Many different species of plants can be planted via hydroseedixing a slope, but such mixes aren't typically using the same sorts of plants most often used as ground covers planted as rooted cuttings from flats/4" pots/one gallan cans. Hydroseeding still requires certain procedures and follow up care and watering/maintenance to be successful. If you were to plant fast growing wide spreading more shrubby plants to cover your slope, it may be competitive price-wise and give you more options of plant types. You may find it worthwhile to hire a landscape architect/landscape designer/landscape contractor to meet with you on-site, review your conditions/expectations/budget and advise you what makes the most sense for your yard. There are always trade-offs for any approach, and hydroseeding may/may not be a good option for you.

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.

QQ thanks again for a good idea. I can visualize that area as a dwarf conifer garden but that is not what I have planned and I am 1/2 way through my plan. The area that I would juniper is only a pie shape 2' wide and 6-8' long section of the slope. If the deciduous and lavender/sage/grass don't work out,I may conifer/rock garden the slope. The slope is only set back from the road 10-15' and it is on the property line. I would need to import a good amount of natural, local rock.

NHBabs, thank you for the photo - it's very helpful to see a visual! Let me show you what I'm dealing with as far as the entire yard is concerned: Not sure if it's entirely clear, but the yard is actually comprised of two slopes, divided by a flat area (that old post and beige rock on the far right are in the middle of the flat area, which is about 6-8' wide). Each of the sloped areas is about 10' and the angle is quite steep. The top slope is fairly evenly angled but the bottom one has a sharp drop off about two feet above the retaining wall. This creates a good bit of flat space behind the wall, where I am planning to plant herbs and peppers (the kitchen opens right out to the back to this area, so this will be very convenient). I am going to rip out that bed of pachysandra in the lower left corner and all of the plantings in the center to create as much space as possible for edibles. The elderberries I mentioned on the other thread could maybe go on the flat area, but I am planning to plant a dwarf cherry there, which I think will eventually dominate the area. I think I'm just going to put all of my berries (raspberry, blackberry, blueberry and elderberry) in pots for now, then transplant all but the blueberries once I have a better grasp of the sun patterns, etc. Anyway, I think we might as well just kill all the grass in one go, mulch and then start with the vinca. It sounds like the grass' root system will keep the topsoil in place even when the grass is dead, and hopefully a fine/shreddy mulch will stay put long enough for the vinca to get going. I really appreciate your advice on this technique!