A 100% slope is actually 1:1 or a 45deg slope, but I have donned my rappelling gear and planted on vertical faces. Yes, the mulch will have to be replenished from time to time. I wish I could tell you how often but there are some variables. I also can't tell you costs because I don't know about either your plant or labor market. You can also get mulch for cheap or free from tree trimmers and some municipalities offer free composted mulch from green bins. You will also have to weed regardless, but the mulch makes it easier. Aggressive groundcover can itself become a weed, especially if it self sows. Plus, weeds are by their nature aggressive and can compete with a lot of aggressive groundcovers, at least for a little while. When I say shrubs, they can be groundcovers as well. They are just low growing shrubs. You can also mulch heavily (8-12") and sow perennial shrub seed. That is a proven restoration technique, although weeds will still come up. Since you are new to gardening, I will recommend this site, The Informed Gardener, especialy the bits about mulch.

The reasons could be soil related: is your soil adequate for the things you planted? Did you research them prior to planting? I know my soil composition and tend to plant things I know will thrive in it. Second, does the sun/shade pattern give the plants optimum growing conditions? It's certainly a factor along with moisture levels. Be patient and focus on improving your soil. Mulching with overlapped layers of corrugated cardboard will attract worms which will improve your soil. Healthy soil = healthy perennials. Just a suggestion but if you're serious about gardening, do a shade study, close to the Summer Solstice, and mark where your garden areas get full-/part-sun as well as full/ part shade. Then stretch your imagination to plant things that adapt to multiple conditions. I have many things growing in part shade & part sun beds that are generally related to either shade or sun. In healthy soil, they've done well wherever they're planted. I have a patch of garden in my yard that nothing has grown in for years That statement suggests poor soil where even weeds don't grow, which also suggests there aren't enough worms thriving in it to enrich the soil. Worms = worm poop which = worm poop in the soil which = healthy soil along with healthy plants. Last comment is gardening means having patience for plants to do what they do naturally. You aren't in control and fertilizers won't change that. Ma Nature calls the shots and as a gardener, your job is to wait until she's ready. Your job is to plant the right things in healthy soil where they've a chance to survive/thrive. They'll take it from there...

B.B.---we use a 1/2 inch--15 mil drip tape with 12 inch spacing. we use a main line in-line filter. our pump pressure is to high so we have a inline regulator to obtain 25 lbs.avg. we have a dripper-flow rate of .7 gallons per hour. Also a vert. green machine pro fert. injector with a 20 gallon tank set with 1/2 inch I/O which works with our min. flow rate of .5 gallon per minute and max of .8 gallon per minute with a max. water pressure of 28lbs. mark. I buy the tape in 500 ft rolls for around 40 bucks. we do not use fabric but we use blk. plastic 1 mil. 48 inch wide mulch.

Any decent independent nursery can help you pick out a suitable ground cover. And if you do some Googling, you can turn up various sites that will provide assorted lists. Here is just one: CA native groundcovers. "Prostrate cultivars of rosemary would be dense and thick enough to swallow the olives. " Not if that rosemary gets shaded by the tree canopy very much. Rosemary is a full sun plant. It will be much less dense and robust if grown in even partial shade. My GC of choice would be Rubus calycinoides (syn. Rubus hayata-koidzumii), creeping raspberry. A tough, layering evergreen GC to 12" that is both shade and drought tolerant