You definitely have a problem. The only thing that will completely stabilize your slope is clump forming grasses - they have roots that go at least 15 feet down, virtually everything else has surface roots. And many clumping grasses will tolerate clay soil and full sun. But they don't give a groomed garden look. They can look neat and tidy but that involves a lot of work on your part, and slopes are hard to work on. If this was my problem I would aim for a wildflower meadow look with plenty of tall grasses to try to keep things stable and I would add in the daylilies, black eyed susans and such to keep something blooming all summer long. I have seen slopes covered in Verbena so some people make that work. It always dies out after a few years in my yard. Some forms of it are amazingly winter hardy and pretty much bloom year round. I have seen slopes covered in blue rug junipers also. I love the easy maintenance but you have to remove every speck of grass or weeds because they can overpower the shrubs and weeding on a steep slope is difficult. Also, any shrub is going to need a lot of watering to get it established and if this summer is a dry one, that will be hard to do. If the clay is holding up and not washing downhill you may not need to worry about erosion control at first. It all depends on just how steep your slope is. If you live in a neighborhood where a wildflower meadow out front won't go over well you could choose one type of plant and cover the entire slope with it. That mass of uniformity will look managed and planned and hopefully solve your problem and depending on what you grow could be virtually maintenance free.

Northern Lights Azaleas are hardy to Zone 3. The whole series is amazing, and definitely worth looking into. I have three of them: Mandarin Lights, Northern Hi-Lights and Tri-Lights. They are very floriferous and fragrant too. They are in different, but all difficult points in the garden. I have had them now for five years now, and the Mandarin azalea (the one on the northern slope)is the biggest- about 1.2 meter high and likewise in diameter. Regarding Hydrangea, I will probably go for Endless Summer. It is also blue, it's hardy to zone 4 and blooms for the whole summer supposedly. Yes, irises don't like it in the shade, regardless what the experts say. Thanks for the daylilies idea- I think that they will look good there. I already have the euonymous and vinca there, some hostas, clematis (three types)out of which Guernsey Cream is the most amazing in that shade, because it is big and white. I might try Astilbe. Thanks!

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.

Ceanothus, lithodora, prostrate rosemary, helianthemums and most of the typical cascading rockery plants would prefer a full sun location although a south facing wall, despite the shade cast by large trees, may be enough of an exposure to encourage the earliest bloomers to flower (before the trees fully leaf out). But none have an especially long bloom season anyway. Creeping phlox will tolerate a shadier situation as will the creeping campanulas like C. poscharkyana or portenschlagiana and their hybrids. Some hardy geraniums might work, as well.