Dahlia plants grown from seed will produce tubers. When I grew dahlia seeds at home the plants developed a small set of tubers at the end of their first year. As dahlias are tender perennials they do need to be lifted and stored to overwinter dahlias in US hardiness zone 7 and below. They can be planted outside again each spring or summer and the tubers will develop in size year-on-year. The resulting flowers each year will be the same as the first year’s blooms.