If you’re looking for a fall-blooming flower and don’t mind if it takes over your space a bit, perennial Ageratum is a good choice.
It wants to spread and does so via rhizomes, so it’s perfect for filling in a large area quickly. Nondescript throughout the summer, it comes into its own in late September or early October, and will bloom straight through the first frost. The color is quite vibrant, contrasting nicely with russets and oranges of autumn. Right now mine is blooming next to bright pink asters, and I can almost pretend I’m warm.