Dahlias are grown primarily for their large, showy flowers that emerge year after year. The dahlia plant grows from an underground root called a tuber. As the plant matures, its tubers will branch from the plant's stem and spread underground. This growth can be removed from the parent plant to form an entirely new plant. Separating the tubers is relatively easy and offers an effective way to propagate dahlias.
Prepare to dig dahlia tubers in the fall before the first frost. Remove any existing above-ground growth that remains on the plant after the growing season has ended, a few inches above the ground to preserve some of the stem.
Dig around the dahlia plant with a shovel or garden fork to loosen the crown of tubers underground. Be careful not to damage the tubers. Remove the plant from the ground and rinse off any soil with a hose.
Identify the tubers to be removed. Look for tubers with "eyes," beginning sprouts that the plant will grow from. The tuber to be removed must also be connected to the stem directly.
Remove the tuber, using a sharp knife, from the rest of the plant by cutting the stem that the tuber is attached to. Retain a portion of the stem attached to the tuber. Apply a fungicide powder to the cut portion to reduce the risk of rot.
Store separated dahlia tubers over the winter for replanting in the spring. Allow the tubers to dry thoroughly and wrap each individual tuber in a sheet of newspaper. Store the tubers in a cool, dry, dark place to keep the plants dormant until spring.
Plant the separated tubers in the spring, after the last change of frost has passed, with the stem end up at the surface in rich, well-draining soil. Water frequently to keep the soil moist. Examine the tuber for growth. The first signs of a sprout will often appear near the joint between the remaining stem and the tuber.