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:

Size classifications (by flower diameter)

Per NC State Extension:

"Dinnerplate" dahlias (flowers 8 inches across) are the category most widely shown in dahlia societies and the largest commonly grown type. Per Penn State Extension, dinnerplate varieties often produce flowers that exceed their advertised size in ideal conditions (rich soil, consistent water, disbudding). 'Kelvin Floodlight' has been documented at 15+ inch diameter under optimal show conditions.

Cultivar selection by use

Per Cornell Cooperative Extension, 'Café au Lait' is the most popular dahlia for home cut flower use because of its unusual muted peach-cream color, large flower size, and long vase life (5–7 days).

'Bishop of Llandaff': the case for single dahlias

'Bishop of Llandaff' is a peony-form dahlia with deep scarlet semi-double flowers, a visible central disc, and nearly black foliage. Per Missouri Botanical Garden, it is a highly award-winning cultivar and a garden standard because of its early bloom, striking foliage, and value for pollinators. Single and semi-single dahlias are significantly more accessible to bees than the tightly packed decorative and cactus forms.

Planting and disbudding

Per Clemson HGIC, dahlias should be planted in full sun after the last frost when soil temperature reaches 60°F. Plant tubers 4–6 inches deep. Space according to mature plant height: 12–18 inches for small types; 24–36 inches for large and dinnerplate types.

Disbudding for large flowers: remove the two buds flanking the central terminal bud on each stem. The plant concentrates energy on the single terminal bud, producing a larger flower. This is standard practice for show-quality flowers.

Overwintering tubers

Per Clemson HGIC, in zones 3–7, dig tubers after the first killing frost:

1. Cut foliage to 4–6 inches above ground
2. Carefully dig tubers with a garden fork
3. Wash off soil; allow to dry upside down for 24 hours
4. Store in barely moist peat moss or vermiculite at 40–50°F
5. Check monthly for rot; discard affected sections

Per Oregon State Extension, the most common storage failure is either too moist (causes rot) or too dry (causes tubers to desiccate). The medium should be barely moist — enough to prevent desiccation, not enough to allow fungal growth.

Frequently asked questions

What is the easiest dahlia for beginners? Per Penn State Extension, decorative types and ball types from the medium-size class (4–6 inch flowers) produce abundantly with minimal management and are more forgiving than dinnerplate types, which require disbudding and staking for best results. The 'Mystic' series (dark foliage, single flowers) is particularly easy and requires no staking.

How do I prevent dahlias from getting powdery mildew? Per NC State Extension, powdery mildew (Erysiphe cichoracearum) is nearly universal on dahlias in humid climates in late summer. It does not kill the plants but reduces appearance. Spacing plants 24+ inches apart improves air circulation. Potassium bicarbonate spray applied preventively reduces severity. The 'Mystic' series reportedly has some mildew resistance.

Can I grow dahlias from seed? Open-pollinated dahlia species and some single-flowered cultivars can be grown from seed, but named cultivars must be grown from divided tubers — they do not breed true from seed. Per Missouri Botanical Garden, seed-grown dahlias are highly variable in color and form and are best treated as unnamed garden dahlias.

When should I pinch dahlias for more blooms? Per Clemson HGIC, when plants reach 12 inches tall (typically 4–6 weeks after planting), pinch the growing tip back to a set of leaves. This promotes branching and produces a bushier plant with more flower stems. For cut flower production from a large plant, pinching is standard practice. For dinnerplate-size flowers, some growers do not pinch, preferring fewer but larger flowers.

Sources

1. Missouri Botanical Garden — Dahlia
2. NC State Extension — Dahlia
3. Penn State Extension — Dahlias
4. Clemson HGIC — Dahlias
5. Cornell Cooperative Extension — Cut Flower Dahlias
6. Oregon State Extension — Dahlia Tuber Storage