I have an annual Dahlia and I would like to save some seeds for next year. I've heard it is easy, but I don't know how to do it. Most of the flowers have dropped their petals. The plants are about 8" tall. What do the seeds look like and how do I collect them?
In zone 6 and below they are treated as an annual.
Instead of collecting seeds (which can be done, see below), it's generally recommended to dig up the tubers (roots) in the Autumn (Fall), clean them, store them, then replant them in the Spring a week or two before the average last frost date (or start earlier in pots indoors).