Dahlias (Dahlia pinnata) offer some of the greatest variety in terms of flower shapes and colours of any ornamental species. You can start dahlias from seed or grow them from tuberous roots.
Dahlias produce tuberous roots, underground structures used to store sugars over the winter that will power the next season's growth. Unlike the tubers found on plants like potato and caladium, the tuberous root functions as both a storage organ and the dahlia's root system.
Plants grown from bulblike structures, such as the tuberous root of the dahlia, do produce and grow from seeds. Tuberous roots allow for vegetative reproduction: A single plant generates multiple plants identical to itself. Seeds result from sexual reproduction, and resulting plants have traits of both parents.
Dahlias start easily from seed indoors or out. Plant seeds a 1/2 inch deep and keep the soil moist. You will see shoots emerge in five to seven days. Once the plant grows and matures, it will form a tuberous root that will begin to multiply itself. You can divide this root to obtain more plants.