Dahlias provide large, bright blooms in the summer garden. There are giant varieties that produce flowers that are 6 inches or more across, as well as dwarf types that have small, delicate blossoms. Dahlias grow from a tender perennial tuber, a structure often mistaken for a bulb. The tubers aren't cold tolerant, so you need to dig them up and store them indoors during the winter. Splitting, or dividing, these tubers allows you to propagate more plants from your existing dahlias. Divide the tubers in spring before replanting the dahlias in the garden.
Examine each tuber and locate the buds, or growing eyes. Split only the tubers that have more than one visible bud.
Cut apart the tubers with a sharp knife, leaving at least one eye on each tuber. Dispose of any tuber pieces that are rotting or show any signs of disease.
Place a fungicidal powder, available at garden centers, in a shallow bowl. Dip the cut ends of each tuber into the powder, which lessens their susceptibility to fungal infections while the cuts are healing over.
Plant each dahlia tuber into a pot of sterile soil mix or back into their previous garden bed. Plant each tuber so the top of the tuber is just beneath the soil surface. Plant with the buds facing upward. Water potted dahlias as soon as the top 1 inch of soil begins to dry, watering from the top until the excess moisture drains from the bottom of the pot. Water bedded dahlias once weekly, moistening the soil to a 6-inch depth.