Dahlias are grown as annuals in most parts of the United States. They may be wintered over in the ground in the deep south or Pacific northwest where the ground does not freeze. In colder climates, dahlias should be allowed to die back after the first frost, then the dead tops are trimmed off to leave the roots and tubers. Dahlias are propagated from the tubers, which can look like roots. They are stored over the winter and planted again in the spring when the soil has warmed.
Prepare the planting bed by tilling or digging the soil several inches deep. Plant dahlias in an area that has good drainage where they will receive at least eight hours of sun each day. Work the soil until it is finely textured. Add humus to improve clay soil, if necessary. Dahlia tubers are large, maybe four inches or more in length, and you will need to till to a depth of 6 to 8 inches to provide optimum growing conditions.
Locate the eyes of the tubers, and make sure to plant the eyes facing up. Not all dahlia varieties have tubers that look alike, but they do grow in clusters. You will see the old stalk from last year's plant on the "up" side of the tubers.
Tubers can be divided for additional plants. Do this in the spring. With a sharp knife, cut the tubers apart at the crown into smaller clusters, making sure that there are eyes on each section.
Plant the tubers horizontally in the ground and cover them with soil. You may wish to mark the planting site for each dahlia, or label the variety.
Fertilize dahlias three to four weeks after planting, when there is leafy top growth. Use a low nitrogen fertilizer. Feed every three to four weeks.