Dahlia is a herbaceous perennial plant that grows from tuberous roots, and like most tuberous flowers, it is easily propagated by digging up and dividing the roots. Since dahlias are hardy only in zone 7 and higher, gardeners living in colder climates who want to keep their tubers must dig them up in the fall anyway, and this offers a good time to divide the roots. The division can be done either immediately before you store them for winter, or in the spring when the growth eyes are more visible. Spring division also gives you the opportunity to discard any tubers that haven't survived the winter.
In warmer climates where the tubers remain in the ground, you can dig up, divide, and replant the tubers in early spring before growth has begun.
In the demonstration shown here, we are dividing the tubers in a cold-weather scenario, allowing the foliage to die in fall before digging up the dahlias for division and storage.
Equipment / Tools
- Shovel or trowel
- Sharp knife
- Mesh storage bags
Cut Back Foliage
Wait until the plants have been hit by frost and begun to die back. At the point where the foliage and stems are turning yellow, the plant is ready to be dug up.
Cut the top growth down to about 4 to 6 inches and leave the plant in the ground for another 10 to 14 days to allow it to cure and prepare itself for winter, and for the new growth eyes to begin development.
Dig Up the Tubers
To lift the dahlias, begin digging about 1 foot away from the plant, loosening the soil. A shovel is generally better to use than a fork since it is easier to avoid stabbing tubers. 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 immediately, you can hose off the remaining soil for a better view of the eyes.
Divide the Tubers
Although a clump of dahlia tubers looks like multiple fingers, you can't just separate the fingers and grow more plants. Each dahlia division needs to have at least one eye and a nice, fat tuber.
The dahlia root clump typically has one large central tuber—the original tuber—as well as small offshoot tubers that developed during the growing season. This central tuber is normally discarded, as it has already expended its energy. It is offshoot tubers that are normally used when dividing a dahlia. These offshoots do not usually have growth eyes, which are instead located around the base of the stem. The eyes are the white or pinkish dots at the base of the stem or top end of the tubers. They are the growing point, the place where the stem will come from next year.
The goal when dividing is to separate the clump into sections, each including a section of the stem base containing an eye, as well as one or more offset tubers extending down from the stem section. Carefully examine the clump, and cut sections that include one or more offset tubers as well as a portion of stem containing more or more eyes.
If you are storing the tubers for the winter, allow them to dry out just slightly before storing. When the skins of the tubers just start to wrinkle, they are ready to be stored.
Store for Winter
Set the tubers somewhere cool, dark, and sheltered. A temperature no higher than 50 degrees Fahrenheit is ideal for storing dahlia tubers for the winter. Allow them to get good air circulation, which will prevent the tubers from rotting. A mesh bag is a good container for storing dahlia tubers.
Over the winter, periodically inspect the tubers. If any develop mold or rot, discard them. If they begin to send out vigorous growth shoots (this can happen in late winter), it's best to plant them in pots and grow them in a sunny window, then transplant them out into the garden once the weather warms and all danger of frost has passed.
Replant the Pieces
Whether it is into indoor nursery pots in late winter or into the garden in spring, dahlia roots should be planted 4 to 6 inches deep. Lay the tuber horizontally, with the eyes or shoots facing up. Lightly pack the soil over them, and water lightly each day until shoots emerge from the soil. Avoid letting the soil dry out completely, but also avoid saturating it. Once ample above-ground growth is present, weekly watering is sufficient.