Dahlias are grown from underground tubers that cannot survive freezing temperatures. While they are a garden staple for their late-season blooms, they require specific preparation for dormancy to survive the winter. The initial step in this winterizing process is cutting back the plant’s spent foliage and stems once the growing season concludes. This action signals the plant to prepare for its dormant phase and protects the tubers from cold weather.
Identifying the Optimal Cutting Time
The precise timing for cutting back dahlia stalks is determined by the first “killing frost” of the season. This is a temperature drop severe enough to cause the above-ground plant tissue to freeze and collapse. When this happens, the foliage will visibly blacken and shrivel, indicating that the season’s growth is over.
Waiting for this natural event is important because the plant uses the freezing of the top growth as a signal. This prompts the dahlia to redirect remaining starches and energy reserves from the stems and leaves back down into the underground tubers. This final transfer of energy maximizes the tubers’ nutrient storage and ensures their viability for the following growing season.
Cutting the stalks prematurely, before the foliage has blackened, interrupts this natural energy transfer. After the killing frost occurs, the stems can be cut down immediately. It is recommended to wait approximately 10 to 14 days before digging the tubers. This short waiting period allows the plant sufficient time to complete the energy drawdown process and helps the “eyes,” or future growth points, to become more visible.
Proper Technique for Cutting Stalks
The physical act of cutting the dahlia stalks requires using the correct tools and making the cut at a precise height. Sharp, clean cutting tools, such as garden shears or loppers, should be used to make clean cuts. Sanitizing the cutting blade with a diluted bleach solution or alcohol between plants helps prevent the accidental transfer of fungal spores or viruses between different dahlia varieties.
The stalk should be cut back severely, leaving only a short section of the main stem standing above the soil line. A height of approximately four to six inches is commonly recommended for the remaining stem. This stub serves a dual purpose in the subsequent steps of winterizing the tubers.
The remaining stalk acts as a handle, offering a convenient point to grasp when carefully lifting the tuber clump from the soil with a digging fork. Leaving this protective stub also helps prevent moisture and pathogens from traveling directly down the hollow stem into the crown and tuber cluster below the soil. This short stem cap provides a barrier until the tubers are fully removed and prepared for storage.
Essential Post-Cut Winter Preparation
Following the cutting of the stalks, the next steps focus on the long-term survival of the dahlia tubers through the cold months. In regions with mild winters (USDA Hardiness Zones 8 and above), tubers can often remain in the ground if protected with a thick, insulating layer of mulch. However, in most climates where the soil freezes, the tubers must be dug up and stored indoors to prevent freezing damage.
Once dug out, the tuber clumps need to be dried and cured in a well-ventilated, frost-free location. This drying allows the outer skin to toughen, which helps reduce the risk of rot during storage. After this initial curing, the tubers can be cleaned of excess soil and labeled to track the variety for the next season.
Successful indoor storage relies on maintaining a cool, dark, and humid environment. The ideal temperature range for dormancy is between 38 and 50 degrees Fahrenheit; temperatures above this range can cause premature sprouting, and colder temperatures risk freezing. Humidity should be kept high, ideally between 75% and 85%, to prevent the tubers from shriveling and dehydrating.
To help regulate moisture, tubers are typically stored in a medium such as peat moss, vermiculite, or wood shavings. These materials cushion the tubers and help retain moisture without causing excessive wetness, which would lead to mold or rot. The storage container should allow for some air circulation while protecting the tubers from direct contact with freezing air.