Dahlias produce some of the garden's most spectacular flowers. Indeed, the "dinner plate" type of dahlia often produces flowers 12 to 14 inches in diameter if given proper care. Although dahlias flower throughout the summer, the warm days and cool nights associated with fall causes them to produce greater numbers of flowers with more intense, vivid color. However, since hardy chrysanthemums and asters also make their annual displays of color at this time of the year, dahlia often is overlooked as a fall flower. September is a good time of the year to enjoy the explosion of color this popular garden flower is known for and to start planning for dahlias in the garden next spring if you do not already grow them.
Dahlia is both the common name and genus to which this native of central Mexico belongs. Although it is a perennial in its native habitat, we treat dahlia as an annual because of its sensitivity to cold temperatures. Dahlia is a member of the Asteraceae (Composite) family and is the National Flower of Mexico. It was being grown by the Aztecs when the Spanish conquistadors lead by Cortez arrived on the scene in the 16th century. The Spaniards brought with them botanists who were to select plants to take back to Spain; dahlia was one of the plants selected. It originally was given the Genus name Georgina in error, a name by which it still is known in many parts of Eastern Europe. The current name of dahlia was given by Abbe Cavanille in the late 1800's in honor of Andreas Dahl, a Swedish botanist and environmentalist. Early dahlias imported into Europe were of the single flower type and probably belonged to the species D. pinnate, D. rosea and D. coccinea.
As hybridization work began between the more than 30 species of the genus Dahlia, the first fully double-flowered forms made their appearance along with new color combinations. Two hundred years later, dahlia has one of the largest arrays of flower forms, colors and sizes of any cultivated plant and today more than 50,000 cultivars in 795 classes have been named and registered. Dahlias are classified according to flower size, type, and color. The flower itself actually is a compound inflorescence known as a head which contains both ray florets and disk florets. There are nine different classifications of size, fifteen of color, and twenty of size accounting for the 795 different classes. Readers are invited to visit the web site of the American Dahlia Society (www.dahlia.org/) for a complete description of the various classes of dahlia.
When most people think of dahlias, the large-statured types propagated from tuberous roots each year come to mind. Indeed, the majority of named dahlia cultivars are of these types which are popular for the cutting garden or the backdrop of borders. However, because of their height and large flower size staking or some sort of additional support is often required. In recent years, dwarf or bedding dahlias propagated from seed have gained much popularity. Bedding dahlias produce small flowers in great abundance on bushy plants making them ideal for annual beds or in the forefront of borders. Although these dahlias also form tuberous storage roots they usually are not saved from year-to-year because of the ready availability of inexpensive plants in the bedding plant market each spring.
The tuberous roots of dahlia can be planted about 14 days before the frost-free date for an area . If plants have been started indoors, do not set them out until after the danger of frost has passed. Plant dahlias in a location that gets six to eight hours of direct sun in an airy location protected from high winds. Dahlias are fairly heavy feeders and do well in fertile, well-drained garden soils high in organic matter. If soil lacks the latter, incorporating up to four inches or well-rotted manure, compost or other forms of organic matter before planting is a good soil management practice. When preparing the soil for dahlias, incorporate about one-fourth pound of a general purpose garden fertilizer (e.g. 12-12-12) for each 10 square feet of garden area. Top dressing with an equal amount of fertilizer in July will help fall blooming. Dahlias grow rapidly and consume large amounts of water. Soil should be kept moist but not extremely wet. Organic forms of mulch can help to conserve water while at the same time controlling weeds.
Dahlias benefit from pruning which is preformed according to the intended use of the plant. Plants destined to producing exhibition type flowers should be pruned to one main stem. Plants whose purpose is to produce a lavish display in the garden should be pinched after initial growth in the spring reaches a height of about one foot. A second pinch after emerging shoots achieve a length of one foot will delay flowering but make for a more spectacular display late in the growing season.
There are a number of diseases and insects that plague dahlias. Botrytis (gray mold) blight and powdery mildew are two foliage disease that can be discouraged through sanitation, proper site selection and keeping foliage as dry as possible. Additionally, fungicides such as thiophanate methyl are effective in preventing these diseases. Since most dahlias are vegetatively propagated and gardeners tend to save their tuberous roots from year-to-year, there is the tendency for dahlias to develop virus diseases such as dahlia mosaic virus. Sanitation, insect control and selection of tolerant cultivars can help to control the latter. Troublesome insects to watch for include aphids, leafhoppers, spider mites, stalk borers and thrips.
Dahlias are not frost tolerant. Therefore, in the Midwest the tuberous storage roots must be dug and stored each fall following the first light frost. Dig them with a spading fork and take special care not to injure the necks of the tuberous roots. Most cultivars have long storage roots connected to the main stem by a thin neck. This neck contains the "eyes" that are needed to produce shoots next growing season. The root alone cannot produce new growth if the neck is broken or badly damaged.
After digging, wash off as much of the remaining soil as possible and allow the roots to dry, taking care not to dry them in direct sunlight. Tuberous roots may be separated in the fall by cutting them from the main stem, taking care to allow the portion of the stem attached to the containing the eyes to remain. Dust the cut ends with a fungicide. Conversely, the entire root system can be left whole and separated the following growing season. Pack the roots in moist peat moss, sawdust or other inert organic material and place in a wooden or cardboard box. Tight containers promote excess moisture retention which encourages storage rots. The temperature range ideal for storing dahlia roots is 40 to 45 degrees F.
With over 50,000 from which to choose, cultivar selection can be a daunting task. The American Dahlia Society list the Fabulous Fifty each year on their web site. Additionally, readers might look for cultivars that have won awards sponsored by the society including the Hart Award, Dudley Award, Gullickson Award and Johnson Award.
REVISED: September 29, 2015