Dahlias originate from the tropical and subtropical highlands of Mexico and Central America, meaning their underground storage organs, the tubers, are not cold-hardy. If left in the ground in most climates, the tubers will freeze and rot. To successfully cultivate dahlias year after year, gardeners must lift and store the tubers indoors, mimicking the plant’s natural dormant period. This process requires a careful balance of temperature and moisture to ensure the tubers survive the winter without dehydrating or succumbing to fungal infections.
Preparing Tubers for Winter Storage
Preparation begins after the first hard frost, which signals the end of the growing season. Frost blackens the foliage and triggers the tubers to enter dormancy, toughening their outer skin for storage. Once the top growth is killed, cut the plant stems back, leaving a stalk four to six inches above the soil line. Leave the tubers in the ground for an additional week or two after cutting back the foliage, allowing this natural curing process to continue before lifting.
When lifting the tubers, use a pitchfork or garden fork and dig widely around the plant’s circumference, about a foot from the main stem, to avoid piercing the delicate tissues. Handle the tuber clump gently, as the neck—the fragile connection point to the stem—is susceptible to breaking, which can make the tuber non-viable. Whether to wash the tubers depends on the soil type. Heavy, clay-based soil should be washed off with a gentle stream of water to remove potential pathogens, while lighter, sandy soil can often just be brushed off.
After cleaning, the tubers enter the curing phase, a short-term drying period that seals minor cuts or abrasions. Lay the tubers out in a cool, dark, and frost-free area with good air circulation for one to two days, or up to several days in humid climates. The goal is to dry the surface enough to create a protective callus, but not so much that the tubers shrivel from moisture loss. Trim any remaining stem back to about an inch to prevent water from collecting inside the hollow center, which would lead to crown rot.
Essential Storage Environments and Mediums
Successful overwintering hinges on maintaining a precise environmental balance of temperature and humidity. The ideal temperature range for dormancy is between 40 and 50 degrees Fahrenheit (4–10 degrees Celsius). Temperatures below this risk freezing the tubers, which is irreversible. Conversely, temperatures consistently above 50 degrees Fahrenheit can cause the tubers to break dormancy prematurely and sprout, expending stored energy too soon for spring planting.
Humidity is the second half of the equation, ideally maintained between 75 and 90 percent relative humidity. This high moisture level prevents the tubers from dehydrating and shriveling. However, the tubers themselves must not be wet, as excess surface moisture promotes fungal and bacterial rot. The storage container and medium work together to buffer the tubers against extreme swings in the surrounding air.
Common storage mediums are chosen for their ability to regulate moisture around the tuber surface.
Storage Mediums
- Vermiculite is popular due to its sterile nature, light weight, and capacity to retain moisture without becoming saturated.
- Peat moss is widely used, but it must be barely moist. If too dry, it wicks moisture from the tubers; if too wet, it induces rot.
- Fine wood shavings, such as pine or spruce, are often sterile and can absorb any moisture released by the tubers.
Nest the tubers into the chosen medium within containers like plastic totes, cardboard boxes, or milk crates. If using plastic containers, ensure ventilation by drilling small holes or leaving the lid slightly ajar to prevent moisture buildup and rot. The tubers should be layered and completely covered by the medium to prevent exposure to air, which causes them to dry out and shrivel.
Checking and Addressing Common Storage Issues
Even with careful preparation, stored tubers require routine inspection throughout the winter. A monthly check is recommended to assess the condition of the tubers and the storage medium. The two most frequent issues encountered are rot and desiccation, both symptoms of an imbalance in temperature or humidity.
Tuber rot manifests as soft, mushy, or discolored spots, typically caused by too much moisture or high storage temperatures. If rot is discovered, immediately remove the affected tuber to prevent the spread of fungal pathogens. Small, soft spots can be surgically removed using a clean, sharp knife, cutting away damaged tissue until only firm, healthy white flesh remains. Allow the newly cut area to dry and callous for a day before returning the tuber to storage, possibly in a new, drier medium or a cooler location.
Desiccation occurs when the storage environment is too dry, causing the tubers to lose internal water and shrivel. A dry, wrinkled appearance indicates low relative humidity. To address this, lightly mist the storage medium with water to increase surrounding humidity, but do not spray the tubers themselves. Moving the container to a more enclosed space, such as a sealed plastic bag within a crate, can also help trap existing moisture. Increasing humidity can halt the process for tubers that have just begun to shrivel, but severely shrunken tubers are often no longer viable.
Common storage locations include an unheated basement, root cellar, or insulated garage that remains consistently above freezing. The storage area must also be protected from pests, such as mice, who view the starchy tubers as a food source. As spring approaches, typically four to six weeks before the last expected frost, move the tubers to a slightly warmer location to encourage sprouting, preparing them for the transition back to the garden.