Dahlia tubers are specialized storage organs that hold the plant’s energy reserves, allowing them to survive periods of dormancy. These fleshy structures are highly sensitive to freezing temperatures and excessive moisture. For gardeners in most temperate climates, harvesting the tubers is necessary to ensure their survival and guarantee blooms the following season. This process protects the tubers from the destructive effects of a cold, wet winter ground.
Timing the Harvest
The proper time to begin the harvest is signaled by the first hard frost of autumn. This abrupt drop in temperature causes the dahlia’s foliage to turn black, indicating that the plant has ceased active growth and is entering dormancy. The frost directs the remaining starches and sugars from the stems and leaves down into the tubers for winter storage.
Resist the urge to dig immediately after the foliage blackens. A waiting period of 10 to 14 days is recommended before lifting the tubers. This brief time allows the newly dormant tuber skin to thicken and toughen (suberization), which reduces moisture loss and vulnerability to disease during storage. The delay also allows the small growth points, or “eyes,” to become more visible on the crown, making identification easier.
The Lifting Process
Before digging, cut the dahlia stalks back, leaving a collar of the main stem that is approximately four to six inches in height. This short stem acts as a handle and serves as a location to attach a permanent label identifying the variety. Delaying the cutting until just before digging prevents the hollow stems from collecting rainwater, which could lead to crown rot.
The removal of the tuber clump requires careful technique to prevent damaging the delicate “necks” or piercing the storage tissue. Using a garden fork or spade, begin loosening the soil in a wide circle, starting about 12 inches away from the central stem. This distance ensures you are outside the main mass of the tuber clump.
Once the surrounding soil is thoroughly loosened, gently lever the entire root mass upward, maintaining support to avoid snapping the connection points. Tubers that are sliced or bruised are highly susceptible to rot during storage, so a slow approach is best. After the clump is out of the ground, carefully shake or brush away the bulk of the loose soil adhering to the mass.
Preparation and Curing
Following lifting, the tuber clumps should be gently cleaned to remove remaining soil particles, which can harbor disease pathogens and pests. A gentle stream of water from a hose or a quick dunk in a bucket is typically sufficient. Thorough inspection is then required to identify and remove any tubers that are visibly damaged, soft, or showing signs of disease, as these will rot and contaminate others in storage.
The clumps can be divided in the autumn, provided each separate tuber possesses a piece of the original crown tissue with at least one visible “eye.” Use a sharp, sanitized knife or shears for division to create clean cuts that heal quickly. While division is often easier in the spring when the eyes are actively sprouting, dividing in the fall reduces the storage space required.
The initial curing phase allows the outer skin to dry and the fresh cuts to scab over. Place the cleaned tubers in a protected area with good air circulation for one to seven days, depending on ambient humidity. This area should be cool and out of direct sunlight, which could scorch the skin. An advanced curing process involves maintaining high humidity (around 90%) at a slightly warmer temperature for up to two weeks, encouraging rapid wound healing and a durable winter skin.
Overwintering Storage Methods
Successful overwintering requires a storage environment that maintains a cool, dark, and consistently frost-free temperature. The ideal range for keeping dahlia tubers dormant without encouraging rot or premature sprouting is between 40°F and 50°F (4°C and 10°C). Maintaining high relative humidity, ideally around 80% to 90%, is necessary to prevent the tubers from shriveling due to moisture loss.
Storage mediums are used to moderate humidity and protect the tubers. Common materials include peat moss, vermiculite, wood shavings, or shredded newspaper, all of which should be only slightly damp—not wet—to prevent fungal growth. The tubers are layered in ventilated containers, such as cardboard boxes or plastic crates, ensuring they are completely surrounded by the medium.
The chosen medium prevents direct contact between tubers, which helps limit the spread of rot or mold. Throughout the winter, check the stored tubers periodically (perhaps once a month). Monitor for signs of shriveling, which indicates a need for more moisture, or the development of rot, which requires the immediate removal of the affected tuber.