It is always less expensive and also horticulturally wise to buy small plants of chrysanthemums and asters and plant them in the spring. Pinch them back as they grow in your garden so they are nice and bushy when they flower next fall. Remember to stop pinching after the 4th of July. You can also start new plants from cuttings from your chrysanthemums and asters.
Sedums come in many varieties and are good fall bloomers, though must be sprayed often in gardens visited by deer. Grow the tall sedums by your asters and the creeping ones to edge your beds. They don't mind hot dry conditions.
Helianthus and helenium are also good fall bloomers, so be sure to include some of those perennial plants. I rely on Helianthus multiflorus for September bouquets, and these double-yellow flowers combine well with sweet little wood asters that spring up like weeds in many gardens.
Annuals such as blue salvia, marigolds and zinnias, (all deer resistant for me), are staples for providing autumn color, as are the pink and white Japanese anemones, which are perennial.
As for a good shrub for fall, plant Callicarpa dichotoma, sometimes called beautyberry, as it has yellow foliage and lustrous purple berries. The red berries on Viburnum triloba (zones 2-8) are also good for fall color.
Get lots of these September favorites planted in your garden now.