Your clematis MIGHT be dying but that bleeding heart is just going dormant in the heat of summer and will back x2 next spring. Once it has gone completely yellow you can cut it back to the ground.

Your dahlias ought to look smashing by Sept. 6th.

You almost have to do it with annuals and hope for no frost, or try some late blooming perennials, such as:

Asters
Garden Mums
Heuchera (coralbells) or anything else with color in the leaf doesnt’ need to actually be blooming for the event. How about variegated shrubs?
Toad Lily
Dendrathema
Ornamental Grasses
Japanese Anemone
Turtlehead
Sedum Autumn Fire or Neon

It’s pretty hard to predict bloom dates for anything as it is very weather dependent. Here’s what I would do. Remember that garden centers get plants all year long. I know you have a lot of things to do just before the event but I would get to an independent garden center or nursery about 2 weeks before your event to plug in some last minute color. The closer you are to your event the easier it will be to predict just what will be blooming then. Sometimes the plants for sale will not be in sync with their normal bloom time and it could work to your advantage.

Congratulations !

Mums and asters are the most obvious choices, they always bloom in the fall and are most readily available during that time at garden centers. (you could probably find some smaller 1 gal size ones at garden centers now…I work at Earl May, which is in the Midwest, and we just got our first shipment.) There are also a few varieties of sedum that bloom or turn red, yellow, and purple in the fall that you could check into. Also, don’t forget about ornamental grasses. You can plant them any time of year but during the fall they turn those gorgeous fall colors (reds, browns, yellows, oranges)

There are also some perennials that bloom all summer and that you could probably push into the fall (with some good sun, water, and fertilizer) like cone flowers, shasta daises, and black eyed susans. Also consider witch hazel and scotch heather for a fall flowering shrub. And to add some foliage color, there’s always barberry and burning bushes that turn bright reds and oranges.

For vines, bittersweet gives really sweet orange blooms but they can be kind of temperamental.

Hope those suggestions give you a few ideas.