Dahlias are prized for their vibrant blooms, offering spectacular color late in the growing season. Though often mistakenly called a bulb, the dahlia’s underground storage organ is botanically a tuber, serving as a reservoir for starches and nutrients. These tubers are sensitive to cold and will not survive freezing soil temperatures. Gardeners in regions below USDA Hardiness Zone 8 must lift and store them indoors for winter protection. Safe overwintering preserves the plant’s energy, allowing the tubers to sprout and produce flowers again the following spring.
Timing is Crucial Preparing for Lift-Off
The process of preparing dahlia tubers for winter storage begins with the first hard, killing frost. This frost causes the foliage to wilt and turn black, which signals the plant to enter a state of dormancy. Waiting for this event is important because the plant responds by relocating energy and starches from the stems and leaves down into the tubers, helping them mature for storage.
Immediately after the top growth has been blackened by the frost, the stems should be cut back, leaving a short stalk four to six inches above the soil line. This short stem acts as a handle for later lifting and marks the exact location of the tuber clump. Some growers recommend waiting one to two weeks after cutting back the foliage before digging, as this allows the skin of the tuber to toughen slightly and the interior moisture to stabilize.
Lifting and Cleaning the Tubers
When the time comes to lift the tubers, use a garden fork or spade, inserting the tool several inches away from the remaining stalk to avoid damaging the cluster. The dahlia’s root system includes a delicate area called the “neck” where the eyes, or growth points, develop for the next season. Any damage to this neck can prevent the tuber from sprouting, so digging carefully in a circle around the plant is necessary before gently prying the clump loose.
Once the clump is out of the ground, carefully shake away the loose soil. Remaining dirt should be washed off gently using a hose, as soil contains microorganisms that can cause decay during storage. While some growers choose to divide large clumps at this stage, the focus of cleaning is to remove all soil and inspect the tubers for any signs of disease or damage.
The Overwintering Phase Drying and Storage
The first phase after cleaning is curing, which involves allowing the outer skin of the tuber to dry and harden. Tubers should be placed in a cool, dark, and protected location with good air circulation for 24 to 48 hours immediately following washing. This curing period is necessary for any small wounds or abrasions to form a protective layer, which reduces the risk of rot in storage.
For long-term storage, the goal is to maintain a balance of temperature and humidity to prevent both desiccation and mold. Tubers need to be packed in a medium that provides insulation and absorbs minor moisture fluctuations. Acceptable storage containers include ventilated cardboard boxes or plastic tubs, where the tubers are layered and completely covered by the chosen packing material. Common packing materials include:
- Lightly moistened peat moss
- Vermiculite
- Wood shavings
- Dry newspaper
The ideal storage environment is a dark, frost-free location where the temperature remains consistently cool, specifically between 40°F and 50°F (4°C and 10°C). Temperatures much higher than this range can cause the tubers to sprout prematurely, while temperatures near or below freezing will kill the plant material. Common locations that meet these criteria are unheated basements, root cellars, or temperature-stable closets.
Monitoring During Dormancy
A monthly inspection of the stored tubers is necessary throughout the winter months to ensure successful dormancy. The two main threats are excessive moisture leading to rot, and a lack of moisture causing desiccation. If a tuber feels soft, mushy, or shows visible signs of mold, it indicates conditions are too moist; the affected tuber should be removed immediately to prevent spread, and the storage medium adjusted to a drier state.
Conversely, if the tubers appear shriveled or wrinkled, they are losing too much internal moisture. This is corrected by lightly misting the storage medium or adding slightly damp packing material to the container. Regular inspection allows for these minor adjustments, which are often the difference between a successful winter and a complete loss of the dahlia stock.