Dahlias come in a huge variety, so they are classified in groups. These are decided according to flower shape and size, although categories can overlap a little. When choosing the best dahlia varieties for a long life, though, the main types are pompon, ball and cactus dahlias.
All work well in small gardens and compact spaces. Pompon dahlias have small, tightly packed, spherical blooms. They are 2in across, with individual florets rolled into a tube and flared at the mouth. Ball dahlias are similar, but a bit larger. Small ball types are 4-6in across and miniature balls are 2-4in. Finally, cactus dahlias have petals rolled into tubes, giving a spiky look. Some miniature varieties are less than 4in across.