How to Grow and Care for Dahlias

Introduction to Dahlias

Dahlias are quick-growing, heat-loving plants that produce abundant flowers on sturdy stems. There are hundreds of varieties with flowers in a wide range of sizes, colors, and shapes.

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.

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 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.

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.

Special Features of Dahlias

Good for cut flowers

Choosing a Site 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.

Growing Dahlias in Cold Climates

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.

Planting Instructions for Dahlias

Plant dahlia tubers in spring after all danger of frost has passed and the soil has warmed. Prepare the garden bed by using a garden fork or tiller to loosen the soil to a depth of 12 to 15 inches, then mix in a 2- to 4-inch layer of compost. Dig a hole about three times as deep as the diameter of the tuber, usually between 3 and 8 inches. Set the tuber horizontally in the hole with the buds facing up, cover it with soil, and press firmly.

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.

Ongoing Care of Dahlia Plants

Apply a 2- to 4-inch layer of mulch around dahlias to retain moisture and control weeds. Dahlias prefer consistently moist soil, so water plants during the summer if rainfall is less than 1 inch per week, and water container-grown plants frequently to keep soil moist but not saturated. Remove flowers as they fade.

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.

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.

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 of Dahlias

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.

Digging and Storing Dahlia 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.

Propagating Dahlia

The usual way to multiply a dahlia is to divide the tuberous roots after winter storage. But dahlia cuttings are also very easy to root, almost as easy as sweet potatoes. If the supplier will ship tubers at least two months before your planting date, you can do it. It's a great way to get five or six plants from a single tuber-immediately, before the season even starts.

There are other advantages, too. Dahlias from cuttings (with no tuber to rely on) grow bigger root systems quickly. By bloom time, there is no difference between plants from cuttings versus those from tubers. Cutting grown plants later produce tubers with thicker skins that are better for storage.

Here's how to proceed. Eight weeks before you plant dahlias outdoors, fill a 2- to 3-inch-deep container with a light potting mix. Set the tuber near the surface, covered lightly with the mix. The "eye" on the narrow end of the tuber should protrude from the soil. Put in a bright, warm spot, where temperatures won't go below 50oF. The tuber will sprout in two to three weeks.

When sprouts are 3 inches high, cut them with a sharp knife of scissors just above the connection to the tuber. Plant cuttings in small containers of mix, up to the bottoms of the lowest leaves. Keep soil moist and cuttings will root in about two weeks. Transplant them to 3- to 4-inch pots and grow on until planting time, after frost. One tuber can produce up to six sprouts, and still have enough strength to be planted outdoors itself.

Types of Dahlias

Dahlia Flower Forms

The American Dahlia Society recognizes 16 different groups or types of flower forms and 15 different color categories

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 or research through The American Dahlia Society. Here we have pictured 10 of the most popular types and have listed a few of the proven performers in each group.

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

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.