Dahlia

Prepare a planting site in full sun with rich, well-drained soil amended with generous amounts of compost. Dahlia tubers should not be planted until the soil has warmed to at least 60 degrees Fahrenheit, typically two to four weeks after the last frost. Dig a hole six to eight inches deep, lay the tuber horizontally with the eye or growing point facing up, and cover with two to three inches of soil. Do not water until shoots emerge, as wet tubers are prone to rotting.

As plants grow, gradually fill in the remaining soil around the stems. Install stakes or support cages at planting time to avoid damaging tubers later. When the main stem has three to four sets of leaves, pinch out the center growing tip to encourage bushier growth and more flower stems. Remove the two smaller side buds that form below each terminal flower bud for larger, showier blooms.

Water deeply two to three times per week during active growth and flowering, and apply a layer of mulch to conserve moisture and keep roots cool. Feed every two to three weeks with a low-nitrogen, high-phosphorus fertilizer to promote flowering over foliage growth. After the first frost blackens the foliage, cut stems back to four inches, carefully dig tubers, and store in a cool, frost-free location for winter.

The dahlia (Dahlia spp.) is native to the mountainous regions of Mexico and Central America, where it grows in volcanic soils at elevations between 1,500 and 3,700 meters. The Aztecs cultivated dahlias for centuries before European contact, using the hollow stems as water conduits and the tubers as a food source called 'acocotli.' Wild dahlia species, of which there are approximately 42, typically bear single flowers in shades of lavender, red, and orange.

The first dahlia tubers arrived in Europe in 1789 when Vicente Cervantes, director of the Botanical Garden of Mexico City, sent seeds and tubers to Antonio Jose Cavanilles at the Royal Gardens of Madrid. Cavanilles named the genus Dahlia in honor of the Swedish botanist Anders Dahl, a student of Linnaeus. Initially cultivated as a potential vegetable crop, the tubers proved unpalatable to European tastes, but the striking flowers quickly captured the attention of ornamental gardeners.

By the early 1800s, dahlia mania had swept across Europe, with collectors paying extravagant sums for novel varieties. The first double-flowered forms appeared around 1806, and intensive hybridization throughout the 19th century produced an astonishing range of flower shapes, sizes, and colors. The pompon and cactus-flowered types emerged in the 1870s, and the enormous dinnerplate varieties became popular in the early 20th century. Today, dahlia societies exist worldwide, and competitive dahlia showing remains a beloved tradition in many countries, with the American Dahlia Society classifying blooms into fourteen distinct flower forms.

Dahlias demand rich, well-drained soil with a pH of 6.5 to 7.0. Heavy clay soils should be amended with compost and coarse sand to prevent tuber rot. Work a generous amount of aged compost into the planting bed before setting tubers. Use a fertilizer low in nitrogen and high in phosphorus and potassium, such as 5-10-10, applied every two to three weeks during the growing season. Too much nitrogen produces lush foliage with fewer, smaller flowers.

Cut dahlia stems for arrangements in the early morning when flowers are about three-quarters open. Choose blooms where the outer petals are fully unfurled but the center is still tight. Cut stems long, placing them immediately into hot water (about 160 degrees Fahrenheit) for one hour to sear the stems and extend vase life. Deadhead spent flowers regularly by cutting back to a main branch to encourage continuous blooming until frost.

Dahlia tubers must be dug and stored in cold climates where soil freezes. After the first frost kills the foliage, cut stems back and carefully lift clumps with a garden fork. Rinse soil from tubers and let them dry for a day in a sheltered spot. Store in boxes of slightly damp peat moss, vermiculite, or wood shavings at 40 to 50 degrees Fahrenheit. Check monthly through winter and mist lightly if tubers appear shriveled. Fresh-cut dahlias last four to six days in a vase.