What I learned about gardening on slopes is: first and foremost, you need to be able to walk around on them. You need a path of some sort. The path could be stones, situated like icebergs with most of their mass below the surface, or it could be a formal path zig-zagging up and down, or big concrete blocks discreetly placed at reasonable intervals and hidden by foliage, or a stairway from which one can reach the whole area, or terraces, but to have a satisfying slope experience (I speak to gardeners), something to walk on safely makes a huge difference, even if you only work on the slope once a year.
The above photo is of a slope down the road recently completely re-landscaped. There were eight or so 30' (9 M) pine trees and a thin layer of weeds. The pines were cut down; the weeds removed. The stumps, visible in the photo, were left. The pines were replaced with Crepe Myrtle trees and the weeds were replaced with a small ice plant ground cover. Rings of Agapanthus were planted around the Crape Myrtles, and a row of shrub conifers were planted below the wooden deck (Great fire strategy, guys! The conifers will light that deck up instantly in case of wild fire!) A crew of men was out there for a week doing all that, sliding up and down that very steep slope. I think the Agapanthus may under perform on a slope because they like their water. So, how are the homeowners going to take care of their new plants? Succulent ice plant is not something to walk on. Apparently they plan on no maintenance at all.
The next photo shows the slope a couple of months later. Several times I've seen the crew crawling up and down the slope pulling weeds and re-cutting the pine stumps, which sprouted prodigious new growth due to a lot of irrigation. If you look carefully you can see white circles on the slope, which are the pine stumps. I hope they poured some stump killer on them,else they will be back. There have been lots of weeds, which they've pulled, but the workers so disturbed the ice plant crawling around the slope they exposed soil which will probably sprout more weeds.
The fire-hazard hedge of junipers at the base of the wooden deck has grown. The local Fire Authority sent out some helpful hints about not planting resin-rich conifers below your wooden deck, but I guess they didn't get that letter.
The property next door to that has the same slope. They stopped irrigating years ago, and let their iceplant die back for the summer. It revives just enough with winter rain to stay alive. Because they are not irrigating and the slope is not much disturbed, they don't get many weeds. It doesn't look all that great, but they are not wasting any water on it, either.
Many other slopes on the street stick with invasive trees that have haphazardly sprouted on their own, like Brazilian Pepper and Nicotiana glauca. To maintain views, the weed trees are hacked and topped in random fashion. Too gruesome to photograph, sorry.
A little farther down the street, these homeowners decided to make use of their slope. They terraced it. Or rather they started to terrace it, and for unknown reasons have stopped and started on the project for thirteen years now. It was a half-finished project when we moved into the neighborhood in 1999, and it's still not finished. They've done various things, and then abandon the effort for a year or two, then try again. This year saw the most progress since 1999, but work has ceased in the past few weeks. Will it sit again for a couple of years?
The top terrace was originally flattened out and they had an unsupported wall of soil(!) with pine roots sticking out of it. This year they sloped the top terrace and pulled this year's crop of weeds. There are now drain pipes sticking out with attractive and elegant plastic sheeting laid underneath the pipes to protect the slope. Big 40 foot pine tree right at the edge there. Whew. When they started working on it again this spring, they had a plastic Halloween-decoration type skeleton displayed hanging off one of the walls, showing a sense of humor about the situation.
Two proper terraces plus a solid stairway down to each would have given them a wonderful vegetable or cutting garden--a great use of space. A+ for the idea, D- for execution? Not every great idea works out. At least they tried to make their slope truly usable. And tried, and tried. It's good to try, even when you fail.
Unrelated pretty Clematis attempting to make up for all the text-dreck and ugly slope photos: