Dahlias are tender perennials that cannot withstand freezing temperatures in many parts of the world. Gardeners who experience cold winters must follow a specific, multi-step process to prepare these plants for dormancy and ensure their survival until the following spring. Successfully overwintering dahlias involves careful timing, proper pruning, and controlled storage of the underground tuberous roots. This guide details how to prepare dahlia tubers for their winter rest.
Identifying the Right Time to Begin Winter Prep
The winter preparation process begins with the arrival of the first hard or killing frost. This event causes the foliage to turn black and collapse, marking the end of the growing season. Waiting for this natural signal is important because the plant uses the time immediately following the frost to transfer energy reserves from the dying stalks and leaves back into the tubers below ground.
These reserves, primarily starches and sugars, are essential for the tubers to survive dormancy and fuel initial growth the next season. Prematurely cutting the foliage before this transfer is complete can weaken the tuber and reduce its viability. Gardeners should wait approximately one to two weeks after the foliage has been blackened by the frost before proceeding with the physical cutback. This waiting period ensures the tubers have accumulated maximum resources and entered true dormancy.
The Physical Cutback: Pruning Dahlia Stems
Once the plant is ready for dormancy, the next step involves reducing the above-ground biomass. Use clean, sharp pruning shears to make a quick, precise cut, minimizing the risk of introducing disease to the remaining tissue. The entire plant structure should be cut down, leaving a short stem stub protruding from the soil.
This remaining stalk should measure approximately four to six inches in height above the soil surface. Leaving this short handle provides a convenient marker for the tuber clump when it is time to dig and lift them. It is also helpful to affix a durable label to this stub, clearly marking the dahlia variety. Because the dahlia stem is hollow, cutting the stalk creates a direct opening to the crown of the tuber clump.
Lifting, Cleaning, and Curing the Tubers
Lifting the tubers requires patience to avoid damaging the fragile tuber necks, which connect the storage roots to the main stalk. Insert a garden fork or spade into the soil in a wide circle, approximately 12 to 18 inches away from the remaining stem. This distance ensures the digging tool does not accidentally slice into the clump. Gently loosen the soil all around the circumference before carefully levering the entire clump out of the ground by gripping the main stalk.
After lifting, remove the excess soil from the tuber clump. Brushing off the bulk of the dirt is often sufficient, though some growers rinse the clump gently with a hose, especially if the soil is heavy clay. If washing, ensure the tubers are not bruised or scraped, as any open wound can invite rot during storage. The tubers are then moved to a location for curing, allowing the skin to dry and harden.
Curing involves placing the tubers in a cool, dry, and shaded area for one to three days, until the surface feels dry to the touch. This period allows a protective layer, called the periderm, to toughen and heal any minor nicks sustained during digging. Avoid placing the tubers directly on concrete surfaces, as concrete can draw out too much moisture and cause shriveling. Proper curing ensures the tuber’s surface is resistant to fungal pathogens and moisture loss during storage.
Ensuring Successful Winter Storage
Successful overwintering relies on maintaining a stable, cool environment that prevents both freezing and excessive dehydration. The ideal temperature range for dahlia tuber storage is consistently between 40 and 50 degrees Fahrenheit (4 to 10 degrees Celsius). Temperatures below freezing will cause the tubers to turn to mush, while temperatures too high can signal them to break dormancy and sprout prematurely.
A suitable storage medium is necessary to regulate humidity and prevent the tubers from shriveling. Materials such as peat moss, vermiculite, or wood shavings are commonly used to pack the cured tubers. The tubers should be nestled within the medium, ensuring they do not touch each other, which prevents the spread of potential rot or mold. The surrounding material should be slightly moist but never wet, maintaining a humidity level of about 80 to 90 percent.
Throughout the winter, a monthly inspection of the stored tubers is necessary. During this check, examine the tubers for signs of shriveling, which indicates the medium has become too dry. A slight misting of the packing material can restore humidity without soaking the tubers. Any tubers showing signs of mold or rot must be immediately removed and discarded to prevent the issue from spreading to healthy stock.