Dahlias are generally propagated by dividing the root tubers. You can do this in the fall, before you store them away for the winter, or in the spring, when you are ready to replant. I prefer to divide in the spring because the eyes are more visible and because I know they've survived the winter. Dahlias also have a tendency to develop more eyes during winter storage, which means more divisions.
Wait until the plants have been hit by frost and die back. Cut the top growth down to about 4 - 6 in. and leave it in the ground for 10 - 14 days, to allow it to cure and prepare itself for winter and for the eyes to begin to develop.
To lift the dahlias, begin digging about 1 ft. away from the plant, loosening the soil. I prefer to use a shovel, rather than a fork, because I stab less tubers that way.
Once the soil is loosened, lift the plant out of the ground. The tubers have a tendency to snap off, so handle the plant gently. Gently shake and brush off the excess soil. If you are going to be dividing in the fall, you can hose off the remaining soil, for a better view of the eyes. If you are going to divide in the spring, set the tubers somewhere cool and sheltered, so the remaining soil will dry out and fall off. Once you've removed the soil, you can cut the stems back to just above the crown and inspect for rot.
Although a clump of dahlias looks like multiple fingers, you can't just separate the fingers and grow more plants. This is something very important that I learned from Hans Langeveld, of Longfield Gardens. Each dahlia division needs to have at least one eye and a nice, fat tuber. The eyes are the white or pinkish dots on the tubers. They are the growing point, where the stem will come from next year. Dahlia eyes are all located around the base of the stem. The tubers growing off of tubers will not have eyes and will never grow into plants. They can be left on, for extra food, or composted. The big, central tuber is the original that you planted in the spring. It may have another good year left in it, but many gardeners prefer to start with fresh divisions each year.
A sharp knife is the best tool to use. It can get between the mass of tubers and will make a clean cut. Hans recommends slicing through the stem, to make the divisions. Let the cut tubers dry somewhere cool (50 F.), dark and dry, for at least 3 days before you store them away. For more on storing them, read Over-Wintering Tender Bulbs.
- Growing Dahlias
- Classifying Dahlias