Dahlias (Dahlia spp.) stop growing and flowering in fall, and the leaves and stems die, but the thick dahlia tubers -- which look somewhat like bulbs -- remain healthy under the soil. Digging up the tubers and storing them helps prevent damage from frosts over winter. When spring comes, you can divide the tubers to increase the number of plants. Dahlias can potentially stay in the ground in U.S. Department of Agriculture plant hardiness zones 7 through 10.
Time to Dig Up Dahlia Tubers
The best time to dig up dahlia tubers is after the first frost in fall. Frosty weather causes dahlia leaves and stems to die down, but the first frost should not damage the dahlia tubers. In areas that don't experience frosts, you can dig up dahlias toward the end of fall, when the plants have stopped growing. In USDA zones 7 through 10, dahlia tubers can often survive winter when left in the ground.
How to Dig Up Dahlias
Sterilize the pruning shears by wiping the blades with a cloth that was soaked in rubbing alcohol.
Prune the remaining stems on dahlia plants to 3 to 4 inches long. Sterilize the pruning shears again when you've finished.
Tie name tags around the stems to identify dahlia varieties for next spring.
Push a garden fork or spade into the soil 6 to 9 inches from the base of a dahlia plant, and gently lever the fork or spade upward_,_ which loosens the tubers in the soil.
Push the fork or spade into the soil at the same distance around the plant base until the set of tubers lifts out of the soil. Dahlia tubers look somewhat like groups of fingers. Work carefully to avoid breaking those fingers off.
Break the loose soil from the tuber with your fingers.
Place the tuber upside down in a dry place for one day, or until the soil around it is dry.
Brush off the remaining soil with your fingers.
How to Divide Dahlias
Dahlia tubers must be stored in a cool, dry spot over winter, ready for dividing in spring. Store the tubers in moist peat to help prevent them from shriveling. The storage temperature must be above 32 degrees Fahrenheit. Divide dahlia tubers after the final local average frost date in spring.
Wipe a sharp gardening knife with a cloth soaked in rubbing alcohol. Cut the dahlia tubers into pieces that have at least one eye. The eyes are the small stubs where shoots sprout.
Dust the cut ends of the dahlia tubers with sulfur dust.
Plant the dahlia tuber pieces 6 to 8 inches deep in sunny spots with organically rich, freely draining soil.