Dahlias add a dramatic statement to the garden with a tall stalk and a stunning, disc-shaped flower. The dahlia root system consists of tubers, often called bulbs. Tubers are elongated roots instead of the more rounded bulbs many gardeners readily recognize. Learning how to split dahlia tubers involves understanding how the plant grows as well as how to put the plant into its dormant mode for the cooler months of the year. For best results, divide dahlia bulbs when you dig the tubers up for winter storage.
Cut down the dying plant stalks to 6 inches from the ground. This allows you to locate the tuber for removal before the first frost. Dig carefully to remove the tubers from the ground since a shovel or trowel can easily damage the bulbs.
Place the tubers on a worktable or the ground for cutting. Dahlia tubers look like a clump of long potatoes stuck together at a central point. Before splitting, you'll need to identify important parts of the bulb. The stem extends upward to form the plant stalk. Tubers connect to the stem and "eyes" located at the ridge where the tuber joins the stem. Eyes can be a little shoot or a circular dot on the ridge.
Position the dahlia bulb on the cutting surface with the visible eye sprouts on one side. Before cutting, turn the bulb clump over to make sure you aren't cutting any eyes on the other side.
Select the bulbs with sprouting eyes first since these plants will most likely produce flowers the following year. Place the utility knife on the stem and cut down towards the bulb section you want to remove. Cut slowly and include part of the stem, the ridge containing the eye and the individual bulb.
Set the cut bulb aside and make the next cuts by selecting another ridge containing an eye. If there aren't any more sprouts, look for the circular area indicative of an eye formation. Make your second cut, being careful to include part of the stem, ridge and the elongated tuber.
Trim off the stem area on the split bulbs to about 2 inches to retain the ridge and eyes. Removal of excess dried growth will prevent rotting during winter storage.