Types of dahlias by flower form
Dahlias (*Dahlia* spp. and hybrids) are classified by the American Dahlia Society into 18 form classifications based on petal arrangement, plus additional classifications by flower size. For garden planning, the most useful distinctions are the main flower forms, size groups (which affect plant.
Dahlias (Dahlia spp. and hybrids) are classified by the American Dahlia Society into 18 form classifications based on petal arrangement, plus additional classifications by flower size. For garden planning, the most useful distinctions are the main flower forms, size groups (which affect plant height and staking needs), and the division between dinnerplate types for display and smaller forms for cut flower work.
Dahlias are frost-tender perennials grown from tuberous roots. They are perennial in USDA zones 8–10; in cooler zones, tubers must be dug and stored over winter.
The American Dahlia Society form classifications
Per Missouri Botanical Garden, the main flower forms:
| Form | Description | Examples |
|---|---|---|
| Single | 8 petals around a central disc; fully open for pollinators | 'Bishop of Llandaff' |
| Mignon single | Dwarf single-flowered type | 'Topmix' series |
| Anemone | Double center tubular florets ringed by flat petals | 'Wilhelma' |
| Collarette | Single ring of petals plus inner ring of smaller petals | 'Knockout' |
| Water lily | Broad, flat, loosely arranged layers; subtle appearance | 'Gerrie Hoek' |
| Decorative | Fully double; broad, flat or slightly rolled petals; no visible center | 'Kelvin Floodlight', 'Café au Lait' |
| Ball | Tightly formed sphere or globe; small to medium size | 'Jomanda', 'Wootton Cupid' |
| Pompom | Very tightly formed small sphere; under 2 inches diameter | 'Small World', 'Moorplace' |
| Cactus | Fully double; narrow, rolled, quill-like petals | 'Sea Urchin', 'Hamari Gold' |
| Semi-cactus | Between cactus and decorative; petals partially rolled | 'Doris Day', 'Hayley Jane' |
| Orchid | Open-centered; narrow twisted petals | 'Giraffe' |
| Peony | 2+ rows of petals around an open center | Many unnamed types |
| Formal decorative | Fully double; petals in precise spiral arrangement | Most large formal varieties |