Bidens cernua L.
Photos by meSept. 2012 in West Des Moines
Despite the name marigold, I don't see much resemblance to the garden variety of marigold.
I had a request to spend some time on the sandbars of the Raccoon River, so when we finally had some time to kill this last weekend, I drove down to a state park with quick access. We found this flowering plant when not much else is in flower. It definitely likes sandy soil. I believe that many variations of this plant exist based on my field guides and internet searches. All my readings have stated that the flowers nod with age.
To study on your own, try reading these two places.