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).
Tuberous roots must be lifted in fall in the St. Louis area. Although generally winter hardy to USDA Zones 7-10, many gardeners in those areas still lift and store the roots each winter because such care usually enhances plant performance...
Dahlias may also be propagated from seed. This method is used most often for bedding dahlias. Because of cross-pollination, they will not come true to cultivar. Harvest seeds in September and October. Cut the ripened flower heads open and lay them out to dry on shallow trays. When dry, the seeds will readily separate form the chaff. Store them in sealed containers. Seeds can be sown indoors in February or March; germination takes about one week.