Dahlias are tender perennials that cannot survive freezing temperatures, meaning their fleshy tubers must be lifted from the ground and stored indoors. Because the tubers contain a high percentage of water, they are susceptible to both freezing damage and desiccation during the winter months. Successfully overwintering these roots requires a controlled dormant period that protects them from extreme cold while managing moisture loss and preventing rot. This process ensures the viability of the tubers, allowing them to produce the next season’s blooms.
Preparing and Lifting Tubers
The process of safely removing dahlia tubers begins once the foliage has been killed back by the first hard frost, signaling the plant to enter dormancy. Waiting a week or two after the leaves turn black allows the tuber skins to toughen slightly, improving their resistance to damage and disease during storage. Before digging, the main stem should be cut down to a “handle” of about four to six inches above the soil line.
To avoid damage to the fragile connection between the tuber and the neck, a pitchfork or spade should be used to dig a circle approximately 12 inches away from the remaining stem. This distance accounts for the typical spread of the clump underground. The entire clump must be lifted carefully, gently rocking it out of the soil to prevent snapping the tubers or their necks, which must remain intact for the tuber to sprout next spring.
Once the clump is free from the ground, excess soil should be gently shaken or brushed off, and the clump should be turned upside down. This allows any water that may have collected inside the hollow stem to drain completely. Remove the bulk of the soil but avoid aggressively scrubbing the tubers at this stage, as this can cause small abrasions that invite fungal pathogens.
Curing and Initial Assessment
After lifting and cleaning, the tubers require a short curing period to prepare their protective outer layer for long-term dormancy. The clumps should be placed in a cool, dry location away from direct sunlight for 24 to 48 hours. This brief drying time allows the skin to heal any minor nicks sustained during digging and helps prevent moisture from encouraging rot in storage.
If dealing with heavy clay soil, a gentle wash with a hose nozzle may be necessary to remove stubborn dirt, but the tubers must be completely dry afterward. Following the curing period, a thorough inspection is necessary to identify and remove any compromised material. Using a clean, sharp tool, trim away and discard any thin feeder roots and any tubers that appear soft, mushy, diseased, or broken.
The clean tubers can be dusted with a fungicide or sulfur powder if desired, which provides extra protection against mold and rot, particularly in humid storage environments. This coating is not mandatory but can be a helpful preventative measure before the tubers are packed away.
Selecting Storage Conditions and Mediums
The success of overwintering depends on maintaining a specific, consistent storage environment. The ideal temperature range for dahlia tubers is between 40 and 50 degrees Fahrenheit (4 to 10 degrees Celsius) to keep them dormant without risking freezing. A high relative humidity, ideally between 75% and 85%, is also important, as it prevents the tubers from dehydrating and shriveling.
Breathable containers are suitable for storage because they allow for necessary air circulation. Sealed plastic containers should be avoided unless holes are drilled, as trapped moisture can quickly lead to fungal growth and rot. Placing the storage containers off a bare concrete floor is also recommended, as concrete can draw moisture out of the tubers. Suitable containers include:
- Cardboard boxes.
- Paper bags.
- Slatted milk crates.
The tubers must be nestled in a storage medium that regulates moisture and prevents them from touching each other. Peat moss and vermiculite are popular choices; peat moss holds moisture well, making it suitable for drier climates, while vermiculite offers excellent aeration, reducing the risk of rot. Finely shredded newspaper or wood shavings can also be used, but wood shavings may draw too much moisture from the tubers in a dry setting.
The packing technique is straightforward: a base layer of the chosen medium is placed in the container, and the tubers are arranged on top, ensuring no two tubers are in contact. They are then fully covered with the medium, and this layering process is repeated until the container is full. Each tuber must be completely encased in the medium to protect it from air exposure, which can cause rapid desiccation.
Mid-Winter Inspection and Care
A monthly inspection of the tubers is an important maintenance step during the storage period. The goal of this check is to catch any emerging issues, such as rot or excessive desiccation, before they spread throughout the container. Tubers should feel firm and plump, indicating they are retaining sufficient moisture.
If a tuber appears shriveled or wrinkled, it signals low humidity and drying out. To correct this, lightly mist the storage medium with water from a spray bottle, being careful not to directly wet the tubers. Conversely, if a tuber feels soft, mushy, or shows signs of mold, it must be removed immediately and discarded to prevent the spread of decay.
Temperature monitoring is a continuous concern, as a drop below 32 degrees Fahrenheit will ruin the tubers, while temperatures exceeding 50 degrees Fahrenheit can prematurely break dormancy. If a significant rot issue is discovered, change the storage medium entirely for the remaining healthy tubers to remove any lingering pathogens.