Dahlia puts on dazzling color show as nights begin to cool.

  • Dahlia 'Karma Corona.'Photo by Loïc Evanno, shared under a Creative Commons (CC-BY-SA 2.5) license, via Wikimedia Commons.
    Dahlia 'Karma Corona.'Photo by Loïc Evanno, shared under a Creative Commons (CC-BY-SA 2.5) license, via Wikimedia Commons.
Your Show-Me Garden: MU Extension brings you gardening tips from experts around the state.

COLUMBIA, Mo. When many garden flowers show the ravages of a long and sometimes brutal summer, dahlias are most spectacular, said University of Missouri Extension horticulturist David Trinklein.

Fall’s warm days and cool nights cause dahlias to produce more flowers with intense, dazzling color, said Trinklein. If you do not already grow them, start planning now for dahlias in your garden next spring.

Dahlia, a member of the aster family, is native to Mexico, where it is the national flower. The Aztecs were growing it when Spanish conquistadors arrived in the 16th century. Spanish botanists brought dahlia and other plants back to Spain.

Early dahlias imported into Europe were of the single-flower type. Hybridization produced the first fully double-flowered forms along with new color combinations. Today, dahlia boasts one of the largest arrays of flower forms, colors and sizes of any cultivated plant. To date, more than 50,000 cultivars have been named and registered.

When most people think of dahlia, the tall types propagated from tuberous roots each year come to mind. “They are popular for the cutting garden or the backdrop of borders,” said Trinklein. But because of their tall stature and large flowers, this type of dahlia requires staking or some other form of support.

Dwarf or bedding dahlias propagated from seed have gained popularity in recent years. Bedding dahlias produce large numbers of small flowers on bushy plants. This makes them ideal for annual beds or the fronts of borders. Although these dahlias also form tuberous storage roots, most gardeners do not save them from year to year because of the ready availability of inexpensive plants each spring, Trinklein said.

Plant dahlias in an airy location where they receive at least six to eight hours of direct sun and protection from high winds. Plant tuberous roots of large types of dahlias about 14 days before the frost-free date for the area. Dahlias can’t endure freezing temperatures, so do not set out plants started indoors until the danger of frost has passed.

Dahlias grow well in fertile, well-drained garden soils high in organic matter. If soil lacks organic matter, add up to 4 inches of well-rotted manure, compost or other forms of organic matter before planting.

Prepare soil by incorporating about one-half cup of a garden fertilizer high in phosphorus and potassium but low in nitrogen (such as 5-10-10) in a 2-foot circle where each plant is to be located. Top-dress with an equal amount of fertilizer in July. Dahlias grow rapidly and need large amounts of water. Keep soil moist but not wet.

Dahlias benefit from pruning. The amount of pruning depends upon the flower’s intended use. Prune exhibition-type flowers to one main stem. Pinch plants for lavish garden display after first growth in the spring reaches a height of about a foot. A second pinch after emerging shoots achieve a length of 1 foot delays flowering but makes for a more spectacular display late in the growing season.

Botrytis (gray mold) and powdery mildew are two foliage diseases that affect dahlia. Avoid these diseases by strict sanitation, proper site selection and keeping foliage dry. Fungicides containing thiophanate-methyl also provide effective disease prevention.

Since most gardeners save dahlia’s tuberous roots from year to year, viral diseases such as dahlia mosaic virus can occur. Sanitation, insect control and selection of tolerant cultivars control these.

Dig roots and store them each fall after the tops have died back. After digging, wash off remaining soil and allow the roots to dry. Do not dry them in direct sunlight. Separate tuberous roots in the fall by cutting them from the main stem. Allow the portion of the stem attached to the part containing the eyes to remain. Dust cut ends with a fungicide to discourage storage diseases.

You can also leave the whole root system intact and separate the next growing season. Pack the roots in moist peat moss, sawdust or other inert organic material and place in a wooden or cardboard box. Ideal temperature for storing roots is 40-45 F.

Find more information about dahlias and their care on the American Dahlia Society website at www.dahlia.org.

For more gardening tips from MU Extension, go to extension.missouri.edu/topics/horticulture-and-gardening.

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