so with spare time, I'm perusing other forums at Gardenweb and happened upon your post, and oh, how familiar it sounds!
We have a retaining wall 2.5 feet high, and about 100 feet in lenght at the bottom of a 10 feet tall slope with a 45 degree angle. Above the retaining wall, we created perennial gardens 3 feet deep, and then more retaining walls in stages so it would break up the horizontal line in the visual when looking at the gardens.
Your willow sounds like it might be far enough away to play with a variety of plants, and with full sun, you have a great potential.
What we planted includes everything from Phlox to Rudbeckias to Echinaceas to Lilies to Echinops to butterfly bushes, etc. These are all plants that hummers and butterflies love. On the edge to cascade over the walls are;
Creeping Phlox - Can take part shade, but mine are in full sun doing well. Water deeply once a week during really dry spells in the summer. Hummingbird moths love these! And they are wonderful to watch. Butterflies also enjoy them.
Aubrieta (Rock Cress) - Full sun plant which is drought tolerant. I winter sowed (posting a link below in case you might like to explore this forum) a lot of these last year. Very nice. Will cascade with time.
Creeping Baby's Breath - Beautiful cascader! I have white and pink. Great for butterflies!
Alyssums, both Basket of Gold and Mountain Alyssum. Basket of Gold is more of a clump and Mountain is a trailing plant. Basket of Gold is my favourite with silverish leaves and the brightest yellow flowers early in the season. Tolerates dry hot sun places.
If your soil is rich, you might like to try Lipstick Fragaria or Fragaria Variagata. These are members of the strawberry family, and their foliage looks great at the edge of our rock wall. The lipstick is deep green with hot pink flowers, and the variagata has white and green leaves with a white flower. I remove the spent flowers to keep them blooming all summer. If I forget one... well, the squirrel gets the berries and leaves the REAL strawberry patch alone! LOL!
There are a lot of other plants out there that would be great for your area. Try to pick something which is somewhat drought/heat tolerant as the soil above these walls tends to dry out and get hotter than other ground level gardens, especially at the edge. Having said this, if you like something, try it.
Nicole.

Here is a link that might be useful: Winter Sowing Forum

: ) I think you're doing better than you think you are, actually...

now- I've seen things as fragile as lilac and butterfly bushes growing in what amounted to a wind tunnel before, and while they tend to 'lean' real breakage isn't all THAT much of a problem...

so let's back to design, eh?

Lavander is a favorite landscaping plant of mine- takes a while to establish itself- but it really does live forever, and will like the extra drainage by being near the wall...and if you can get the lupines to take around here, please tell me- I tried a few years back, but they weren't happy.

packing the gaps in the wall with 'stepable' groundcovers like Thyme, short sedums, stonecrops, and other alpine plants is never a bad idea- that's what they like best, and 'living' walls are wonderful things, indeed.

for the first year, definately look at the cascading habits of petunias, and sweet potato vines (which come in a stunning deep purple) as well as woodland and creeping phloxes, which will also take well to stone walls...

I think you have a good mix of verticals , clumps, and ground-huggers...maybe a nice burning bush on either side of the stairs leading to the willow?

chinacat_sunflower,

I wanted to do a couple of butterfly bushes and have seeds already started, so I may have to go ahead and try them there just to see. I guess I could transplant them if they dont do well. And would act as a little bit of a wind break for the rest of the plants.

My wall is the commercial type. Windsor stone. And dry stacked. Would planting in the gaps(which there are very few) widen the gaps as the plants grow?

I like the ideas that you have. Anymore? LOL
Symantha

Here are just some suggestions--some are probably duplicates of what you've already gotten.

Sedums--there are so many varieties, just go to a nursery and pick out what you like. Golden Acre spreads rapidly, and is very hardy. Sedum oreganum and Sedum spathifolium are 2 others that are doing well for me. Butterflies are attracted to the flowers.

Thyme--many different varieties, many low growing and mat forming. Flowers are attractive to butterflies.

Prostrate Rosemary--just like the upright shrub, but low-growing and spreading.

Wild Strawberry--evergreen, white flowers in spring, followed by tiny red berries that are attractive to birds. I have this growing over my retaining wall.

Lavenders--there are the tall kind (3 ft tall), and dwarf varieties (eg, Hidcote and Munstead Dwarf; Jean Davis is another dwarf, with white to very pale lavender flowers). Tolerant of sun, poor soil and drought. Butterflies and bumblebees like the flowers.

Hen-And-Chicks--Sempervivens, Jovibari, Escheveria (sp?). Good plants to wedge into small spaces. Tolerant of poor, thin soil. These work well to fill in gaps between stones, or between a wall and pavers. Variety of colors and sizes.

Miniature Bulbs--Botanical (species) tulip and miniature daffodils (some only 6 to 8 inches tall), Muscari armeniacum, and miniature iris. I've planted M. armeniacum between the strawberry plants on my retaining wall, and plan to put mini daffodils along the side of my footpath.

Alliums--ornamental garlic and onions. Tolerant of poor soils and heat, flowers attractive to butterflies.

Dwarf conifers--many species are available as dwarfs; Juniper communis, Lawson's cypress (Port Orford Cedar), balsam fir, Hinoki cypress, etc. Will give some very attractive greenery in winter.

Something I've never tried, and honestly don't know how well it works--but I've heard of growing Clematis as a ground cover. Instead of training it to climb a trellis, just let it trail along the ground.