Dahlias, with their vibrant blooms, are a favorite in many gardens, but they are not tolerant of cold temperatures. Because the underground storage organs known as tubers are tender and cannot survive freezing soil, they must be lifted and protected each winter in climates experiencing hard frosts. Storing these tubers indoors allows gardeners to preserve their preferred varieties and ensures the perennial return of these beautiful flowers.
Harvesting the Tubers
Lifting dahlia tubers begins once the foliage has been killed by the first hard frost of the season. This frost signals the plant to stop growth and directs energy into the tubers, preparing them for dormancy. Waiting for this natural cue is important, as the tubers continue to mature and toughen their skin while still in the ground.
Before digging, cut the main stems back, leaving a stalk of four to six inches above the soil line to serve as a handle and marker. Use a garden fork or spade to carefully loosen the soil in a wide circle, approximately 12 to 18 inches away from the stem, to avoid accidentally spearing the fragile tubers. The tubers form a clump radiating from the main stem, and their necks are particularly susceptible to breaking if handled roughly. Gently lift the entire clump from the earth, ensuring the delicate neck connecting the tuber to the crown remains intact.
Once the tuber clumps are out of the ground, carefully brush or shake off the loose soil clinging to them. Immediately label each clump with the variety name using a waterproof marker or tag tied securely to the remaining stem. Proper labeling prevents mixing varieties, ensuring you know which flowers you are planting the following spring. The tubers are now ready for the next phase of preparation.
Cleaning and Curing
After harvesting, the tubers require immediate cleaning to remove excess soil, which can harbor moisture and disease-causing microorganisms that lead to rot during storage. Use a gentle stream of water from a garden hose to wash away the remaining dirt. While washing, inspect the tubers closely for any signs of damage or disease, such as soft spots or cuts.
Any damaged or diseased parts should be cleanly trimmed away with a sharp, sterilized knife to prevent the spread of infection to other tubers during storage. Remove the fine, hair-like feeder roots at this time. Once cleaned, the tubers must undergo a short curing period to allow the skin to toughen and surface wounds to heal.
Place the cleaned tubers in a cool, sheltered, and well-ventilated area for 24 to 48 hours, protected from direct sunlight and rain. This drying process helps seal the outer layer of the tuber, reducing moisture loss and making the surface resistant to fungal or bacterial entry. The skin should feel slightly tougher and drier before moving on to long-term storage.
Long-Term Storage Requirements
Successful winter storage requires a consistent environment that prevents the tubers from shriveling or rotting. The ideal storage temperature range is 40 to 50 degrees Fahrenheit (4 to 10 degrees Celsius), which keeps the tubers dormant without risking freezing. Temperatures below 32 degrees Fahrenheit will destroy them, while temperatures above 50 degrees Fahrenheit can cause premature sprouting.
A moderate to high level of relative humidity (75% to 85%) is necessary to prevent the tubers from drying out. If the storage location is too dry, the surrounding medium must be slightly moistened to regulate humidity. If the environment is too damp, adequate air circulation is needed to prevent mold and fungal growth.
The choice of packing material regulates the moisture level around the tubers. Effective storage mediums include materials like peat moss, vermiculite, or dry wood shavings. These materials absorb excess moisture while maintaining a humid microclimate. The tubers should be layered in containers, such as vented cardboard boxes or plastic bins, and completely covered with the chosen medium. Ensure individual tubers do not touch each other, as this facilitates the spread of rot or disease.
Throughout the winter, check the stored tubers periodically, typically once a month. If the tubers appear shriveled, the storage medium is too dry, and a light misting of water may be needed. If any tubers show signs of mold or rot, they must be immediately removed and discarded to prevent infection from spreading.
Waking Tubers for Spring Planting
The storage period ends four to six weeks before the last expected frost date. This is the time to bring the dormant tubers out of storage and prepare them for the growing season. The first step involves inspecting the tubers and dividing the large clumps into smaller, viable sections.
Division is often easier in the spring because the small growth buds, known as “eyes,” are more visible as the tubers wake up. Each new division must include a portion of the crown (the area where the tuber meets the stem) and at least one visible eye, as this is the point from which the new plant will sprout. Tubers without an eye will not grow. Discard any sections that are soft, mushy, or completely dry and shriveled.
To give the plants a head start, the divided tubers can be “pre-sprouted” indoors. Place them horizontally in trays filled with a lightly damp, sterile medium, such as potting mix or peat moss. The eyes should be facing upward and lightly exposed or barely covered. Placing these trays in a warm, bright location encourages the eyes to sprout and form shoots, preparing the young plants for planting outdoors once the danger of frost has passed.