Dahlias are tender perennials in most growing zones, meaning their fleshy, water-storing root structures, known as tubers, cannot survive freezing temperatures in the soil. The vibrant blooms that last through the summer and fall rely entirely on these tubers for their energy and survival. Proper care in the autumn is a necessary process to ensure the tubers mature correctly and remain viable through a long winter dormancy. Successfully overwintering these underground energy reserves guarantees the return of your favorite varieties the following spring.
Winding Down Growth Before Frost
Preparation for winter storage begins several weeks before the first expected frost, while the plant is still actively growing. This period shifts the plant’s energy from producing foliage and flowers to maturing the tubers underground. To signal this change, gradually stop all fertilization, particularly those high in nitrogen, about 4 to 6 weeks before freezing temperatures are likely. Nitrogen promotes soft, leafy top growth, which is detrimental to the hardening-off process required for long-term storage.
Reducing the frequency of watering also signals that the growing season is ending. Allowing the soil to dry slightly encourages the plant to draw moisture and starches back down from the stems and leaves into the tuberous roots. This concentration of carbohydrates and reduced moisture content prepare the tubers for dormancy and enhance their resistance to rot during storage. The foliage will naturally begin to yellow and fade as this process occurs.
Cutting Back Stalks and Waiting
The true end of the dahlia’s growing season is marked by the first hard or “killing” frost, which causes the foliage to blacken and collapse. This indicates that the above-ground portion of the plant is dead and preparing for dormancy. Once the stems and leaves are blackened, cut the stalks down to a height of four to six inches above the soil line.
This remaining stub serves as a handle for lifting and prevents precipitation from entering the hollow stem, which could cause the crown to rot. After cutting the stalks, wait seven to fourteen days before digging up the tubers. This brief time allows the eyes (the tiny growth buds for next season) to fully set on the crown and for the tuber’s skin to toughen or “cure” while still in the ground. This in-ground curing also permits the final transfer of starches from the stem stub down into the tubers, maximizing energy reserves for the winter.
Lifting and Curing Dahlia Tubers
Once the waiting period is complete, the physical process of lifting the tubers must be done with care to avoid damage, especially to the fragile neck where the tuber attaches to the stem. Use a pitchfork or garden spade, inserting it wide and deep around the remaining six-inch stalk to loosen the soil, typically at least a foot away from the central stem. Gently leverage the tool to lift the entire clump out of the ground, taking care not to snap the necks, which are necessary for next season’s growth.
Next, clean the clumps by carefully brushing off the bulk of the soil or by rinsing them gently with a hose. Washing the tubers makes it easier to inspect them for damage or disease and removes soil pockets that retain excess moisture in storage. After cleaning, the tubers must enter a short-term curing phase in a cool, dark, and well-ventilated location for 24 to 48 hours. This brief drying period allows any cuts or abrasions to form a protective layer, which seals the skin and prevents moisture loss and pathogen entry during winter storage.
Creating the Optimal Winter Storage Environment
Successful overwintering depends on maintaining a dark, cool, and slightly humid environment that keeps the tubers dormant without letting them dry out or freeze. The ideal temperature range for dahlia tuber storage is between 38 and 50 degrees Fahrenheit (4 to 10 degrees Celsius). Temperatures below freezing will destroy the tubers, while temperatures consistently above 50 degrees Fahrenheit can encourage premature sprouting and shriveling.
The storage area should also maintain a high humidity level, ideally between 75% and 85%, to prevent dehydration. To achieve this balance, place the cured tubers in a storage medium such as peat moss, coarse vermiculite, or wood shavings. The medium should be barely moist, like a wrung-out sponge, to buffer the humidity without saturating the tubers.
Store the tubers in ventilated containers, such as open cardboard boxes, bulb crates, or paper bags, and avoid using airtight plastic containers, which trap moisture and promote rot. Checking on the stored tubers monthly is necessary to look for signs of shriveling, which indicates the environment is too dry, or mold, which suggests it is too wet. Adjusting the humidity or promptly removing any compromised tubers will help preserve the rest of your stock until the spring planting season.