Dahlias, which grow from a cluster of tuberous roots, are classified as tender perennials because they are native to the mountainous regions of Central America. They cannot tolerate the sustained freezing temperatures of a cold winter. The question of whether you must dig them up annually depends entirely on the climate where they are grown. The survival of the underground tubers is directly linked to the severity of the frost and the soil temperature, making the need to lift them a geographical decision.
Climate Dictates the Necessity of Lifting
Dahlia tubers are fleshy and full of water, which makes them highly susceptible to freezing. The internal cell structure of the tuber ruptures when the surrounding soil temperature drops consistently below 40°F (4°C). This freezing and thawing process destroys the tuber, causing it to rot and preventing the plant from regrowing the following spring.
The United States Department of Agriculture (USDA) Hardiness Zone system provides the clearest guidance on whether you need to lift the tubers. Gardeners in Zones 8 through 11, where the ground generally does not freeze solid, can often leave their dahlias in the ground year-round. Even in these milder areas, applying a thick layer of insulating mulch is recommended to protect the tubers during occasional unexpected cold snaps.
In contrast, those gardening in Zones 7 and below must treat the dahlia as an annual or lift and store the tubers indoors to ensure their survival. This effort transforms the dahlia into a perennial that returns each year, rewarding the gardener with earlier and more prolific blooms.
Lifting and Initial Preparation of Dahlia Tubers
The correct timing for lifting dahlia tubers occurs after the plant has experienced its first hard frost. A hard frost is when the temperature drops low enough to kill and blacken the foliage and stems above ground. This freezing signal forces the plant to stop growing and channel energy reserves back into the tubers, helping them mature and prepare for dormancy.
It is beneficial to wait for approximately one to two weeks after the foliage has been killed by frost before digging. This brief curing period in the soil allows the tuber skins to toughen, creating a protective layer that resists damage and dehydration during storage. When ready to dig, cut the main stalks down to about four to six inches above the soil line.
Digging must be done with care, starting about a foot away from the main stem to avoid puncturing the tubers. The most vulnerable area is the “neck,” where the tuber joins the stem, as damage here often leads to rot. Once the entire clump is gently lifted, remove the bulk of the soil by hand or with a gentle rinse from a hose. The cleaned tubers must then be cured further by drying them in a cool, dark, and well-ventilated location for 24 to 48 hours. This final drying firms the skin and ensures the surface is dry before they are packed away for winter.
Steps for Winter Storage
Successful winter storage depends on maintaining a consistent, cool environment that prevents both freezing and premature sprouting. The ideal temperature range for dormant dahlia tubers is between 40°F and 50°F (4°C to 10°C), with a moderate to high humidity level of about 80 to 90%. Temperatures below freezing will destroy the tubers, while temperatures consistently above 50°F can cause them to break dormancy and begin to sprout prematurely.
The tubers should be stored in a medium that helps regulate moisture and allows for air circulation. Suitable packing materials include:
- Slightly moistened peat moss.
- Vermiculite.
- Shredded newspaper.
These materials prevent the tubers from shriveling due to excessive moisture loss without trapping so much moisture that they begin to mold or rot. Tubers can be placed in perforated plastic bags, cardboard boxes, or plastic totes, ensuring the container allows for air exchange.
Throughout the winter months, periodic inspection of the stored tubers is necessary to monitor their condition. If the tubers appear shriveled or wrinkled, the storage environment is too dry, and a light misting or addition of slightly damp packing medium may be needed. Conversely, if any tubers show signs of mold or become soft and mushy, they should be removed immediately to prevent the spread of decay to the healthy stock.