Originally created 07/31/98

Pruning key to get maximum dahlias

WASHINGTON -- Dahlia enthusiasts are busy pinching back the budding stalks of their favorite flower now. Some start disbudding earlier; others stagger it over six weeks or so. The idea is to gain fewer but significantly larger blooms; the resulting delay leads to an extended flowering season.

Bigger has been one of the objectives in hybridizing this unusual plant, originally grown in Mexico, Guatemala and other parts of Latin America for its bulbs, which taste a bit like potatoes. The stalks of giant varieties reach a height of 8 feet, and flowerhead sizes truly take on the proportions of a dinner plate or a cereal bowl. The foliage is dense, handsome and reliable -- a perfect complement to the flowers' dramatic beauty.

Staking is essential; hammer stakes into the ground when tubers are planted. If you stake after they are growing, you risk damaging the tubers, which also grow as the season progresses. Soft string and strips of old bedsheets are the best tying material.

For admirers of big, fluffy blooms, which can be cut for vases indoors to last a week, the dahlia is the right plant, a steady producer. (The best cutting time is early in the morning or late in the day.) Additional advantages include the broad and vivid color spectrum -- virtually every tint of the palette except those in the blue range -- as well as diversity of form.

Some dahlias look like peonies; others imitate anemones and buttons. Many gardeners go for varieties that offer flawless formality, with each layer of petals perfectly symmetrical. Others are drawn to the engaging shagginess of the so-called cactus type. Single dahlias -- with only one row of ray florets -- are classic: the simple pinwheel form at its best.

But all this exuberance demands a price: greatly improved soil, worked at least to the depth of 18 inches but, if possible, to 2 feet. A pH reading of 6.8 is ideal. Fall is the best time to test the soil and to add lime.

The soil must drain very well, and the soil's organic content requires replenishment every year. Feeding dahlias is an art unto itself. A side-dressing of 5-10-5 or 5-10-10 granular fertilizer is recommended for mid-July. But fertilizer should be lightly scratched in and watered, not touching the stem. Whenever flower buds appear, more feeding is needed: three parts bone meal (3 to 4 pounds per 100 square feet), one part each of phosphate and potash.

Dahlias must have six hours of sunlight a day and at least 1 inch of water a week, Maintaining a thick mulch, preferably well-decomposed compost, a few inches deep helps the plant by conserving moisture and cooling the soil.

They are subject to a host of diseases and pests. Prudence calls for prompt destruction and removal of the affected plants.

For most gardeners, dwarf and medium-height dahlias are easiest to fit into a mixed border. A few basic types are available in market packs and in 4-inch-square plastic pots. Bedding dahlias, smaller than a foot, bloom nonstop all summer long, often until the first frost.

The dahlia is a tender bulb whose tubers must be dug up every fall after frost withers the foliage and browns the stalks. Storage of dahlia tubers is tricky: in a box filled with sawdust but with a little water periodically sprinkled on the tubers so they do not dry out completely. A dark, dry, cool room is essential, no colder than 35 degrees or warmer than 40 degrees.

Next spring, gardeners searching for rarer and choice varieties should consider ordering tubers from mail-order dahlia nurseries.

Though demanding and finicky, the dahlia's flowers are worth the work invested in them.

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