Location #1

Very-very steep slope (I'd say 60 degrees or more) of 100'+ in lenght. Width is 12' in its narrowest point and 36'+ in its widest one. Direction of strip is strictly S-N, W is the top, E is the buttom.
Observation points mostly from the E (W side is bound by the street, E side is bound by 2-3' high retaining wall and a driveway).

What is there:
Grove of MATURE (and relatively healthy) spring blooming trees and shrubs.
I have no clue how the previous owners managed to plant and initialy water them on a such steep slope, but here is the list of what is there:
9 Cornus florida,
1 Cornus kousa,
1 Kwanzan cherry,
2 Weeping Yoshino cherry,
25+ different azaleas,
7-8 different mid-size rhododendrons.
Technicaly it's an unobstructed full sun location (from sunrise to sunset). However high and not very dense canopy of above mentioned trees creates a very bright part-sun conditions for the understory plants.
As perennials goes Geranium sanguineum 'Max Frei', Dicentra eximia and Platycodon grandiflorus 'Sentimental Blue' (dwarf balloon flower) absolutely adore such conditions and being let to selfseed created substantial colonies here and there. That's it. No other perennials.
To my surprise not too much weeds either despite the fact that whole slope-bed is not mulched except by natural mulch of the tree's fallen leaves.
No sprinkler or any other means of watering cause closest water bib is about 100' away.
Sorry for the long description, but I was trying to give as much info upfront.

Now is my question:
What kind of perennials I could/should introduce to this bed that a) will like such conditions, b) will bloom in mid and late summer, c) could produce an impact from the distant view d) will be self-sufficient in a term of maintanance, meaning fall and/or spring cleanup would be the most they'll receive?
My fantasy is not going beyond the usual suspects: rudbeckia, echinacea, shasta daisy, monarda, but even with those my experience is limited to my well watered beds, so please help me!

coreopsis,gaillardia,salvias,cat mint,per.mums,soapwaort,candytuft. I have these planted on a section of town owed land and never water them!

I have lots of sedums,succulents and prickly pear growing in a spot that I never water. I also have eryngium blue hobbit in there. I have a few dark purple and a few var. sedums that really are very pretty that I could share cuttings of. I think liatris,yarrow,iris and daylily would do well after initial watering in.

If your interested I could share several pads of my yellow prickly pear cacti. They are very pretty in bloom and require no care. I also have a few salvia framboise that I removed, potted and ready for the taking. OH, I just thought of salvia artemis,pretty silver foliage and a nice surprise after year one of funky white flowers.

I have daylilies in a similarly steep west-facing slope and they thrive. It's a bit sunnier than your son's and gets some roof runoff, but no supplemental water at all and now after a month with less than 1/2" rain total they seem perfectly happy. I find daylilies grow well in conditions ranging from full sun to full bright shade, though fewer flowers in shade. At the bottom of the slope where there will be more runoff, you can try Siberian iris which, though it is a spring bloomer, will look nice with your other spring bloomers and has tidy foliage the rest of the season. I have one of the goundcover thymes growing under shrubs where it gets perhaps 4 hours of sun daily, if that, so you could try that in one of the sunnier spots to get as many flowers as possible for impact. Look into some of the other geraniums (I think G biokovo has been recommended here several times) to see if any bloom later in the season. Think about hosta, especially some of the more sun-tolerant gold forms, which will add both color and texture. Mine never get watered and don't seem to suffer.

You could scatter seed of some of the self-seeding annuals and half-hardy perennials and see where they are happy, like annual allysum, Verbena bonariensis, annual poppies, nicotiana, California poppies, etc. My Nicotiana was started from seed years ago and has perpetuated itself since then in shades of red, purple, pink, and white starting early summer and continuing to frost. As a bonus, their scent that wafts through the gardens and house in the evenings is a true delight.

Depending on how rural it is, you could also try some of the native field wildflowers like asters and goldenrod or their cultivated counterparts if they will tolerate the part shade and dryness as their tendency to seed around will be an asset here.

Some of the easy bulbs disliked by the critters like daffodils or the really large alliums can go in the sunnier spots and will need no care at all (though they will be spring and early summer blooming . . . ) Colchicum will put out leaves in the spring and has very bright pink fall flowers. Although relativley expensive, I find that they will increase slowly over the years and you will have some bright blobs of fall color.

I might also add some mulch once it is planted to help keep moisture in and weeds down. I use coarse chipped tree trimmings on my steep daylily slope and renew it about every 4 years or so.

My gut reaction when I read 25+ azaleas and steep slope was a rhododendron/azalea dell with falling water as well. There is an extraordinary young one at Coastal Maine Botanical Gardens that I have some photos of (in some file) that I would be happy to share if you�re interested.

Babs made a very good suggestion to incorporate biennials and half-hardy perennials as a scattering of seeds now will produce plants the will start the cycle of re-seeding for years to come with little or no effort. Allen Armitage has a great book Armitage�s Manual of Annuals, Biennials, and Half-Hardy Perennials that is a really wonderful resource. As you are looking for distance impact with flowers I personally believe that one flower blooming at a time rather than watering down the distance viewing with multiple plants blooming at the same time makes the biggest impact and of course the larger and brighter the bloom the better.
If you find yourself frustrated by the lack of water 100' away Joe and I brought water to our woods (100� away) relatively inexpensively by install basically a manual irrigation system that we use at our discretion.

Black pipe leading to 4" black pipe (lays above ground) brings the most volume of water the furthest distance.

We created different zones using zone valves that need to be turned on and off manually.

Using soaker tubing on my berm

Direct tubing to specific shrubs in the woods

The smaller tubing can be re-designed or repaired easily.

In my garden, daylilies need to be dead-leafed on a regular basis or they look terrible, and I've got very low standards as far as tidiness in the garden goes. Maybe it has to do with the varieties I grow?

Much of my slope is planted with old-fashioned orange ditch lilies which were in the regular gardens when we moved here since I wanted to keep many of the existing plants to keep something of the garden history of this old farm. I find that their foliage looks all right all season (at least from the distance I observe them) and I am interplanting some more recent varieties to extend the season. In general their fading foliage is hidden by the slightly taller older varieties' foliage. My lemon lilies (I don't know if this is just a local term for an older type of tall yellow daylily that I got at a plant swap) also have nice looking foliage for most of the season. The one thing that looks messy about my daylily bed is the leftover stalks after bloom, but in my case, no one is going to be standing in the full sun of the back field to look at the back of the house in mid-August . . . However, they seem to pull out easily with just a tug, so it's only about 20 minutes of work the few times I actually cleaned them up.