Certified GKH Gardening Expert
You can plant seeds up to two months before a frost. The flowers you have should self-seed, unless they are a hybrid. In that case the seed probably isn't viable.
Some sources say germination is higher if they go through the cold treatment.
I once cut down some stalks in the fall, placed them in an area I wanted coneflowers, and the next spring they popped up and bloomed that season.