Dahlias are one of several perennial plants that grow from tubers. The tubers provide the food source for the plant before it starts to photosynthesize and continue to multiply underground during the growing season. Dahlias are easy to propagate by digging up and dividing the current year's tuber clumps to produce more plants next year. Unlike daylilies, however, dahlias are tender plants in USDA hardiness zones 3 to 6 (and sometimes tender in zone 7), so if you want to grow them as perennials in these colder regions, you must dig up the tubers and store them indoors in a cool place for the winter.
In warmer climates where dahlia tubers can remain in the ground, you can dig up, divide, and replant the tuber clumps in early spring before new green growth emerges.
What Is a Tuberous Root?
A dahlia tuber system consists of a clump of fleshy nodules that resemble small sweet potatoes that extend beneath the central stem and crown. It is different than a true tuber, like a potato, in which the fleshy root structure has multiple "eyes," any one of which can create a new stalk. Nor is a tuber the same as a bulb, corm, or rhizome, each of which has a unique identifying structure. All these structures are sometimes mistakenly described as bulbs, but in reality, they are quite distinct from one another.
When to Dig and Divide Dahlias
The right time to dig dahlia tubers in cold climates is to wait until the plants have been hit by a true frost and the foliage has turned to black mush. Cut back the blackened foliage and main stem to 4 to 6 inches tall. Leave the tubers in the ground for at least 10 days. Don't dig them up too early because you want the tubers to start to cure and be as fully developed as possible. But you also don't want to wait too long because dahlias are hardy only in zones 7 to 10, and soil temperatures below 20 degrees Fahrenheit will kill the tubers.
It's best to dig tubers during a week when rainfall is not predicted and the soil is dry. The foliage and stems will be cut days before digging up the tubers, and it's best to avoid water filling up the hollow stems, which can introduce rot.
Before Getting Started
Dividing dahlias is a trickier operation than for many perennial plants, because it's critical that each tuber division contains a piece of the crown which is where the growth eyes are located. The crown is located at the base of the stem. To produce a plant next year, a tuber must have a piece of the crown with at least one eye.
Most experienced growers discard the center mother tuber and keep only the well-developed tubers. The mother tuber is a large tuber in the middle of the clump and it will look different than the others (it has a darker, crinkled texture). The mother tuber produced the plant this year and won’t have any energy left to produce a plant next year. It won’t store well over the winter.
Equipment / Tools
- Garden spade or pitchfork
- Sharp knife
- Mesh storage bags or cardboard boxes
- Fungicide powder
- Newspaper, vermiculite, or peat moss
Cut Back the Foliage
After a freeze blackens the dahlias foliage, cut the plant down to about 4 to 6 inches. Then, leave the plant in the ground for another seven to 14 days. During this time, the growth eyes will be stimulated, making them easier to see when you dig up the tubers.
Dig Up the Tubers
This task is more easily accomplished if the soil is on the dry side; it's made more difficult when soil is wet and heavy. To lift the dahlia tubers from the ground, use a pitchfork or spade and begin digging and loosening the soil about one foot away from the main stem around the entire plant. A shovel is generally better to use than a pitchfork because there's less chance of stabbing the tubers.
Once the soil is loosened, carefully lift the plant out of the ground getting underneath the tuber clump as best you can. The tubers have a tendency to snap off from the crown, so handle gently. Carefully shake and brush off excess soil then hose off the remaining soil for a better view of the crown and its eyes.
Divide the Tubers
Dividing means cutting apart the crown and retaining one or more tubers along with it.
The goal when dividing a clump of tubers is to separate the crown into viable sections, each including one or more fleshy tubers. You can usually assume that if you have a big piece of crown, it has an eye. The eyes will look like white or pinkish dots. Carefully examine the crown, and use a sharp knife or scissors to cut it into sections that include one or more tubers and at least one eye, but two eyes are better. The center tuber is the mother, and this is normally discarded.
Inspect each of the sections you have divided, looking for soft or damaged tubers. Remove those with a sharp pruner or knife; also remove the tiny feeder roots extending out from the tuber.
Treat the cut edges with a fungicide powder. Allow the divided sections to dry out for a day or two in open air before storing them. When the skins of the tubers just start to wrinkle, they are ready to be stored.
Store for Winter
Place the tuber divisions in a mesh bag, or wrap them in newspaper and place them in a cardboard box or empty milk crate. Never store them in a sealed plastic bin. Store the container somewhere cool, dark, and sheltered. Many growers prefer to add in a handful of vermiculite or peat moss to stabilize the moisture during storage. A temperature between 40 and 50 degrees Fahrenheit is ideal for storing dahlia tubers. Allow them to receive air circulation, which will help to prevent rot.
Over the winter, periodically inspect the tubers. If any develop mold or rot, discard them. If the tubers begin to send out vigorous growth shoots (this can happen in late winter), it's best to plant them in pots immediately and grow them by a sunny window. Then, transplant them into the garden once the weather warms and all danger of frost has passed.
Replanting the Tubers Next Season
Whether it's into indoor nursery pots in late winter or into the garden in mid-spring, dahlia tubers should be planted 4 to 6 inches deep. Lay the tuber horizontally with the eyes (or new green shoots) facing upwards. Backfill the planting hole with soil and tamp down lightly.
Do not water tubers at planting time. A tuber can’t absorb nutrients or water until its feeder roots begin to grow. If soil is totally dry, water sparingly and don't water the tubers until shoots emerge from the soil. When green growth appears, start watering deeply every week with at least one to two inches of water. Moisture must be able to reach roots that are six inches deep. Dahlias are thirsty plants. Tall dahlias will need to be staked, and you can anchor install the stake at planting time.
“Digging, Dividing, and Storing Tubers.” Dahlia.Org.