Dahlias are prized for their spectacular late-summer and autumn blooms. As a native of Mexico, this plant is not naturally equipped to survive freezing temperatures. Whether you must dig up the underground storage organs, known as tubers, for winter depends entirely on your climate. Lifting these tubers is necessary to preserve the plant for the following growing season.
Climate Determines the Answer
The need to lift dahlia tubers is a direct response to ground temperature and moisture levels during the colder months. In areas where the soil freezes solid, typically USDA Hardiness Zones 7 and below, lifting the tubers is required for survival. Freezing temperatures cause ice crystals to form inside the tuber cells, rupturing the cell walls and destroying the tissue.
Even in regions with mild or intermittent freezing, cold, soggy soil presents a significant danger. Tubers left in poorly drained ground during a cool, wet winter are highly susceptible to fungal diseases and rot. The combination of low temperatures and excessive moisture creates the perfect environment for decay organisms to destroy the stored starches.
Gardeners in warmer climates, generally Zones 8 and above, can leave the tubers in the ground year-round. The soil usually provides enough insulation to protect the tubers from lethal cold. If your soil is well-drained and the ground does not freeze, you may safely allow your dahlias to remain dormant in place, often benefiting from a thick layer of protective mulch applied in late autumn.
The Process of Lifting Tubers
The timing for removing dahlia tubers should occur after the first hard frost has killed the foliage. This frost signals the plant to enter full dormancy and ensures the tubers have maximized their energy storage. Waiting about a week after the leaves have turned black allows the tuber skins to toughen slightly, improving their resistance to damage during handling.
Before digging, cut the tall, hollow stalks down to about four to six inches above the soil line. These remaining stem sections serve as a handle during lifting and help identify the crown where new growth buds will form. It is important to label the stem with the variety name at this point, as the tubers will be indistinguishable once cleaned and stored.
A garden fork or pitchfork is the preferred tool for lifting, as it minimizes the risk of slicing into the brittle tubers compared to a sharp shovel. Begin digging a wide circumference, roughly 12 inches away from the center stem, to stay outside the tuber clump’s perimeter. Gently prying the soil upward on all sides will loosen the entire clump, allowing it to be carefully lifted out without breaking the delicate neck connecting the tuber to the stem.
Once removed, shake off the majority of the loose soil manually. The next step is curing, where the tubers dry in a cool, sheltered, and well-ventilated location for several days. This initial drying, typically lasting two to five days, helps the skin cure and creates a protective layer against pathogens before winter storage.
Essential Steps for Winter Storage
After the initial curing period, the tubers must be thoroughly cleaned to remove any remaining soil, which can harbor disease spores and insect pests. Rinsing the clumps gently with a hose is effective. Follow this with an inspection for any sign of damage, rot, or shriveling. Discard any tubers that are soft, diseased, or broken immediately to prevent the spread of decay to healthy material.
The goal of proper winter storage is to maintain cool dormancy without allowing the tubers to dry out completely or accumulate excessive moisture. They are best stored in a container that allows for air circulation, such as a ventilated cardboard box, wooden crate, or plastic bin with air holes. Avoid concrete surfaces, as they can draw moisture directly out of the tubers.
The tubers must be packed in a medium that helps regulate humidity, preventing both desiccation and mold growth. Suitable materials should surround the tubers to prevent them from touching one another. Separating the tubers minimizes the risk of a single rotten tuber spreading decay throughout the batch during the storage period.
Packing Mediums
- Slightly dampened peat moss
- Fine wood shavings
- Vermiculite
- Perlite
The ideal storage environment should be dark, frost-free, and maintain a stable temperature between 40 and 50 degrees Fahrenheit (4 to 10 degrees Celsius). High humidity, around 80 to 90 percent, is often necessary to prevent the tubers from shriveling and losing viability. Throughout the winter, check the stored tubers periodically. Remove any that show signs of mold or rot, and lightly mist the medium if the tubers appear wrinkled or shriveled.