I plant coneflower and asters and mums for fall color, and blackeyed susan.

You could try amsonia (which flowers relatively early,but would give great backdrop colour in the fall. Bee balm is also nice with a relatively long bloom time (and I love having it in the garden to make tea too....). Golden Marguerite would be pretty or any of the coneflowers too. I don't have a whole lot of experience with full sun gardens, but I know my mom's backdrop plants are purple phlox, black eyed susans and a white penestemon. They work together and then you can do some other highlighter plants

I like the Golden Marguerite idea. I did a gardenweb search and came up with some pics where it was paired with Knautia. They looked really great together.

Baptisia australis/false indigo blooms about the same time as lupine so that's another early season flower. Something I grew last year I thought was fab through the entire season right up until frost and went nicely with the Russian sage too--gaura lindheimeri/wand flower. It's not hardy colder than Z5 but it blooms non-stop with NO fertilizer or even water. The cultivar I grew--a white variety called 'The Bride'--got about 36" tall. Mine are beginning to show signs they're coming back this year but I winter sowed them and WS plants are tough as nails.

Euphorbia polychroma/cushion spurge is another under-used perennial for full sun that requires zero care/maintenance--just comes up reliably every year and does its thing. I've had mine for 20+ years--it's drought tolerant and well-behaved. Adds a different texture to the bed as well plus it can be grown from seed via winter sowing. It's short, however and only grows about 20" tall--nice for filling in between taller perennials.

Japanese anemones are a very tough, reliable late summer bloomer. I assume you also have loads of spring bulbs in there to extend the season at the other end.

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I LOVE anenome, just bought a pink one!!! I call em "My Poor Girls dahlia!" Just cuz I have such a hard time growing dahlias, but these grow invasive good! I have so many "invasive" flowers , it boosts my confidence!ha!

Not specific plant suggestions, but would suggest whenever you feel/see a lull in your garden, go to a display garden (e.g., botanical gardens) or even drive around your neighbourhood, and check out what is blooming. Snap some photos, ID the plant (easier at the botanical gardens where they are labelled, of course), and get some of that for next season.

Even minor variances in zone can impact timing, so (very) local info will always be the best help.

Love the anemones! Are those Whirlwind? I also like Robustissima.

No, they are plain old Honorine Jobert.

Flora, I had Honorine Jobert in my last garden and absolutely loved them. For 10 years I've wondered why I haven't planted them in my gardens now. Your picture has made me put this on my list as a MUST purchase this year. I have a local nursery that has tons of them so I have no excuses.

Zaphod42, how big an area are you talking about? There are some good 3' grasses for fall interest as well as some great asters, goldenrod and sedum. How about peony? Those bloom at the same time as false indigo and the blue with a pink peony is great. My peony foliage turns a nice bronze color at the end of the season.