Dahlias are tender perennials that cannot survive winter in most climates, requiring gardeners to lift their tuberous roots each fall. These roots store energy for the following season’s growth and must be protected from freezing temperatures and excessive moisture to remain viable. Digging and storing the tubers is an annual practice for gardeners outside of USDA hardiness zones 8 and above, where the ground does not consistently freeze. Success depends on proper timing, careful handling, and maintaining specific environmental conditions during dormancy.
Determining the Optimal Time for Lifting
The single most important signal for lifting dahlia tubers is the first hard frost of the season. A hard frost causes the above-ground foliage to turn black and collapse instantly. This freezing event forces the plant to stop active growth and begin moving starches from the dying stem down into the tubers for winter storage.
Gardeners should not rush to dig immediately after this initial frost, as the tubers require a short period of “curing” while still in the ground. Wait approximately 7 to 14 days after the top growth has been killed by the frost. This waiting period allows the outer skin of the tuber to toughen and cure slightly, providing a protective layer against dehydration and decay during storage. It also gives the growth buds, or “eyes,” time to mature and become more visible. The tubers must be fully lifted before the ground freezes solid, which would make digging impossible.
The Digging and Initial Preparation Process
Once the waiting period is complete, cut the main stalks down to 4 to 6 inches of stem above the soil line. This remaining stem acts as a handle for lifting and helps mark the clump’s location. Since dahlia stems are hollow, delay this cut until just before digging, as an open stem can collect water and funnel it down to the crown, leading to rot.
To lift the tuber clump, use a garden fork or a sharp spade, inserting it wide and deep around the plant to avoid slicing the delicate roots. The necks of the tubers, where they attach to the main stem, are particularly fragile and prone to breaking. Gently lever the entire clump out of the ground, using the remaining stem as a guide, and carefully shake off any large, loose soil.
Next, the tubers need thorough cleaning to remove soil, which can harbor disease organisms and pests that promote rot in storage. Rinse the clumps gently with a garden hose or swish them in a tub of water, taking care not to puncture the skin. After cleaning, allow the clumps to cure by drying the outer surface in a cool, shaded, and well-ventilated location for 12 to 48 hours. The goal is to dry the exterior completely without allowing the interior to shrivel and dehydrate.
Requirements for Winter Storage
Successful long-term storage depends on replicating dormant conditions that prevent both freezing and premature sprouting. The ideal storage temperature range is consistently cool, between 40 and 50 degrees Fahrenheit (4 to 10 degrees Celsius). Temperatures below 38°F can cause the tubers to freeze, while temperatures above 50°F can signal them to break dormancy and sprout prematurely.
Maintaining adequate relative humidity is equally important to prevent the tubers from shriveling, a sign of dehydration. An ideal humidity level is high, aiming for 75% to 85%. To achieve this balance, tubers are often packed in a slightly moist, non-soil storage medium that helps regulate moisture and prevent direct contact between roots, which can spread decay.
Suitable storage media include peat moss, vermiculite, and wood shavings, which should be slightly damp but never wet. The tubers can be layered within cardboard boxes, plastic bins, or crates. Ensure the container allows for some air circulation, especially if using a non-breathable plastic bin. Check tubers periodically throughout the winter, and remove any roots that show signs of mold, rot, or shriveling to prevent spreading the problem.