Dahlias are tender perennials grown from tuberous roots. Highly sensitive to cold, winterizing is an annual necessity for most gardeners. In USDA Hardiness Zones 8 and colder, where the ground freezes, the tubers must be lifted and stored indoors to survive the dormant season. This process ensures the plant’s return and robust growth the following spring.
When to Cut Back and Lift Tubers
The timing for preparing dahlias for winter is signaled by the first hard frost. Frost causes the above-ground foliage to blacken and die back, indicating the plant has stopped its growth cycle. Allowing this natural dieback is beneficial because it triggers the plant to send energy and nutrients down into the tubers, helping them mature for storage.
After the foliage is killed by frost, cut the stalks back, leaving a stem of four to six inches above the soil line. Wait 5 to 14 days after the frost before digging the tubers out of the ground. This delay allows the protective outer layer of the tubers to cure slightly in the insulating soil, which helps prevent desiccation and rot during storage.
Cleaning and Curing the Tubers
Lifting the tubers must be done carefully to avoid damage, especially to the delicate “neck” connecting the tuber to the main stem. Use a garden fork or spade to loosen the soil in a wide circle, starting at least a foot away from the main stem to protect the sprawling tuber cluster. After lifting, remove the majority of the excess soil by hand or by lightly shaking the root ball.
Washing the clumps with a hose removes heavy soil and allows for better inspection for damage or disease. If washing, immediately place the clumps upside down so water trapped in the hollow stem can drain, preventing rot in the crown. Following cleaning, the tubers must enter a curing phase, drying in a cool, dark, and well-ventilated area for 24 to 48 hours. This curing helps the skin heal and seal minor nicks, forming a protective barrier before long-term storage.
Ideal Conditions for Winter Storage
Successful long-term storage requires a consistent environment that prevents freezing and excessive moisture loss. The ideal storage temperature is between 40 and 50 degrees Fahrenheit (4 to 10 degrees Celsius). Temperatures below this range risk freezing, destroying the tissue, while higher temperatures can cause premature sprouting or rot.
Humidity is important, requiring 75% to 85% relative humidity to keep the tubers hydrated. Low humidity causes shriveling, while high humidity encourages mold and fungus growth. To regulate moisture, pack the tubers in a slightly moistened, inert medium such as peat moss, vermiculite, or wood shavings. These materials buffer against environmental fluctuations and prevent contact between individual tubers, minimizing the spread of rot.
Pre-Spring Inspection and Division
Throughout the winter, periodically check the stored tubers, ideally once a month, to assess their condition. Look for shriveled tubers, which indicate the storage medium is too dry and may require misting. Tubers that appear mushy, soft, or show signs of mold should be promptly removed and discarded to prevent infection from spreading to healthy stock.
In late winter or early spring, the tuber clumps can be divided to increase the number of plants before replanting. Division is easier in the spring because the small, pinkish “eyes,” or growth points, are more visible as the tubers break dormancy. Each division must be carefully separated, ensuring it includes a viable eye and an undamaged piece of the neck connecting it to the main crown. This prepares the individual tubers for planting once the threat of frost has passed.