Dahlias are grown as annuals in many regions because their fleshy, underground storage organs, known as tubers, cannot survive freezing temperatures. Outside of the mildest climates (typically USDA Zone 8 and warmer), lifting the tubers from the soil and providing an artificial dormancy period indoors is the only method to preserve the plant. This process protects the tubers from winter frost and excessive moisture, which can cause them to rot. Successful overwintering requires careful management of temperature, humidity, and handling throughout the storage period.
Preparing Tubers for Dormancy
Timing the excavation should occur after the foliage has been blackened by the first hard frost. This killing frost signals the plant to stop active growth and directs energy and starches down into the tubers, ensuring they are fully mature for storage. Waiting a few days to a week after the tops have died back allows the tubers to properly enter dormancy and makes the eyes—the tiny growth points for next year—more visible.
Before digging, cut the stalks down, leaving a short stem handle of about two to six inches above the soil line. This trimmed stalk acts as a handle for lifting and helps reduce the chance of rot by allowing remaining moisture to drain from the hollow stem. To lift the clump, begin digging about a foot away from the main stem to avoid slicing into the tubers.
A pitchfork or garden fork is a better tool than a shovel for this task, as it minimizes the risk of damage during lifting. The entire clump must be gently pried from the ground. Take extra care to protect the delicate neck area where the tubers attach to the stem. Wounds to the tuber’s skin or neck create entry points for pathogens, increasing the likelihood of rot during storage.
Cleaning and Curing Techniques
Once the tubers are out of the ground, remove the bulk of the soil clinging to the clump. While brushing off excess dirt is an option, a light rinse with a garden hose or a dip in water is often necessary, especially for tubers grown in heavy clay soil. The goal is to clean the tubers enough for a thorough inspection for damage or disease, without puncturing the skin.
Immediately following cleaning, the tubers must be cured by drying the outer skin to a slightly hardened state. This initial drying period helps prevent fungal and bacterial growth that thrives on surface moisture. The clumps should be placed in a cool, well-ventilated, and frost-free location, such as a garage or covered porch, for 24 hours to a few days.
During curing, place the clumps upside down to allow any remaining water inside the hollow stem to drain away, further reducing the risk of rot. The tubers are ready for packing once the surfaces feel dry to the touch and any cut or broken areas have formed a protective callus.
Choosing Storage Mediums and Environments
Long-term storage success relies on maintaining a precise balance of temperature and humidity. The ideal storage temperature range is narrow, falling between 40 and 50 degrees Fahrenheit (4 to 10 degrees Celsius). Temperatures below this range risk freezing, while temperatures above it can cause the tubers to break dormancy and sprout prematurely.
Equally important is the relative humidity, which should be maintained around 80 to 90 percent to prevent shriveling. The storage medium plays a significant role in regulating moisture around the tubers. Common packing materials include:
- Peat moss
- Fine vermiculite
- Sawdust
- Shredded newspaper
The tubers should be layered in a container, such as a plastic bin or cardboard box, with the chosen medium placed both below and above them. The material must surround the tubers completely, ensuring they are not touching each other. This prevents localized rot from spreading throughout the batch. Vermiculite and peat moss are popular for their ability to buffer humidity swings, though dry peat moss can sometimes draw too much moisture from the tubers.
Monitoring and Addressing Storage Issues
The dormant period necessitates regular monthly checks to ensure the tubers remain healthy and viable. The two most frequent storage failures are desiccation (excessive drying) and rot or mold. Tubers suffering from desiccation will appear shriveled and wrinkled, indicating the storage environment is too dry and the tuber is losing internal moisture.
To remedy shriveling, move the tubers to a more humid location, or lightly mist the packing medium with water. Note that once a tuber has lost significant water, it cannot fully plump back up, but the process can be halted. Conversely, soft or mushy spots, often accompanied by mold, signal that the environment is too warm or the humidity is too high.
If mold is present but the tuber is still firm, wipe the mold off and replace the packing material with a fresh, drier medium. Any tuber that has completely rotted must be removed immediately and discarded to prevent the problem from spreading. Maintaining the ideal temperature and humidity, which may require using a thermometer and hygrometer, is the most effective preventative measure against desiccation and rot.