It seems odd to associate autumn with flowering but Cosmos, a naturalised weed in our country, is always the first sign. I don't know why, but it is the inspiration for bad "art" everywhere. It seems almost mandatory for people with no taste to have a bad watercolour or similar by a hobbyist painter in their house.
At this time of year, every disturbed patch of ground turns into a meadow of mostly pink or white flowers all mixed up with each other. Occasionally, you can see the odd maroon flower but those are rarer.
Bidens formosa is indigenous to Mexico and Southern USA. I think it first arrived here in animal fodder.