In warm climates, dahlia tubers will survive if they are left in the ground over the winter. However, even ground-wintered dahlia bulbs should be divided every three to four years. Each year that they grow, dahlia tubers multiply and produce more shoots. A dahlia tuber with too many shoots will suffer from weak stems and small flower size. By dividing your dahlias periodically, you not only strengthen the plant but increase your supply. If you don't need any more dahlias in your own flower garden, consider giving them away as gifts.
Dig up your dahlias after the first fall frost. Once the foliage dies and blackens, cut the stalk down to 6 inches above the ground. Then leave the dahlia tubers in the ground for another week before digging them up. This will allow the bulbs time to develop eyes, which will help you determine where to cut the tubers. If it does not freeze by early November, cut the dahlias back anyway and dig them up the following week. When digging, work at least a foot away from the dahlia tubers to loosen the soil before pulling them out to avoid breaking any tubers.
Use water to wash off the dahlia tuber clump to remove all the dirt. Then trim off all roots attached to the tubers.
Spread the tuber clumps out indoors in a cool, dry location. Allow them to dry overnight.
Use garden shears to shear off any protruding tubers that are easily separated.
Cut the stalk off of the tuber clump. Cut as close to the crown (the swollen area where the stalk and the tubers meet) as close as possible without cutting into the tubers.
Split the dahlia tuber clump vertically in half with a sharp knife.
Separate the tubers from the divided clump halves with your knife. Make sure that each whole tuber division that you make has at least one eye on it. The eyes are located near the crown of the bulb near the stalk. Any broken, damaged or very skinny tubers should be discarded.
Trim as much of the stalk tissue away from the individual tubers as possible without cutting the eye(s). Stalk tissue is likely to rot during the winter.
Soak the divided tubers for 20 minutes in a bleach solution of 1 cup bleach per 3 gallons water. This will kill any fungi or bacteria on the tubers.
Allow the dahlias to dry in a cool dry place for 24 hours. Then replant them or store them until spring.