I'm joining in because I've encountered this problem this year - right in the centre and not so easy to get into faff about planting anything. I almost broke my neck trying to cut them back!
Never noticed this problem before as I'm sure it's never been sunny enough for the plants to go over so quickly!
I thought about a couple of decent sized pots of something just to sit on top of the soil, the pots would be hidden by the plants in front but everything sizable was so expensive.

Its a difficult one Scottish, little private nurseries near here are, like Bilbo says selling off leftover summer plants, and some garden centres have a half price section where the plants need tlc.

Pots of either may help......

Back in the early spring I planted lily bulbs around my poppies. Now the poppies have been cut back, I leave them with stalks around 12" high, the lilies have come through and filled the area. Once they are in flower they will fill that empty area with colour.

I have annual poppies in between fuchsias - they are ready for pulling up just before the fuchsias come into flower. If you cut back many geraniums immediately after flowering you should get a second lot of flowers - not as many as first time but enough to make a difference. Gladiolas are in flower now and should help.