Chapel Hill, N.C. — There's nothing subtle about dahlias. Today's hybrids are vivid, striking, bold composites that provide the most eye-catching displays at flower shows coast to coast. And they can do much the same thing in home gardens, too. The theatrical giants with their 12- to 15-inch blooms are spectacular additions to any large perennial garden. For small gardens or low borders, there are also mini-dahlias with 11/2- to 2-inch flowers produced on foot-high, shrubby plants. And dahlias come in all sizes between these two extremes.
Dahlia gardeners also have a wide choice in both colors and forms. These plants come in white, pink, rose, red, yellow, orange, lavender, purple, blends, and bicolors -- every color but a true blue. And while one grower will prefer the spiky cactus types, another might choose softly rounded decoratives, singles, or fully double balls, or anemone-flowered, peony-flowered, or orchid-flowered dahlias.
When I had a garden in Connecticut, I thought of dahlias as labor-intensive plants because, like glads and tuberous begonias, these summer-flowering tubers had to be dug each fall, stored carefully, and replanted in the spring. But while gardeners in the Northern tier of states must do this, it's not so for those in the South. Here in the Piedmont of North Carolina, dahlias can remain in the ground year after year, surviving nicely with only a light mulch for winter protection.
Prof. August De Hertogh, chairman of the department of horticultural science at North Carolina State University, contends that the dahlia ``is one of the most underused plants in the North Carolina landscape.''
The same may be true nationwide. Like hemlines and car sizes, plant popularity comes in cycles. Dahlias have not been in vogue for a while, but I suspect they will benefit from the recent nationwide resurgence of interest in perennials -- particularly in the warmer parts of the United States, where they can be treated like herbaceous perennials.
Culture for dahlias is reasonably simple. Plant the tubers two to three inches deep in a well-tilled bed enriched with generous amounts of compost or rotted manure. Stake the tall varieties at planting time. Water and feed generously throughout the growing season. Professor De Hertogh recommends using a balanced fertilizer at least once a month and preferably twice.
If you are gardening in one of the Northern states, plant your dahlias in full sun. Southern gardeners, on the other hand, should try to provide some midday shade.
When the new plants have two or three sets of true leaves, pinch out the tip to promote branching and produce a more compact, attractive plant. The tall varieties will benefit from a second pinch when the side shoots produce three sets of leaves.
Several flower buds will form at the tips of each growing shoot. While these buds are small, you have a choice. As with peonies, mums, and roses, you can leave all the flower buds on the plant and enjoy a colorful display of numerous, small flowers. For cut flowers with long stems or for exhibition dahlias, remove the side buds on each shoot and leave only the terminal bud to produce one magnificent flower per stem.
Dahlias respond to pruning with renewed growth and more floral production. In the North, gardeners can enjoy bloom from midsummer until frost. Here in the South, we have the first flush of flowering in early summer. When the summer ``muggies'' set in, and the dahlias begin to look miserable, they should be cut back drastically. They regrow readily and produce another wave of flowers for the fall season.
Occasionally dahlias will be bothered by aphids or Japanese beetles. Sevin is a most effective pesticide; it is deadly, however, to beneficial insects such as bees. If used, it must be administered with extreme care.
Professor De Hertogh prefers one application of systemic Di-syston pesticide for his home crop of dahlias. ``My feeling is that homeowners should spray as seldom as possible,'' he says. A systemic pesticide is generally applied only once. If gardeners plan to use any pesticide, they should read the labels carefully.
An organic alternative to these pesticides is to use Safer's insecticidal soap, which is harmless to beneficial insects. It should do away with the aphids. A nonpesticide method of getting rid of Japanese beetles is to knock them into a pail of water. This method is effective if your dahlia plot is not too large.
One Connecticut dahlia specialist found the corn borer to be the worst pest in her garden. She would watch for small holes in the stems. If she suspected borers were at work, she would cut into the stem and drop in some insecticide.
In addition to using dahlia tubers, it is also easy to raise dahlias from seed. Treat small border varieties ``Rigoletto,'' ``Border Jewels,'' ``Early Bird,'' or All-American winner ``Redskin'' as annuals like zinnias and marigolds. By frost, they will have produced small tubers, and one or two may be worthy of harvesting.
Whether you are raising the dwarf dahlia varieties from seed or tubers, remember that they will perform as well in containers as in the garden.
Professor De Hertogh is particularly involved with dahlias in containers -- specifically dwarf cultivars produced in six-inch pots for the commercial greenhouse industry. Gardeners can buy ``Baby Moon''; rose-colored ``Berliner Kleene''; another decorative, fawn-colored ``Fantasia''; ``Bonne Esprit,'' a deep red semi-cactus with rose tips; or orange ``Prefere'' or ``Bern.''
These first successful commercial varieties have several advantages. Their compact size makes them ideal as potted plants, and they still have relatively large flowers. Cut them back and plant them in the garden for additional bloom and then save tubers for next year.
Look for these containerized dahlias in garden centers and grocery store displays during the summer. For larger sizes, plan ahead and order some tubers early next year for a rewarding and beautiful summer and fall display.
For those who want to order tubers from a specialist firm, several reliable ones are:
Douglas Dahlias, Route 1, Box 91, Myrtle Creek, Ore. 97457.
Swan Island Dahlias, Box 800, Canby, Ore. 97013, catalog $1.50.
Ruschmohr Dahlias, Box 236, Rockville Center, N.Y. 11571.
Another excellent way to learn more about dahlias is to join the national plant society. For information about the American Dahlia Society, write ADS, 2044 Great Falls, Falls Church, Va. 22043.