1006 Dahlia spp.Common Names: dahlia, garden dahlia Family: Asteraceae (aster/daisy Family)
There are about 30 species of dahlias known from the wild, but this profile is about the more than 20,000 garden cultivars that have been created by hybridization and selection. Few of the wild species dahlias are cultivated. It seems likely that many, if not most, of the modern dahlia cultivars came from hybridization and back-crossing D. coccinea, D. rosea, and/or D. pinnata with other wild dahlia species. The garden dahlias are bushy plants that grow from underground tubers. Most have strong, erect stems and attractive toothed or featherlike leaves. The larger cultivars can get 4-6 ft (1.2-2 m) tall. Flowerheads (these are composites!) are often quite intricate and flamboyant, some with literally dozens of ray florets, like psychedelic pompoms. Flower colors may be white, pink, yellow, orange, red or purple, and sometimes mixed. The many cultivars range from miniatures with flowerheads 2 in (5 cm) in diameter to giants with flowerheads over 10 in (25 cm) across. Dahlias are classified in ten (Royal Horticultural Society) or twelve (American Dahlia Society) groups, based on characteristics of the flowerheads.
The many dahlia cultivars were hybridized, bred and selected from ancestors that grew wild in the mountains of Central America, from Mexico to Columbia. Native Central Americans were hybridizing and cultivating dahlias before Europeans arrived in the New World.
CultureDahlias are high maintenance plants. Grow them in an open, sunny area. Mulch well to reduce water loss and suppress weeds. They are heavy feeders, and should be fertilized weekly with a fertilizer high in nitrogen in the spring and high in potassium in the summer. Before planting, blend some bonemeal into the soil. Many dahlia cultivars will require staking. To encourage a bushier plant, pinch off the growing end a few weeks after planting, but before blooming. Deadheading encourages more blooms. Dahlias are susceptible to numerous diseases and pests, and usually require frequent treatment with various fungicides and insecticides. Light: Dahlias need full sun and should not be grown where they will be shaded by taller shrubs, walls or trees. Moisture: Dahlias require a lot of water, but still need a well drained soil. The soil should be rich in organics and rotted manure. Hardiness: USDA Zones 8 - 10. Dahlias do best in a cool, moist climate. They are grown in areas that receive hard frosts, but the tubers must be dug in autumn (right after the first frost), dried upside down, treated with fungicide, and stored over winter. In USDA zone 8, tubers should be mulched for winter protection. In warmer climates the tubers can be left in the ground, but will probably do better if lifted and stored in a dry place over the winter. Propagation: Dahlias are propagated from tubers which are divided in spring, before planting out, or from basal shoot cuttings which are produced in late winter in the greenhouse. Bedding dahlias are grown from seed.
Dahlias are long flowering and can be expected to bloom from early summer until first frost. Use the upright forms in pure or mixed borders. Plant them about 2 ft (60 cm) apart. For the best cut flowers, remove the two side buds that form beneath the primary terminal bud, a process called disbudding. The bedding dahlias, grown from seed, are low growing, usually treated as annuals, and useful in massed beds and in containers. Dahlias are among the most beautiful of garden flowers, but they require a great deal of attention and care, and many gardeners, especially those in the South, find it just isn't worth the effort. If you live in the moutains of Central America, dahlias are for you!
The Aztecs bred and cultivated dahlias in their gardens for their beauty, for animal feed, and for medicinal purposes long before the Spanish Conquest. The Aztecs used extracts of dahlia tubers for the treatment of urinary diseases. They used the long hollow stems of the tree dahlia (D. imperialis) as water pipes. Dahlias were introduced to European gardeners in the late 1700's. By the mid 19th century, Europeans and Americans were hybridizing dahlias by the thousands and had developed such a passion for them that prices went through the roof, much like what happened with tulips in the 17th century "tulip mania". In the 20th century, Europeans learned to extract inulin from dahlia tubers, then convert the inulin to laevulose, a sugar substitute important to diabetics and useful in retarding crystallization in candy and other sugar products. Today dahlias remain one of the most beloved of garden flowers, with international and regional societies devoted to their culture and display. Check out the American Dahlia Society, for an introduction to dahlias and a directory of other interest groups.
Steve Christman 1/25/06, 3/21/08