Margaret Lauterbach

Dahlias are easily grown — and easy on the eyes

Many gardening friends in the East grow dahlias instead of roses because they’re not subject to the range of diseases that roses are. Here, roses are rarely afflicted with black spot, but some folks love to grow dahlias anyway because they’re beautiful and easily grown in our climate. In our area some of the most prominent dahlia growers are men. Spectacular dahlia blossoms are entered in the Western Idaho State Fair competitions each year, many grown by men. Boisean Tim Garland has even bred a special dahlia, a white one, accepted by the American Dahlia Society and called Garland’s Enterprise.

Dahlias may be grown from seed or tubers. Tuberous dahlias are native to Mexico and Central America, grown as a food crop by the Aztecs, who relished the tubers. They also used wild dahlias medicinally to treat epilepsy, and used the hollow central stem for water pipes. They called dahlias by their name for water pipes, but the director of the Royal Gardens of Madrid named them dahlias, after Swedish botanist Anders Dahl. (In 1805 a German naturalist re-named them Georginas, and some areas still call them by that genus name).

When dahlias were exported to Europe, gardeners saw the blossoms and instantly adopted them as ornamentals, not edibles. The tubers grow somewhat similarly to those of sweet potatoes. You plant one tuber with an eye for a plant, but when the time comes to dig tubers, they’ve multiplied. The growing plant does need a fair amount of water.

Anju Lucas, head of perennials at Edwards Greenhouse, said that when you plant a dahlia tuber, water it as you normally would water a tuber, but don’t water it again until the plant has grown 3 or 4 inches tall. Then and from then on, it thrives on deep watering twice a week. That means irrigating rather than sprinkling, for water on foliage is not a good situation for most plants. Tuberous dahlias are heavy feeders, requiring fertilization once a month, using a product that’s lower in nitrogen than phosphorus and potassium, a product such as an NPK of 4-6-5.

When the plant grows to about 18 inches in height, nip off the top of the center stem so the plant will bush out and produce several blossoms for cutting and use in vases. Then it’s a situation where the more you cut, the more you have to cut, Lucas said. When you cut a flower for an arrangement, tap the hottest water you can into a glass or plastic (NOT metal) container, and settle the newly cut flower in it for an hour. When you remove it you may cut off the now-discolored bottom part of the stem. The hot water should be in a low container, steam not rising to the height of the blossom. This treatment will prolong the use of the blossom in an arrangement.

Slugs, snails and earwigs are their principal predators, so you should take preventive steps to keep them from destroying the blossoms’ beauty. Some blossoms are designated as “dinner plate” dahlias, for that’s the size of the blossom. They form on plants that grow 6 to 8 feet in height, and usually require supporting stakes. Other varieties are shorter. Like other plants that do not emit insect-attracting scents, they instead have bright colors to entice pollinators. Blossoms are categorized by single or double, anemone-flowered, collerette, waterlily, decorative, ball, pompon, cactus, semi-cactus, fimbriated (fringed), single orchid, double orchid, peony-flowered, or just miscellaneous in shape.

Blossoms are shades of yellow to red, and white; but the closest to blue they’ve been able to breed is purple. Blue color needs six hydroxyl groups, but dahlias produce only five of these anthocyanins. That may be changed by genetic modification one of these days.

I haven’t grown dahlias because they’re tender to frost, and when it’s time to lift them and treat tubers for storage we have too many other things that must be done at the same time. Bedding dahlias and tuberous dahlias are easily killed by frost. Tubers are hardy in zone 8 and higher zones, and in those climates a gardener doesn’t have to lift the tubers. We’re really in zone 6 (USDA says zone 7, but we’ve dropped to colder temperatures than that every winter since they upzoned us), so tubers remaining in the ground are destroyed by freezing.

When dahlia plants are blackened by frost, wait a few days before cutting stems about 3 inches above soil level, then dig, with spade about a foot from the stem to avoid damaging tubers. Put tubers in a frost-free and rodent-free location for a few days to dry out, then brush loose soil off and store them in damp sand, peat moss or vermiculite (some cat litters work too) in a location where temperatures remain between 45 and 55 degrees F. Check them periodically for mold and for drying out (sprinkle lightly with water). In spring, each tuber possessing an eye will produce a new dahlia plant.

Did your daffodils bloom as they should have this spring? If not, it’s time to divide them or at least fertilize them with a fertilizer with an NPK rating of about 5-10-10. If you divide your clumps, then you shouldn’t have to fertilize each year until you do divide them. Daffodils are safe from vertebrate damage because they’re toxic.

Send garden questions to or Gardening, The Statesman, P.O. Box 40, Boise, ID 83707.