Getting started early in the season means you'll have your pick of beautiful dahlias. Dahlias come in purples, reds, oranges, pinks and whites and offer ample texture to an arrangement or stand on their own quite well. Sunflowers are also usually around at the very beginning of fall.
If you're a purist, chrysanthemums -- better known as mums -- are the quintessential fall flower. Available in a handful of varieties, mums are long lasting even when they're cut. Goldenrod is a perennial with small, bright yellow flowers that looks great in a fall arrangement.