All About Dahlias

By Diane Bilderback

From cactus dahlias like bursting fireworks stopped in time to pompon dahlias with intensely colored petals, dahlias make a dramatic addition to any garden. All but the smallest kinds provide an abundance of showy bouquets for the home. All are easy-to-grow plants that produce prolific flowers in the summer and fall. In my Missoula, Montana, garden, they bloom for more than six weeks, beginning around July 20. In most milder climates, bloom starts anywhere from early June to August 1 and continues until frost. Cutting the flowers for indoor bouquets encourages more blossoms. Dahlia foliage provides extra landscaping accents, as colors range from light yellow-green to a purple-green.

But many gardeners have never tried dahlias. They may have the mistaken impression that they're tricky. Dahlias are no more difficult than potatoes in most of the United States. Named varieties are propagated by sweet-potato-like tubers that rapidly grow into vigorous, multi-stemmed plants. In fall you divide and overwinter the tubers, increasing your stock each year.

You can also grow dahlias from seed. This works well with smaller kinds for bedding plants, and you'll see flats of these in garden centers in spring. Seed catalogs offer a range of seed-grown kinds, including larger cactus types and single colors that come true. If a seedling looks especially good, you can save tubers from it.

I hope this guide to dahlia landscaping, varieties and culture will encourage you to include these beautiful, prolific flowers in your garden.

Popular formal-decorative dahlias are 'Jess Lynn', 'Steling Silver', and 'Tonya'.

Colors Galore

Dahlias' wide color range -- from the darkest red or purple to all shades of pink, orange, yellow, white, blends, variegated and even bicolors -- makes it easy to include them in any gardening color scheme and mix them with other annual or perennial flowers.

The height of most dahlia varieties ranges from three to six feet, so plant them in the middle to rear of your flower bed. The taller varieties also perform wonders as accents at the edge of a corn patch or around a pole bean trellis. Dahlias also are effective visual screens surrounding a garden nook or behind border plants because of their dense foliage. In perennial beds, use them as color spots along bed edges so you can easily dig the tubers in the fall. Edge a bed of dahlias with early-blooming annuals, such as ageratum, petunias or Hero or Aurora marigolds, to provide color at the beginning of the season.

Thousands of Varieties

The dahlia's great variety in flower form and color results from the crossing of primarily two species. All dahlias are native to Mexico. In 1789, botanical explorers collected native dahlia species that had 1- to 2-inch flowers, a single set of petals and a central disk. The overall effect was similar to common sunflowers. Grown together, these native species naturally hybridized and produced flowers with many more ray flowers and colors. By the late 1800s, intensive breeding had developed most of the various flower forms and colors. Currently, there are nearly 2,000 named varieties in the world.

Dahlia Flower Forms

The American Dahlia Society recognizes 16 different groups or types of flower forms and 15 different color categories (see list at the end of this article). It tends to get overly complicated and you don't need to memorize them all in order to grow beautiful plants. A few minutes with a catalog will make it all more clear.

Dahlia flowers range from less than 2 inches to more than 10 inches in diameter. Some flower forms have five different sizes.

To grow very large (sometimes called "dinner plate") blossoms, gardeners must limit the number of flowers that develop to as few as four. If your goal is a large number of flowers per plant, you will want to grow varieties with blooms less than 6 inches in diameter.

How to Grow Dahlias

In all but the hottest climates, grow dahlias in full sun. In hot summer areas, do best with some afternoon shade.

Dahlias, like potatoes, prefer moist, but well-drained soil. If the soil has not been tilled, loosen it to a depth of a foot. If you have sandy soil, add compost or other organic material to increase water retention. For heavy clay soil, add sand and organic material to increase drainage. Many dahlia growers add a 1- to 2-inch layer of homemade compost to the soil before tilling in early spring.

Large amounts of nitrogen are not necessary, but plants must have adequate phosphorus and potash for growth and flowering. If you use a chemical fertilizer, add one with a rating of 6-12-12 or a slow-release fertilizer.

Dahlia shoots are frost-sensitive; plant tubers in the garden no sooner than two weeks before the last date of spring frost. In the South, plant tubers from April 15th to May 1st. Or, to avoid the intense heat of midsummer, plant in June so that flowers develop in the fall. Dahlias take 8 to 12 weeks from planting to start blooming.

Bench Starting. The easiest way to start dahlias is to plant tubers directly in the garden. However, in very short-season regions, in climates with heavy rains at planting time or where soil is warm and saturated, such as in the South, it's better to start tubers "on the bench".

Begin 4 to 6 weeks before outdoor planting time. Place the tuber in a 2- to 3-inch-deep tray filled with light potting soil. Set the "eye" -- the little "bump" or growing point of the tuber -- slightly out of the soil, and cover the rest. Place the tray in a bright window, cold frame or greenhouse where temperatures range from 50°F to 80°F. Keep soil moist but not wet. Sprouts will appear in two to four weeks.

Once the sprout has two sets of leaves, gradually acclimate it to the garden by taking the tray outdoors to a shaded location for a few hours every day for a week. Then, if possible, plant on a cloudy day, or shade the young plants for another week with a piece of cardboard set in the ground on the south side.

Staking and Planting. Space most dahlia varieties two feet apart, but allow three feet for ones with large-diameter blossoms. Where the growing season is long, use 30- to 36-inch spacing for all varieties. When planting more than one row of dahlias, stagger them so that they don't shade each other.

Dahlia varieties more than three feet tall need staking. This is especially important in hot climates, as the stem is not as strong as when grown in cooler temperatures. Label and place the stakes before you plant. Use 5- to 7-foot stakes (the longer stakes for hot or long-season climates), and drive them about one foot into the soil. Dig a 6-inch-deep hole next to the stake, and place the dahlia tuber with its eye about 2 inches below the surface in cool climates, and up to 4 inches deep in warm climates. Place the eye near the stake with the rest of the tuber angling down into the soil. Then cover the tuber with soil. When planting sprouted tubers, position them so that the sprouts are at the surface of the soil. Use twine to attach the plant to the stake as it grows.

To speed growth and flowering, keep dahlias weeded. Dahlia feeder roots are very shallow, so to avoid disturbing them, pull weeds when they are small. Most growers use a light mulch such as small chip pine bark, pine needles, compost, or a 1- to 2-inch layer of grass clippings around the base of plants to discourage weeds and help retain moisture. In hot or dry climates, mulching is a must to grow the best dahlias.

"Stopping" Growth. During the first month of growth, pinch out the very tip of the plant. This is called "stopping" because it stops the growth of the main stem and diverts growth to side shoots at the base of each leaf.

Dahlia varieties with flowers that are more than 8 inches in diameter need pinching when the plant has 4 leaves; for the 6- to 8-inch-diameter flow varieties, pinch at 6 leaves; and for smaller varieties, pinch at 8 leaves. If all the side shoots develop, there will be 4 to 8 lateral branches, and the plant will become bushy.

If you live in a warm, long-season climate, you can double pinch your plants so the flowers develop in the cooler part of the summer and have better color and form. Pinch as above, and pinch out again if any lateral branches develop flower buds before the third week of July. Pinch out the terminal flower bud and all small buds in the axils of leaves for three sets of leaves down the stalk. For the fourth set, pinch out one side so that the other lateral branch in the axil of a leaf becomes the new growing point.

Disbudding. If you grow dahlias for garden color, disbudding is not important. But if you want long-stemmed, larger-sized flowers, plan to "disbud" your dahlias. The terminal central flower bud is the largest and is flanked by two side buds. With your fingers, snip out these two smaller side buds when the central bud is no larger than a large pea. Then remove the small branches from the angles of the next two sets of leaves below the bud.

Pests and Diseases. Dahlias are susceptible to numerous pests, including spider mites, aphids, corn borers, earwigs, thrips, potato leafhoppers, Japanese beetles, cucumber beetles, and slugs and snails. Although all these pests can be troublesome, slugs or snails early in the season can kill young plants, and mites during the summer can dispatch even large, healthy plants in a few days. Insecticidal soaps work well to limit damage caused by spider mites, potato leafhoppers and aphids. Ladybugs eat both aphids and corn borer eggs. Another corn borer control is the bacterium, Bacillus thuringiensis, sold as Bt. Using a rotenone or pyrethrin spray is often an effective control for beetles.

Dahlias are also susceptible to bacterial wilts and fungal and viral diseases. Light yellow vein banding, wavy yellow lines and rings or spots of yellow on the leaves are common indicators of viral infection. Other viral indicators include short internodes (stunted growth) or abnormally shaped leaves. In all cases, remove infected plants immediately, placing them in the garbage -- not the compost pile.

Dahlias as Cut Flowers. Immediately place cut dahlias into a bucket or vase of water. According to Alan Fisher, an accredited judge for ADS, "always cut with a very sharp knife" to avoid crushing the stems. Recutting the stems and changing the water every two days also extends flower life. Many gardeners who show dahlias use a solution of 1/4 teaspoon of bleach plus 2 teaspoons of sugar to 1/2 gallon of water to prolong blossom life.

Digging and Storing the Tubers. In areas with autumn frosts, dig tubers soon after a hard freeze has killed the tops of the plants. For areas with mild fall weather, cut the stems off about two inches above the ground with sharp pruning shears in late October or early November. Use a garden fork or spade to carefully lift the tubers out of the ground. Wash them with water to remove soil, and air dry. In very dry areas, keep tubers moist until storage.

Bill McClaren, ADS's research chairman, recommends cutting off the best tubers for next year and storing them in newspaper lined and covered flat boxes in your basement or other very cool but frost-free area. On selecting tubers to overwinter, he counsels, "If the tuber is smaller than a pencil, it won't survive a long winter, though it will do fine in short-winter areas." Plants grown from small-diameter tubers produce feeder roots faster and ultimately form more tubers than plants started from larger tubers.

Each cut tuber must have a growing point or "eye." The stem must be cut behind that growing point bump to ensure that the tuber has a growing point for next year. Place the tubers in the newspaper-lined wooden box in a single layer.

If your climate is very dry, place the tubers in plastic bags with small holes punched in them to reduce shriveling. Some growers use moist vermiculite in the bags to help maintain moisture. If possible, take more than one tuber from each plant to provide a selection to choose from the next growing season. Give your extra tubers to friends in the spring so they, too, can enjoy growing dahlias!

Some dahlia suppliers also sell an indelible pencil or permanent marking pen to label your dahlia tubers. Write the variety name directly on the individual tubers before storing, so your storage box does not become next year's dahlia grab box!

Throughout the winter, check the tubers and promptly remove rotting ones. If the tubers begin to shrivel, lightly mist the newspaper with water to increase the humidity.

As you can see, growing dahlias is very much like growing potatoes, except for the minor chores of staking, stopping and disbudding. Plan now to include gorgeous dahlias for an easy accent to your garden landscape!

Dahlia Types

Enthusiasts classify dahlias by flower shape into 16 groups. Here we have pictured 10 of the most popular types and have listed a few of the proven performers in each group, with help from the American Dahlia Society.

The details of classifying dahlias can get a little complicated. Five of the groups, for example, are further subdivided into five flower sizes, ranging in width from 10 inches down to 4 inches. These things are of little practical importance unless you plan to enter your flowers in competitions. But if you want to know more, the best first step is to get a specialty catalog such as Swan Island Dahlias.

Semicactus. Flowers from less than 4 inches to more than 10 inches across. Outer petals pointed and tubular only near tips.

Popular varieties: Inland Dynasty, Irene's Pride, William R., Magic Moment, Hamari Accord, Grenador Pastel, The Queen, Mary Jo.

Pompon. Flowers less than 2 inches across. Round-tipped petals are arranged to form a globe.

Popular varieties: Oreti Duke , Poppet, Glenplace, Moorplace.

Collarette. Flowers are various sizes. Between the outer row of petals and the central disk is a collar of smaller petals.

Popular varieties: Alpen Cherub, Little Showoff.

Cactus. Flowers from less than 4 inches to more than 10 inches across. Petals are pointed and tubular.

Popular varieties: Camano Messenger, Andrew David, Nita, Jessica, Kiwi Gloria, Glenbank Twinkle, Billy.

Waterlily. Flowers are various sizes. Multiple rows of broad petals are flat or slightly curved toward the center.

Popular varieties: Red Velvet , Figurine, Ken's Flame, Nepos.

Formal Decorative. Flowers from less than 4 inches to more than 10 inches across. Petals are broad and round-tipped, arranged symmetrically, with no central disk.

Popular varieties: Jess Lynn, Clyde's Choice, Kidd's Climax, Sterling Silver, Kenora Lisa, Formby Perfection, Hamilton Lillian, Connecticut Dancer, Tonya, Rebecca Lynn, Alpen Jewel, Rose toscano.

Single. Flowers are various sizes. One row of petals around a center disk.

Popular variety: Alta Bishop.

Novelty. Flowers are various sizes. Petals are different than those of other classifications.

Popular varieties: Akita No Hikari , Alloway Candy, Fidalgo Julie.

Peony. Flowers are various sizes. Two to four rows of flat petals around a center of twisted petals.

Popular varieties: Tinker Bell , Japanese Bishop.

Anemone. Flowers are various sizes. Flat petals surround a disk composed of densely packed tubular petals.

Popular varieties: Alpen Embers , Azuma Kagami.

Missoula, Montana-based Diane Bilderback plans on expanding her dahlia collection to include small varieties in pots and more cactus types. And she's hooked on dahlia shows!

Photography by National Gardening Association and Suzanne DeJohn/National Gardening Association

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