Heptacodium miconioides, seven sons flower, is a late summer bloomer that's very fragrant, and fits your bill nicely. Often referred to as a mulit-branched shrub/small tree. More info at the link below. Here is a link that might be useful: Chicago Botanic Garden

I am new to gardening myself and my basil was doing the same. I live in RI and watered every 2-3 days and it looked like I was leaving them in the sun to long but was told I was watering them to much. (Honestly I dk how people can tell.) What I did though and it has improved my plants a little is water every 2-3 days depending on the heat and I started putting them in areas where they had full sun for only 6-9 hrs a day. Once they bulked up a little I put them in brighter spots. This did not fix the problem completely but it did help a lot. Hope it helps you.

One of your first goals will be to slow the water sheeting down the surface, both to reduce erosion and to increase the amount of moisture that soaks in. If you can afford someone to put in a few boulders that you can plant between, that might help. Even chunks of cordwood slightly set into the soil will help and can be left to rot or get covered by the mulch and plants. Use this sort of a shape if possible \__/ (sort of a sideways C with the opening facing into the hill and dug into the slope. You'll want to put down really fibrous bark mulch to help hold soil and reduce runoff while things take root. Another option DH used to hold a very steep slope was getting long wooden stakes (grade stakes) and pounding them into the slope perpendicularly to the slope to brace boards that help hold the mulch and soil while the plants established. Grade stakes are pretty inexpensive and he used slabs (cutoffs from the outside of a tree) from a small-scale sawmill. This wouldn't work with rocky soil, but we have no rocks, so if that's your situation it might help and is inexpensive. I think T2D's slope is a bit steeper in person than it looks in some of the photos, particularly on the far side of the steps. I agree that those beds are stunning! I have a quite steep bank behind my house (SW facing) and at the time I planted it had little time or money to deal with it. I did have a bunch of old orange daylilies crowded into a bed where I wanted to use the space for something else, so I transplanted those about every foot or so and they hold the soil well and have spread. (You may be able to get a bunch of these by offering to dig them on Craigslist or by just looking to see if someone posts that they are available.) I also got another medium height plant that spreads from another gardener (no idea what it is - screaming magenta daisy-like flowers and large coarsely toothed leaves) and the two weave around each other. I put a Korean spice viburnum (V. carlesii) a few feet up from the bottom of the slope and it has done well. A Kousa dogwood is at the bottom of the slope. I mulched it well and renew the mulch every 4 years or so. I'm only now starting to consider what else I might want to plant here, so I'll enjoy seeing what others suggest. When I plant on a slope, I take the extra soil from the hole and use it to make a wall below the plant and create a pocket to help hold water. As far as other specific ideas you mentioned above, you might want to reconsider the wildflower seeds as they tend to need a lot of weeding to get established, need sunlight to sprout (so no mulch) and may well wash to the bottom of that steep a slope. I'd put the Siberian iris near the bottom of the slope as they like it a bit less dry and may need dividing in a few years to bloom well. Bearded iris will tolerate dry soil, but won't help much with erosion as their roots aren't very deep or prolific. Sweet fern will do well anywhere on that slope as it will grow in any poor site IME, but it will sucker and spread, not remain just where you put it. It hasn't been difficult to pull out when I've needed to. I have a white-flowered 'Jet Trail' quince that suckers and is in a relatively dry situation (at least it never gets supplemental water and the soil is sandy.) It seems to top out in height at about 4 feet, but I have to periodically whack it back to reduce the girth as it gets more than 6 feet wide. Some other ideas that occur to me for your situation: Think about sumac - either one of the fancy kinds like fern-leafed or lowgrow or even plain old staghorn. It will sucker also, but is pretty unfussy and likes full sun. Birds like the fruits and the fall color is stunning. Junipers - come in different textures and colors The shrubby Caryopteris have lovely late summer purple-blue flowers. They are low growing and some have variegated or golden foliage, though the original types have bluish foliage. Many bulbs like to be drier in the summer, so they will help add interest in the spring and the roots will help stabilize soil. (daffodils and tulips would be a good start.) Nepeta and Russian sage come to mind as plants that will be OK. Ice plant will work if it's a kind that's hardy enough for you. The western Penstemons or Agastaches like it dry. I planted Agastache rupestris on a steep slope as well as Penstemon pinifolius (pineleaf beardtongue) along with some hybrid forms of both kinds and they have survived most years though they are borderline hardy here. Achillea and lambs ears are happy where it's dry. Look at High Country Gardens website for ideas of plants that like dry and sunny. I have found that many of the plants they sell (like the Penstemons and Agastaches) will grow for me on steep slopes. They tell you what is the precipitation range for each of the plants, but in steep areas I find I can plant things that like it a bit drier than my area is. Regardless of what you plant you will probably need to water some the first year if there isn't regular rain so things have a chance to get established and grow sufficient roots. Soaker hose below the mulch will help with this without increasing erosion potential. Here is a link that might be useful: High Country Gardens

The Miscanthus is behind the daisies which are Anthemis Susannah Mitchell, which is short with small button blossoms. If you can see a little pink on the right, that is a Knock Out Rose next to the Miscanthus, which hasn't had a yellow leaf on it through this dry spell and last year was the last thing blooming in my yard. The Pennisetum Hamlin is very tame and fits right under that BBush. There are a few others that are taller that are nice. Have you seen the pink blossomed 'Karley Rose'? It is just a little larger than the Hamelin. There is also one called Moudry which I think is gorgeous. BTW, here in zone 5/6 I have not seen a seedling from my Pennisetum. I saw a great combo in someone's front yard... Hardy Hibiscus, Shasta Daisies and Joe Pye Weed with Miscanthus gracillimus. It gets huge and probably wouldn't work for you dtd. But the pretty Pink Hardy Hibiscus, with the Joe Pye Weed was eyecatching. Here is that P. Moudry, wish I had room for that! :-) I just love tabs on a browser, don't you? [g] Nandina are very pretty. They have fall color too, right? Bluestem is great and I wish I had room to grow it. I also love Indian Grass which I didn't dare plant so I put it in a pot this year..lol. Not exactly happy there. [g] You know you are talking about prairie plants there dtd! :-) I see from your OP that you mentioned Joe Pye Weed there already that might be staying. I measured my 'Little Joe' just now and it is about to bloom and stands 45" high. First year in, so don't know if it will get larger. I also measured the bed my Butterfly Bush is in and it is 7ft deep at the point the BBush is planted. I did plant it forward, because my neighbor has an evergreen on the other side of the fence that hangs over a good foot and a half. There are about 5 trees right along my fence line behind that whole bed, so that BBush has to deal with dry conditions for sure. I just looked up 'The Blues'...isn't that so pretty! I came across a grass comparison chart in my travels and posted the link below for anyone interested. Here is a link that might be useful: Grasses comparison chart