I think dahlias would probably work. Mine have been blooming continuously for the last month and will continue until the first frost. I water only every week or two and deadhead when I'm picking flowers for the house, every week or so. They need very well-drained soil to survive our wet winters outside. I dig mine every fall, but I know that a lot of people don't and they seem to do all right for them - not me! I have some pretty heavy wet clay that they don't like at all in the winter.

Black eyed susans are great this time of year, and the asters are just starting to kick in. I got some little asters a few years ago at the hardware store. They were in little 6" pots and were about 8" tall. They've come back each year and make a small bush about 2 1/2 feet tall and wide--COVERED with blooms. Some are white and some are purple. When they went to seed, I took some of the branches and dropped them in beds around the yard--now I've got a bunch of baby asters getting ready to bloom this year. They and the black-eyed susans are totally neglected, and then look fantastic when they bloom.

Here's a photo from last year around the 3rd week of September. The Black eyed susans are a little ahead this year. If you look to the left of the rose, there's a purple aster (I'm not sure what type) and to the right is the black eyed susan.

{{gwi:1090305}}

"Relatively shady" puts some limitations on your selection. Black-eyed Susan (Rudbeckia) is a great late summer perennial with some serious color impact, but it is not an ideal choice for shade. Could work OK in light or partial shade. Unfortunately, the bulk of the shade loving perennials tend to be rather early bloomers. Some of the hardy fuchsias will tolerate modest shade and provide color until a hardy frost, toad lilies (Tricyrtis) are shade lovers that have a late bloom season but are best admired up close - not a long distance flower :-) And there are monkshood (Aconitum) that bloom in late summer in shade that will provide very bright blue flowers. Foliage color may be your best bet.....Japanese forest grass (Hakonechloa macra) or coral bells (Heucheras and their relatives) can add a lot of zing.

Or go with something bigger.....like hydrangeas.

well, its not exactly showy, but my Ceratostigma griffithii just started blooming. it gets going a little more in late september, and blooms till frost. smallish blue blooms, but the whole plant reddens up for fall. so blue on red. pretty cool.

I have several hardy fuchsias in partial shade and they do really well though they are dormant a long time in spring.

'Army Nurse' and 'June Bride' are two that have big blooms like the hanging basket varieties on upright plants.

Shade limits your choices, as said. Hostas and pulmonarias would give you foliage color in shade. Dahlias bloom now but they are full sun plants. Fuchsias do well in part shade and bloom now.

"I have some dahlias in the back yard that I need to divide. Would they work"
They will work.I had some dahlias and they are show stoppers and will bloom late.I guess you know that slugs love to consume them as they start sprouting..

Chelone obliqua (turtlehead) are starting into full bloom right now in the relative shade.

Wood asters bloom in a "relatively shady" spot. They're a bit floppy, still pretty.

There are alot of really nice annuals that bloom all season until the first frost in our area. I don,t know what hieght your looking for but Endless summer hygrangeas are really nice this time of year and for something bigger something like pink diamond hydrangea. My lilies just got done and asters are just getting started.

Cimicifuga are very low maintenance, have gorgeous architecture and wonderful fragrance reminiscent of Philadelphus. Nepeta 'Walker's Low' and 'Six Hills Giant' are still blooming with little blue flowers in lovely clumps. There are hybrid musk roses like Lavender Lassie that will bloom in some shade (at least 4-5 hours of sun a day). Feverfew is dependable for a tiny daisy-like effect. It spreads easily which I appreciate, not least because it's simple to remove in spring if it turns up in places where I don't want it. Lots of unique borage blooms right now, hardy fuchsias of course, sedums like Autumn Joy are lovely and if you trim them in May, will manage some bloom in semi-shade.

Japanese anemones would be a good pick for late color in shade. They take a lot of neglect and are very pretty. The only problems are that they can spread more than wanted and the color palette is limited to white and shades of pink.

For a hot dry place with even no summer water, one of my heathers that has electric magenta blooms like little balloons is a 3' square of intense bloom now, I'm not home to try to look up the name.