Dahlias are one of several perennial plant species that create tuberous roots. Like most such plants, including daylilies, dahlias are easy to propagate by digging up and dividing the root clumps. Unlike daylilies, however, dahlias are tender plants in zones 3 to 6, so if you want to grow them as perennials in these regions, the roots must be dug up and stored for the winter. This winter preparation offers a good time to propagate the plants through division.
In warmer climates where dahlia roots can remain in the ground, you can dig up, divide, and replant the root clumps in early spring before growth has begun.
What Is a Tuberous Root?
A tuberous root is a root structure consisting of a clump of fleshy nodules that extend down beneath a central stem. 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 tuberous root 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
Wait until the plants have been hit by a true frost and begun to die back. Once the foliage and stems are turning yellow, the plant is ready to be dug up. Don't dig them up too early, as you want the roots to be as fully developed as possible. But you also don't want to wait too long as dahlias are hardy only in zones 7 to 10, and soil temperatures below 20 degrees Fahrenheit will kill the roots.
It's best to try and plan for a week where dry weather is predicted. The stems will be cut several days before digging up the roots, and it's best to avoid water penetrating the cut stems, which can introduce rot into the root clumps.
Before Getting Started
Dividing dahlias is a trickier operation than for many perennial plants, as it's critical that each piece include a growth "eye." These eyes are typically located near the top of the tuberous structures, near where the plant stem was located. Without an eye, the root clump will not be viable.
Most experienced growers discard the center "mother" root clump, and keep only the well-developed offshoot root clumps to store for replanting the following spring. The mother clump has generally been exhausted by supported the plant's growth, and usually will not produce the following year.
Equipment / Tools
- Shovel or trowel
- Sharp knife
- Mesh storage bags or cardboard boxes
- Fungicide powder
- Vermiculite or peat moss
Cut Back the Foliage
Cut the top growth of the dahlia plant down to about 4 to 6 inches. Then, leave the plant in the ground for another 7 to 14 days. During this time, the growth eyes will be stimulated, making them easier to see when you dig up the roots.
Dig Up the Roots
To lift the dahlias, begin digging about 1 foot away from the plant on all sides, loosening the soil. A shovel is generally better to use than a fork because there's less of a risk of stabbing the tubers. Once the soil is loosened, carefully lift the plant out of the ground. The tuberous nodules have a tendency to snap off, so handle the plant gently. Carefully shake and brush off excess soil then hose off the remaining soil for a better view of the eyes.
Divide the Roots
When you remove all the dirt, you will reveal a root structure with many new fleshy root nodules radiating off a center point—the original "mother" root clump.
The goal when dividing is to separate the clump into sections, each including one or more fleshy offshoot nodules with a growth eye at the top end—where the root section attaches to the plant's stem. These eyes will show as a a white or pinkish dots. Carefully examine the clump, and cut it into sections that include one or more tuberous nodules and at least one eye. The center root structure is the "mother" clump, and this is normally discarded.
Inspect each of the sections you have cut away, looking for soft or damaged areas. Cut these away with a sharp pruner or knife; also remove the tiny feeder roots extending out from the nodule.
Treat the cut edges of the roots with a fungicide powder. Allow the sections to dry out slightly in open air before storing. When the skins of the tubers just start to wrinkle, they are ready to be stored.
Store for Winter
Place the root pieces in a mesh bag, or wrap them in newspapers and place them in a cardboard box. Store the roots somewhere cool, dark, and sheltered. Many growers prefer to add in a handful of vermiculite or peat moss with each root clump to stabilize the moisture during storage. A temperature between 32 and 50 degrees Fahrenheit is ideal for storing dahlia roots. Allow them to get good air circulation, which will help to prevent rot.
Over the winter, periodically inspect the roots. If any develop mold or rot, discard the affected roots. If the roots 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.
Some varieties of dahlia do not react very well to winter storage, and even in the best circumstances, you can expect to lose some of your roots to rot or desiccation over the winter. This is normal and not a reflection on your efforts—even the most accomplished gardeners experience some failures when trying to store dahlia roots over the winter.
Replant the Roots
Whether it's 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 new green shoots) facing up. Loosely pack 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 should be sufficient. Tall dahlias will need to be staked, and you can anchor this stake at the time of planting.