Dahlias are perennials that cannot survive the deep ground freeze common in many temperate climates. Their fleshy, tuberous roots contain a high water content, making them extremely susceptible to freezing temperatures and subsequent rot. To successfully enjoy these vibrant blooms year after year, gardeners must lift the tubers from the soil and place them into protected winter storage. This process requires precise timing and careful technique to ensure the tubers remain viable until spring.
Determining the Optimal Time for Lifting
The timing for lifting dahlia tubers is dictated by specific environmental cues, most notably the first hard frost of the season. This “killing frost” causes the above-ground foliage to blacken and collapse, signaling the plant’s growth has ceased. Waiting for this event is beneficial because the plant shifts its remaining energy and carbohydrates from the dying foliage down into the tubers for storage.
Lifting the tubers immediately after this frost is not recommended. It is advised to wait one to two weeks following the complete dieback of the top growth. This brief period allows the tuber’s outer layer to stabilize its moisture content, a process sometimes referred to as “curing” or forming a “winter skin.” This toughened layer helps the tuber resist dehydration and fungal pathogens during dormant storage.
Lifting too early means the tubers may not have fully matured or developed this protective layer, leading to poor storage success. Waiting too long, especially until the ground freezes solid, will destroy the tubers entirely, as ice crystals rupture their cellular structure. While gardeners in warmer regions (USDA Zone 7 and above) may leave tubers in the ground with thick mulch, indoor storage remains the most reliable method.
The Process of Excavation
Once the curing period is complete, excavation begins by preparing the remaining stem. The stalk should be cut down to four to six inches above the soil line, which provides a convenient handle for lifting and marks the clump’s location. Cover the hollow top of this remaining stem with aluminum foil or tape to prevent water from entering and promoting crown rot.
The goal of digging is to remove the entire clump without piercing or snapping the delicate tubers, especially at the “neck” where the tuber joins the stem. This neck region is where the growth buds, or “eyes,” are located, and damage here renders the tuber unviable for the next season. Using a garden fork rather than a spade is preferred, as the tines are less likely to slice through the roots.
To safely extract the clump, insert the fork into the soil in a wide circle, approximately 12 inches away from the central stem. Working around the perimeter, gently rock the fork back and forth to loosen the surrounding soil without levering the clump itself. Once the soil is loosened, the cluster can be gently lifted using the cut stem as a handle, taking care to cradle the mass to prevent the tubers from breaking their fragile necks.
Preparing Tubers for Winter Storage
After lifting, the clumps require cleaning and preparation before their dormant period. The first step involves thoroughly removing all soil, which harbors microorganisms that can cause rot during storage. A gentle stream of water from a garden hose is used to wash away the dirt, allowing for inspection of the clump for any damage.
Following washing, the tubers must be cured or dried in a cool, shady, and well-ventilated location for one to three days. This allows surface moisture to evaporate and minor cuts to heal or “callus over” before winter storage. Avoid placing the tubers directly onto concrete floors, as this material can wick moisture out too quickly, causing excessive shriveling.
This is the ideal time to divide the tuber clumps, though some growers wait until spring when the eyes are more visible. For a successful division, each separated tuber must contain an intact body, a piece of the crown or neck tissue, and at least one viable growth eye. To protect the tubers from fungal issues like fusarium and botrytis rot, apply a dusting of powdered sulfur or a garden fungicide. This is often done by shaking the tubers inside a bag containing the powder and a storage medium like vermiculite.
Selecting the Ideal Storage Environment
A successful storage environment maintains a balance between temperature and humidity to keep the tubers dormant and healthy. The most suitable temperature range is a cool, non-freezing environment, ideally maintained between 40 and 50 degrees Fahrenheit (4.5 to 10 degrees Celsius). Temperatures below freezing will destroy the tubers, while temperatures above 50 degrees Fahrenheit can cause premature sprouting and depletion of stored energy.
A high relative humidity is equally important, with an optimal range of 70 to 90 percent necessary to prevent shriveling and drying out. Tubers should be stored in a ventilated container (such as a cardboard box, plastic crate, or laundry basket), rather than an airtight container that can trap moisture and encourage rot. This allows for adequate air circulation during their dormancy.
The tubers should be layered within a dry, lightweight, and insulating medium to help regulate moisture and protect them from temperature fluctuations. Common materials used for this layering include:
- Dry peat moss.
- Vermiculite.
- Sawdust.
- Wood shavings.
Throughout the winter, check the stored tubers monthly for signs of rot (soft or mushy spots) or excessive shriveling, which indicates the environment is too dry and may require a light misting.