Cultivars
Single-flowering dahlias
These open flowers are up to 100mm (4in) wide with 1 to 2 rows of ray florets that may be overlapping and surround tiny disk florets.

Waterlily dahlias
These flowers range from miniature to giant and produce double flower heads with a few flattened ray florets that may be in or outwardly curved along the margins and resemble a waterlily.

Collerette dahlias
These flowers grow to 150mm (6in) wide and produce a single row of flat ray florets that may be overlapping and surround a collar of shorter florets that encircle the central disk.

Anemone-flowered dahlias
These double flowers grow to 150mm (6in) wide with or more rows or flattened ray florets that surround a dense group of upwards tubular florets.

Pompon dahlias
These flowers are up to 50mm (2in) wide with incurved florets for there whole length forming a very spherical shape.

Ball dahlias
These fully double flowers range from miniature to small with spirally arranged incurved ray florets for up to half of there length forming a ball-shape.

Semi-cactus dahlias
These fully double flowers range from miniature to giant forms with pointed ray florets that are broad at the base and recurved upwards for half of there length and straight towards the centre. 

Cactus dahlias
These fully double flowers range from miniature to giant forms with narrow pointed ray florets that are strongly recurved for more than half there length on the outside and straight or curled towards the centre.

Decorative dahlias
The fully double flowers have no central disk and range from miniature to giant forms. The ray florets are broad flat or slightly incurved along the margins with a blunt apex and may be twisted. 

Miscellaneous dahlias
The flowers range from miniature to giant forms and consist of dahlias that do not fall into the other groups and include orchid-chrysanthemum type, peony-flowering, star form and lilliput dahlias.