Ordinarily I would be among the first people to recommend bromeliads as a large scale ground cover in a shady, surface rooting overhead tree situation, but large leaves such as your Carrotwood trees(?) will tend to accumulate in the rosettes. Other choices mentioned by Michelle/deviant-duhziner, such as succulents might include particular succulents such as Crassula multicava, Aeconiums, variegated Spider Plant/Chlorophytum comosum or other grassy foliaged shade tolerant and Eucalyptus understory survivors such as Dianealla intermedia of D. tasmanica, or some of the larger growing Liriope species. If the trees were thinned out to allow more light, I might also suggest Oxalis spiralis var volcanicola or Asparagus densiflorus var sprengeri compacta, which both appreciate dappled shade rather than dense shade, but also compete well with surface tree roots. Many bromeliads would also work, but they tend to accumulate dead leaves from trees between their foliage, and look messy unless groomed periodically. Anything you might plant in this area will greatly benefit from some new topsoil laid over the existing exposed roots of at least 4 to 6 inches in depth to allow for new plants to get established. I would also suggest that you convert your existing spray irrigation system to drip soaker hoses or emitter lines with emitters at 12 inches on center and spaced about 18 inches cross wise to the slope to give more uniform coverage to new ground cover plants, at a slower rate of application. You will also need to fertilize and water more frequently to allow these new plants to establish. All of the plants I mentioned are pretty good at absorbing leaf litter without needing to rake it up. Kim's suggestion to consider removal of the trees before they can damage your brick retaining walls may be the most logical choice in the long term, as I agree with her that it is just a matter of time before they do start to damage the walls.

I'm in the same boat. They blasted away part of a mountain to build my neighborhood about 3 years ago, and the steep slope that everyone on my side of the street has is quite intimidating. I have to put on my baseball cleats just to walk to the top!! Anyway, the landscaping was pretty shoddy, and the soiil is very rocky. They planted some grass that was supposed to grow to a certain height and then stop (perhaps some variety of Buffalo grass?), but there were so many weeds that took root (we back up to a forest, more or less). I am south facing, and even with the slope, the ground gets long periods of direct sun. I've read that traditional groundcovers like spurge and ivy are more partial shade to full shade plants, so I'm kind of in the same boat as you. I'm tired of trying to take out the weeds that take hold every spring, although I am quite interested in the insect life that I find in there.

Okay, as a follow-up piece of information, I just discovered that the more bare part of the hill has a few jack-in-the-pulpits in the shadier parts, fi that gives any context as to soil and light conditions (though from what I've read, these wildflowers can grow in a variety of conditions).

I have PLENTY of both regular and variegated kerria japonica if you want some suckers, also clerodendrum trichotoma (harlequin glorybower -- a nice five-foot shrub that blooms in white fragrant flat panicles, then the berries turn blue while the brachts stay maroon, and then later the leaves go golden before they drop -- definitely a three-season shrub, though again it DOES sucker). I also have a large abelia (really a part-sun shrub, but truly blooms all summer) that I would like to dig up and replace with a blueberry. I live in Lanham, near NASA -- is that close enough for you? LynnT