By Paul A. Thomas
University of Georgia
The plants, for instance, need a lot of water. Don't expect good flowers unless you plan to water during dry spells. Water them often enough to keep the soil moist but not saturated.
To conserve water, mulch the plants. Many mulch materials work well, including pine straw, grass clippings, pine bark and black plastic.
The crown of a dahlia plant will usually produce many shoots. For the largest and highest-quality flowers, allow only the strongest shoot to develop from a crown.
Disbudding is important, too, if you want large blooms. (Don't thin the buds of pompon and other small flower cultivars.)
When the flower buds of each terminal cluster are about the size of peas, remove all but one. The buds grow in groups of three, and the central, leader bud will always produce the largest flower. So, systematically remove the side buds.
At the same time, pinch out the small, tender growth buds in the leaf axils of the top set of leaves on the stem.
Too muchA single tuber planted in the spring will multiply into many tubers by late summer. If the clump isn't divided, many stems will emerge the following spring. This isn't good. The most vigorous plants with the best flowers come from single plants, not clumps.
Divide dahlia tubers each year in the spring, when the buds have swollen and are easily seen. Use a clean, sharp knife to carefully cut tubers so that each contains a healthy growth bud. If you break the bud away, the tuber is almost always worthless.
Dahlias are susceptible to bacterial and fungal diseases, including bacterial and verticillium wilt, Botrytis blight, crown gall, powdery mildew and stem rot.
Minimize these by mulching and proper spacing for good air circulation. Trimming or thinning out leaves near the base of the plant will improve air circulation, too. If disease does set in, remove and destroy all infected parts of the plant.
Viruses known to infect dahlias include mosaic and ring spot. Symptoms may include leaf mottling, vein clearing, spotting, distorted growth, stunting or wilting and can vary among cultivars. The best defense is to plant symptom-free or resistant stock and destroy infected plants.
BugsPest problems vary greatly among regions. The most common pests are aphids, spider mites, thrips and slugs. Frequent scouting will help catch problems early.
Dahlia tubers are hardy to U.S. Department of Agriculture zone 8. Growers in this zone may leave the plants in the ground to overwinter. In the colder zones of north Georgia, it's best to dig the tubers.
The most appropriate time to dig is after the first frost kills back most of the herbaceous growth. Before digging, cut the shoot to within a few inches of the ground and remove any stakes. Using a fork, start far enough from the plant to not damage the tubers, usually about a foot from the stem.
Wash tubers under low-pressure water to clean them before storage. Discard damaged or diseased tubers. Allow the good ones to dry slightly before storing. Drying times range from a few hours to more than a day, depending on the local humidity.
After you dry and label them, put the tubers in dry packing material such as vermiculite, peat moss or sawdust. Store them in a cool place like a basement or garage where no light can reach them. The best storage temperature is 45 degrees.
Check the tubers for signs of shriveling a few times during the winter. Add a little moisture may be to the packing media if needed.
(Paul Thomas is a Cooperative Extension horticulturist with the University of Georgia College of Agricultural and Environmental Sciences.)