Dahlias, celebrated for their vibrant, late-season blooms, are tender perennials that cannot survive the deep cold of winter in many regions. Unlike hardy bulbs, their storage organs are thin-skinned tubers, which are highly susceptible to freezing temperatures and excessive moisture in the soil, leading to rot and death. To preserve these valuable plants, gardeners must undertake the process of overwintering, which involves carefully lifting the tubers from the ground and storing them in a protected environment. This effort secures the genetic material of preferred varieties, allowing for the propagation of healthy, mature plants that will bloom reliably again.
Timing the Cutback and Lift
The signal to begin overwintering is the first hard frost, which causes the foliage to turn black and collapse. This triggers the plant’s natural dormancy, prompting the movement of starches and nutrients from the stems down into the tubers. Waiting for the foliage to die back naturally ensures the tubers are mature and have stored enough energy to survive the winter and produce strong growth the following season.
After the foliage is killed by frost, allow the tubers a brief “rest” period, keeping them in the ground for an additional one to two weeks before excavation. This delay allows the tuber’s skin to toughen or “cure” slightly, helping them resist damage and decay during storage. Before lifting, cut the hollow stalks down to four to six inches above the soil line. Cutting the stems immediately before digging prevents moisture from traveling down the hollow center of the stalk, which could promote crown rot.
The Process of Excavation and Curing
The removal of the tuber clump requires caution to avoid breaking the fragile neck connecting the tuber body to the crown. Use a garden fork rather than a shovel, as its tines loosen the soil instead of slicing through the tubers. Begin digging a wide circle, at least six to twelve inches away from the central stem, to remain outside the perimeter of the tuber cluster.
Gently work the fork around the plant, gradually lifting and loosening the soil before prying the clump from the ground. Once lifted, shake or brush off the excess soil. A thorough washing is optional, but if the soil is heavy clay, rinsing can help remove dirt that harbors microorganisms which may cause rot during storage.
The next step is the curing phase, which seals small wounds and hardens the tuber skin, minimizing moisture loss and susceptibility to pathogens. Place the cleaned clumps upside down in a protected, well-ventilated area for three to five days. Positioning them upside down encourages moisture to drain out of the hollow stem.
Curing Conditions
This area should be dark and maintain a cool temperature, ideally between 60°F and 70°F, with good airflow. During this time, inspect the tubers for any signs of damage, rot, or disease. Cut away any compromised sections, as a single bad tuber can quickly spread decay to the entire stored clump.
Choosing the Storage Environment and Medium
Successful long-term storage depends on maintaining specific environmental conditions that keep the tubers dormant without drying out or rotting. The ideal storage location should be dark and frost-free, with a consistently cool temperature range of 40°F to 50°F (4°C to 10°C). Temperatures above this range can cause premature sprouting, while freezing temperatures will destroy the tissue, rendering them non-viable.
Humidity is a major factor, with a target of around 85% being optimal to prevent the tubers from shriveling. Since achieving exact humidity can be difficult, a storage medium is used to buffer the moisture content around the tubers. These media help retain moisture and provide insulation without holding excessive water that promotes fungal growth.
- Lightly moistened peat moss.
- Vermiculite.
- Wood shavings.
- Dry shredded newspaper.
The tubers should be packed into containers, such as cardboard boxes or plastic crates, ensuring they are fully surrounded and separated by the medium to prevent them from touching. This isolation prevents the spread of decay if one tuber begins to rot.
Winter Monitoring
Throughout the winter, periodic checks (usually monthly) are necessary to monitor the health of the stored tubers. If a tuber shows signs of shriveling, a light misting of water can reintroduce moisture to the storage medium. Conversely, if soft spots or mold are present, the affected tuber must be immediately removed to protect the rest of the supply.