At a time of the year when many flowering garden plants show the ravages of having endured a long, sometimes brutal summer, dahlias are their most spectacular. Like peacocks, they really start to strut their colors. The warm days and cool nights associated with fall cause dahlias to produce flowers with more intense, vibrant color. September is a good time of the year to enjoy the explosion of color provided by this popular garden plant, and to start planning to add dahlias to your garden next spring.
Dahlia is a native of Mexico and a member of the Asteraceae (sunflower) plant family. Its genus name also serves as its common name. Dahlia is the National Flower of Mexico where 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 botanists with them who were assigned the task of selecting plants to take back to Spain and dahlia was one of the plants selected.
In error, dahlia originally was given the genus name Georgina, a name by which it still is known in many parts of eastern Europe. Its current genus, Dahlia, was assigned to it in the late 1800's by Spanish botanist Abbe Cavanille. The name honors Anders Dahl, a Swedish botanist and student of Carl Linnaeus, the "father" of plant nomenclature. Early dahlias imported into Europe were of the single flower type and probably belonged to the species D. pinnata, D. rosea, and D. coccinea.
As hybridization work on the genus began, the first fully double-flowered forms of dahlia 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. To date, more than 50,000 cultivars have been registered by the American Dahlia Society. The flower of a dahlia is a compound inflorescence known as a capitulum, or head, which contains both ray florets and disk florets. Dahlias are assigned a 4-digit classification number, depending on their size, form and color. An explanation of the numbering system can be found at https://www.dahlia.org/wp-content/uploads/2018/02/UnderstandingClassificationNumbers.pdf.
When most people think of dahlias the large-stature types propagated vegetatively 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 background of borders. However, many require staking or some sort of additional support. Some of these giants bear flowers up to eight inches in diameter and often are referred to as "dinner plate dahlias."
In recent years, dwarf or bedding dahlias propagated from seeds have gained popularity with gardeners. Bedding dahlias produce smaller flowers but in greater abundance on bushy plants which makes them ideal for annual beds or in the front 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 started plants in the bedding plant market each spring.
Dahlias established from tuberous roots may be planted outdoors 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 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 form of organic matter is a sound cultural practice before planting. Holes for planting should be deep enough so the "eyes" of the tuberous roots are covered by at least four inches of soil.
During the summer, top dress plants with a garden fertilizer relatively low in nitrogen (e.g., 5-10-10) on a monthly basis. Avoid fertilizers high in nitrogen which tend to produce huge green plants with weak stems and few to any flowers. Dahlias grow rapidly and consume large amounts of water. Therefore, 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 flowers. 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.
Several diseases and insects 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 cold hardy at our latitude which means their tuberous roots must be dug and stored each fall after the tops have frozen. This is seldomly applied to smaller border dahlias started from seeds. To overwinter roots, dig them with a spading fork taking special care not to injure the necks of the tuberous roots. Most cultivars have long, plump storage roots connected to the main stem by a thin neck. This neck contains the "eyes" that are needed to produce shoots the following growing season. The root alone cannot produce new growth if the neck is broken or has been badly damaged.
After digging, wash off as much of the remaining soil as possible and allow the roots to dry, taking care not to subject them to 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 containing eyes to remain attached to the roots. 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 (plastic) containers promote excess moisture retention which encourages storage rots. The temperature range ideal for storing roots is 40 to 45 degrees F.
With over 50,000 dahlias from which to choose, cultivar selection can be a daunting task. The American Dahlia Society list the Fabulous Fifty each year it's their web site (www.dahlia.org). Additionally, readers should consider looking for cultivars that have won awards sponsored by the Society including the Hart Award, Dudley Award, Gullickson Award and Johnson Award.