Astilbe comes to mind. May not go all the way to end of September but many varieties and colors. I'm sure I'll think of more.

Plastic flowers popped into my fevered brain. . . :-)

Carl

How about chelone or aconitum or asters?

Agastache fits all your criteria but it does reseed a little and if you didn't want the reseeding, you would want to deadhead once before it set seed. Mine has not reseeded very much at all. I do try to deadhead but a couple of times I have forgotten. I've only found three volunteers in the gar*den. Not too bad.

Achillea would fit all your criteria, but it does spread.

Asters need some pre-bloom trimming and may not like the overhead watering.

Callirhoe is great but is not tall enough.

Coreopsis - the grandifloras . I think they fit all your criteria. The grandifloras will reseed a little bit but I have not found them a problem. I guess whether the fol*iage looks good after bloom is a matter of opinion. They do have a casual look to them. The verticilliatas are not 24" tall.

Sedums would be perfect if they bloomed earlier.

What about some of the tall Cranesbills?

Daylilies need some clean up after bloom and not sure whether their foliage is attractive after bloom. I don't grow them.

Nepeta 'Walker's Low'. Fits all your criteria

Perovskia might work.

Rudbeckia 'Indian Summer' is not a runner and reseeds very gently. Keeps blooming with deadheading. Since I do deadhead, I don't know if they bloom without deadheading.

Veronica Crater Lake Blue?

What about some carpet roses?

I forgot a couple....I grow Penstemons. 'Iron Maiden' has a shorter bloom period then I expected and I leave the stalks up after bloom and I like the look of them, but not everyone might. You could cut them back to basal fol*iage after bloom if you wanted. They clump. ' Pike's Peak Purple' is attractive all the time and the fol*iage takes on a yellow cast in fall. it does clump, it has a good bloom period, no care beyond cutting back dead fol*iage in spring. No reseeding that I've noticed. I water overhead all the time.

Shasta Daisies, I hesitate to recommend because I pulled all mine out. [g] I don't remember why exactly. I did buy another one this year, to give it a second chance.

What about Stachys macrantha?

Baptisia, ornamental grasses, maybe lavender.

This really isn't easy.

George, you didn't say if this was for zone 6, but assuming it is for you, it looks like some of the new salvias and gaillardias might work. Monrovia has a fairly good plant filter/definer on their catalogue page, so you can have a look and tailor it as you want. Check Salvia greggii 'Heatwave Sizzle' et al - depends on how fussy one is about reblooming.

Kindly,
Jane

P.S. The Styrax Japonicus you gave me in 2008, is now 9' tall, blossomed prolifically last year and is beautiful. Many thanks, again.

Here is a link that might be useful: Monrovia plants

After my smart-mouth response at the top here (hey, it was after midnight!),
I'll try a more reasoned response today. . .

Of all the criteria you set forth, it's the overhead watering that concerns me the most - one of my commercial clients has a lawn watering system that douses not only the grass, but a complex perennial/annual garden. . .over the years, I've had to edit OUT many perennials that didn't stand up well to
being constantly "rained" upon.

That said, my recommendations might include many suggested above, particularly
Rudbeckia 'Goldstrum'; Sedum 'Autumn Fire' (which blooms, for me, a week
or two earlier than the others); Coreopsis 'Zagreb' (I haven't had any seeding
problems, and they're not 24" but could they be used at the feet of taller plants?); Hemerocallis (which love all the extra watering!); Solidago 'Fireworks' (yeah, the clump spreads, but it perfectly fills all your
other requirements); Siberian iris; Echinacea (not the fussy hybrids, rather
an old reliable like 'Magnus'; and Eupatorium 'Chocolate', ironically, which could handle full sun with all that extra water, and which would provide some much needed foliage contrast. . .Prairiemoon's suggestion of carpet roses is certainly interesting, but I think you'd want landscape roses, for their greater height, such as in the Meidland series. . .some of the "stiffer" ornamental grasses (such as Pennisetum 'Hameln') could work, but definitely not many Miscanthus varities or Chasmanthium latifolium (mine collapsed from overhead watering!).

Mad Gallica was right - this is HARD ! Are you allowed to incorporate 3'
flowering shrubs, because that would certainly open a whole other range of possibilities, and many of them can certainly take care of themselves.

Can't help my innate curiosity, but what's the reason for this challenging situation?

Carl

mad gallica, of course, baptisia and grasses!

Jane, love salvias and some of the gallardias. Talk about constant bloom, I have one gallardia and it goes until frost. Again, that seems like a casual pl*ant*ing and a coreopsis grandiflora would work with these two.

Carl, I thought your previous response was appropriate and comical. [g]

I would have to argue against Rudbeckia 'Goldstrum'. I grew that in a mixed pl*ant*ing in full sun with echinaceas and coreopsis and lots of other pl*ants and the Goldstrum took over the bed. It crowded everything else out. Three years later, I had a heck of a time getting it all out and had to start over. Never again.

I have an Autumn 'Charm' that is the only sedum in my gar*den that develops mildew. I am planning on sho*vel pruning it this year if it happens again.

Roses -- My experience with Meidland roses was so much blackspot that I sho*vel pruned it. A landscape rose that I have had good experience with that I keep to about 3 ft tall, is 'Rhapsody in Blue'. It has been in the gar*den for 3 years and has been disease free and good looking. I fertilize and prune in the spring and that's about it. The roses are singles and only last a day or two but they are plentiful and have a decent rebloom too. The foliage looks good all season. I think it could easily be kept to a smaller size with little effort.

The more reliable Echinaceas are great and easy. Like 'Magnus', 'Ruby Star' and 'Primadonna Rose' are two I've had good experience with.

If as Carl suggested, you could incorporate shrubs, I wouldn't be without my butterfly bushes. They are offering three dwarf versions this year, that grow to 4-5ft. There is also one called 'Blue Chip' which grows 24-30" high and is said to be hardy to zone 5.

pm2

It isn't the irrigation that kept tripping me up, but the no deadheading. A lot of plants that I consider very reliable, easy care, like peonies, really need to be deadheaded or they look awful.

I guess, some explanation is in order.
This is a relatively small (100-120sf of planting area) portion of the large bed mostly consisting of shrubs. This particular sub-bed is a portion of the large bed and defined from the main bed by 12" English boxwoods hedge. Therefore I want to treat it as a separate entity. Shape of the sub-bed is somewhat weird, kind of a tongue with a widest part of 5' and narrowest is less than a 1'.
Sub-bed area is not visible from nowhere and not attended at all except in a summer when we spend time by the pool.
I know, picture wood be much more helpfull, but....

Formerly it was a home for 10-12 HT-roses, but amount of maintenance they required wasn't justifiable for the end result.
So, my idea was to create STRICTLY perennial bed which will be colorfull and interesting during the pool season
(between Memorial Day and end of September), plant it thickly (clumpers) with plants that have at least presentable foliage after the peak (because to do deadheading etc. is not really convinient in a such small place) and just clean it up in a fall or spring and forget about it till next year. Dividing once in a three years I could manage, but not more often than that, please :-)
Not looking for anything super fancy, just bread-and-butter not fussy workhorses will be just fine.

That really helps to understand what you are trying to accomplish and what the space looks like. Then I guess I might consider the following, as one option.....

Sedums of different colors and sizes, Nepeta Walker's Low, Coreopsis verticilliatas, Penstemon Pike's Purple, Pennisetum 'Hamelin', Maybe a Baptisia in the middle of the widest part. They also have a dwarf that only grows 2 feet. Any of the new colors are nice, including a yellow. Callirhoe and or Geraniums as an edging. You could also leave space for a few annual lantana if you need more color and they will bloom all summer and you can just toss them in the fall. You might not have room for all that but any combination that appeals to you. I've found all of these pl*ants, easy care and attractive for a long time. The only one that would enjoy deadheading is the Nepeta, which would give a rebloom, everything else you can just leave until the fall.

I'm sure there are other options that would be just as nice, that is just what I have the most experience with.

What about campanula? C. persicifolia is about 30" tall. White and purple. Other campanulas from low to medium, pinks, whites and purples. Small, medium and large leaves. They DO seed quite a bit, but I don't find them hard to control as they pull easily when young. By mixing varieties you can get a long bloom season.

The two main plants that come to mind:
* geranium Rozanne
* baptisia (foliage remains stunning).

I also like the daylily "Rosy Returns", but I do pull out the dead leaves and also cut off the stalks that are done. But it repeats and looks great all year.

I concur about nepeta Walker's Low, but I do deadhead it about twice a year.

Good luck!

Changed my mind about baptisia. Although it is a favorite of mine, and the foliage stays beautiful after the blooms, if this is your pool area, I'm guessing the prime viewing time is July and August. So the baptisia blooms will be done before you even spend time there. I think an echinacea, or shasta daisy, or geranium Rozanne, or a repeat blooming daylily would be great.

I have geranium Rozanne lining my two front beds and path and it's incredible...profuse booms that lasted until late October last year. In fact I was ready for them to be done because they were clashing with my fall decor. ;-) However, while it is a clumper, it also spreads with runners - which die back in winter completely. This spring they're coming back in clumps. The foliage died back to the ground in my garden, whereas the geranium St. Ola I have in my two back beds turned beautiful fall colors, and stayed evergreen all winter. It's bloom time starts and finishes earlier than Rozanne.

How about one or more of the smaller clematis on a low obelisk or growing through a low shrub. Or one of the herbaceous, non-vining clematis.

Another possiblity is some of the summer blooming bulbs. I've found that the Oriental-trumpet crosses aren't much bothered by lily beetles. Leucojum aestivum (summer snowflake) and Lycoris squamigera (surprise lily) both are summer bloomers, and I think there may be at least one allium that is as well.

Geranium 'Jolly Bee' is another long-bloomer rather like 'Rozanne'.
Balloon flower (Platycodon) has blue flowers in August; it isn't a long bloomer, but has nice foliage.

Thanks for all replies.
So far sib.irises, c.persicofolia, sedum 'AJ'(or alike) and hibiscus moscheutos (probably only Luna series will stay under 4', am I correct?) made it to the list. Phloxes David and Franz Schubert as a mildew resistant cultivars (at least in my garden) also were considered, but they probably would need to be cut in a half in July in order to stay in my size requirements). Considering salvia 'Caradonna' (it reblooms for me in August).
Is any plants with daisy-like flowers grow strictly in clumps? Any daisy cultivars? Heliopsis? Gaillardia? Helenium?

IMHO, daisies, and all the daisy like flowers I can think of, have very ugly, persistent flower stalks that really should be deadheaded. If I don't get to anything else, I have to cut down the daisies.

personally, I think salvias demand deadheading when they are through blooming. I am not a fan of their dead heads. Also I thought Caradonna seemed even worse than May Night for some reason in that regard.

Helenium is a good choice for no fuss. I have two kinds that bloom a few weeks apart from each other - Coppelia and Indiansummer.

What about no fuss Knockout Roses? Depending on JB activity, really just some spring startup maintenance.

Or even The Fairy groundcover roses. JB's don't touch them. I recently read that the Knockout "inventors" have come up with a groundcover-ish version.

Concur with G. Rozanne as a good choice. Self-cleaning.

How about Sedum 'Novem' - a new culivar has pretty foliage and seems to have more upright habit, clump/mound forming, low maintanence, flowers in fall/foliage rest of year...

I am looking for this if anyone sees it... (only found it locally at Avant Gardens)

Here is a link that might be useful: Sedum 'Novem'

This sedum 'Novem', if lives to the hype, looks very interesting.

George,
who tried 'Purple Emperor' at least 3 times and settled for the 'Postman's Pride' which at least semi-alive in my garden.

That looks like a nice S*edum to me too, Sue. I have 'Purple Emperor' that always opens up on me and I don't enjoy the color of the blooms that much. I really loved 'Bertram Anderson' which was almost black but it didn't come back the first spring in the gar*den. I'll keep my eye out for that one too.

Would sedum xenox fit the bill? I got one last year at the annual spring WFF plant sale. Or matrona? (much hardier than purple emperor)