Dahlia

Dahlias are herbaceous perennials in the aster family (Asteraceae) that are native to Mexico, Central America, and northern South America. The genus is named after Anders Dahl, a Swedish botanist who died in 1789. Dahlias have an erect form and a rapid growth rate and are generally considered high maintenance.

Dahlias need fertile, moist but well-drained soil, and full sun to partial shade. These plants are most often grown as annuals as their tuberous roots can be injured at temperatures below 25 degrees F. Despite their tropical origins, dahlias may languish during the heat of summer. Keep them mulched and provide plenty of water, and they will reward you with a long bloom season, from late summer to first frost. Deadhead spent flowers to extend blooming. Dahlias tend to be tall and may require staking. 

To grow dahlias as perennials in zones below 7, dig up the tubers in fall, dry carefully (do not wash), and store in vermiculite or dry sand at 35 to 45 degrees F. In zones 7 to 10, mulch over in winter. When planting, cover tubers to a depth of 3 inches. Propagation is by division or root cutting. Dahlias can also be grown from seed-germinated selections, but these are typically dwarfs, often not heat tolerant, and are poor bloomers.

The American Dahlia Society recognizes 29 groups based on flower size and shape, including single, anemone, collarette, waterlily, decorative, fall, pompon, cactus, semi-cactus. There are 41 accepted species of dahlias in an array of flower colors, sizes, and shapes, but cultivated dahlias are hybrids of mixed parentage.

Dahlias are at home in cottage, cutting, or container gardens. Add them to a border, as an accent or planted in small groups. They attract bees, butterflies, and hummingbirds. 

Insects, Diseases, and Other Plant Problems: Dahlias are susceptible to phytophthora blight and root rot, powdery mildew, crown gall, viruses, and wilts. They may also be hampered by aphids, leafhoppers, and caterpillars.  

VIDEO Created by Elisabeth Meyer for "Edibles, Bulbs, and Houseplants" a plant identification course offered in partnership with Longwood Gardens.