Some of the most stunningly beautiful blooms of summer come from tubers that were first cultivated by the Aztecs, who also cultivated Imperialis, a tree form of dahlia whose long and hollow stems could be used to carry water. Named after Anders Dahl, a pupil of legendary Swedish botanist Carolus Linnaeus, the dahlia was not introduced to Europe until 1789. By 1815, all modern dahlia shapes and colors were established (with the exception of the cactus variety).
Dahlia tubers or “crowns” (cluster of tubers) become available in February, usually in packages of 1, 2 or 3. Each crown or individual tuber needs to have a piece of stem and at least one “eye” or growing bud. This eye will be on the lower main stem, not on the tuber itself. Handle carefully so this source of new growth stays attached and unbruised.
PLANTING: For best results choose a full sun location (6 hours or more) where the soil is rich and well-drained. Dahlias can grow in a half day of sun, but fewer blooms will result. In the fall or winter, dig in composted manure where dahlias are to be planted the following spring. Plant after the danger of frost is over. Work the soil to a depth of about 9 inches, then make an 8-9 inch diameter hole. When planting a dahlia variety that will grow to 36 inches or more, place a stake in the hole before planting. Place the tuber or crown with the eye(s) closest to the stake. This way the stem will grow closest to the stake for support when the plant is fully grown.
Add a handful of bone meal, a layer of soil, and at a depth of about 6 inches, place the tuber or crown in an almost upright position. The eyes or buds should point upwards. If you feel uncertain when planting, try imagining a clock with the eye at 12 o’clock (top) and the tuber(s) at 5 o’clock. Cover with about 2 inches of soil. Unless it is a very dry season, you won’t need to water until shoots appear above ground.
A good way to get a head start with your dahlias is to pot the tubers 4-6 weeks before the last frost date, then move them outdoors after the danger of frost is over. In this scenario, keep eyes just above the soil line, and the container in a warm, bright-as-possible location. Water once and not again until sprouts appear. Water about 6 hours before transplanting. Then gently transplant the entire contents of the pot to the garden or appropriate container with the tuber tops just below the soil surface. It is important not to disturb the roots.
STOPPING: When the plants have produced about 4 pairs of leaves, snip out the topmost tip (avoid crushing the stem). Side shoots will develop to form the main body of the plant. For optimum bloom size and plant growth, many gardeners "disbud" their dahlias. Disbudding means removing buds from side shoots but retaining the main center bud. Side buds appearing very early can be removed. From planting time to bloom will be about 2 1/2 months.
FERTILIZING: Avoid fertilizing for about 6 weeks after planting. A granular bloom boosting organic fertilizer can be used as directed on the package. Water soluble fertilizers can be used (after buds appear) every 3 to 4 weeks. Fertilizers with a low nitrogen are recommended in order to avoid small blooms and weak stems.
LIFTING & STORING: Dig or "lift" crowns after a frost. This will reduce the possibility of tubers shrinking or rotting while overwintering. Labeling the crowns as you lift them will help you avoid mixing up varieties and/or colors when you replant. Leave about 6 inches of stem attached to the crowns and be careful not to damage the necks of the tubers or crowns, from which next year's shoots will emerge. Wash off all dirt. Check for bruises or soft spots and if you find any, cut them out, dust with yellow sulphur and air dry the remaining tubers (away from direct sunlight). Store the tubers in a well-ventilated, dry, cool (38-45° F is ideal) space. There are many ways to store the crowns: Wrap crowns in several (8 or 9) layers of newspaper and place them in paper bags, cardboard boxes (e.g., shoe box) or wooden crates; or place crowns in barely moistened sand or vermiculite. Check tubers during the winter and if they are beginning to shrivel, barely mist them. Some gardeners use plastic bags with holes cut into them. We do not recommend this system because plastic can retain moisture.
DIVIDING: Crowns may be divided in the fall or in the spring (which we recommend). Spring is the easiest time to see the eyes from which new shoots will form. If the eyes are difficult to find, try cutting the clump or crown division of tubers in half. Allow cuts to dry before planting the crown division. Single tubers which have broken off and have no eye may be discarded. Each root or crown division must have an eye if it is to grow and bloom. Enjoy your dahlias! Try planting taller decoratives as a back border or focal point. Use shorter dahlias in front borders, window boxes or in a container of mixed flowers. Tall dahlias are also well-suited to patio pots. All varieties are good for cutting and using in bouquets.