In 2015, I had one cosmos plant that came up from seed. As the seed heads dried, I collected the seeds and spread them around. In 2016, from that one cosmos and its seeds, I had a multitude of cosmos all over my back yard. The butterflies loved them!
Now, I have a collection of cosmos seeds from all the seed heads that were produced this past summer. I collected them for three reasons. One, so I could plant them next spring in case none of the fallen ones germinated or got froze out. Two, so I could give them to family and friends who might want some. And three, because the seed heads took energy away from the plant and kept them from blooming as long. It was also wonderful therapy for me to go out and wander through the garden collecting them.
The cosmos also bloomed from July through the first frost, after my daylilies were almost done. So I still had a garden full of flowers the rest of the summer.