Hardiness of Dahlias - Dahlias are tender perennials. Their tubers must be dug up and stored over the winter in all but the mildest climates (zone 9 and south).
Locating And Planting - While dahlias can be grown from seed, it is easier to grow them from tubers. Plant tubers much as you would other summer bulbs. If you want to push the envelope see the file Starting Summer Bulbs Early. In the spring when frost danger is past, choose a sunny site where the soil drains well. Dahlias do poorly in wet, clayey soils, and it may be advisable to build dahlia beds higher than the surrounding soil surface to improve drainage. Add lots of organic matter such as peat humus, sand, chopped leaves, or peat moss to the soil for the same reason. Soil should be slightly acidic (pH 6.0 to 6.5). If possible, dig at least 1 foot deep to loosen the soil when preparing the bed for planting.
Plant dahlia tubers on their sides with their eyes (buds) pointing up in holes 6 inches deep, 1 to 3 feet apart. A dusting of garden sulfur on the fleshy roots will help prevent diseases and insect pests. Cover the tubers with 3 inches of soil; as the shoot grows, gradually fill in the holes. Some serious gardeners seek to produce prize blooms. They plant the tubers deeper, up to 12 inches.
Container Gardening with Dahlias - Start dahlias in tubs or standard clay pots in early spring. If they are set out into the garden later, put them 3 inches deeper into the bed than they were in their pots. Water dahlias in containers faithfully. Once allowed to dry out, dahlias are extremely difficult, if not impossible, to coax into bloom again. Dahlias, the dwarf and semi-dwarf types, can be grown to flower indoors. Sow seeds in 2-3 inch pots in February. Kept at a temperature of 65° to 70°F. They will produce blooming plants by summer. Provide as much sunlight as possible, perhaps using a growing light. Dwarf types are also well suited to window boxes or patio planters paired with sweet alyssum or other annuals.
You can jump start dahlias indoors in pots in the early spring. Later transplant them into the garden 3 inches deeper than the pot depth. See Growing Plants In Containers and Containers For Plants in Yardener’s Tool Shed.