Canna lilies are tropical or subtropical perennials valued for their dramatic foliage and vibrant, summer-long blooms. In regions experiencing cold winters, these plants cannot survive freezing temperatures outdoors, making proper overwintering a necessary annual task. The fleshy underground stems, known as rhizomes, must be carefully lifted and stored to protect them from frost damage. This process ensures the rhizomes remain healthy and viable until the following spring.
Determining the Optimal Time to Dig
The timing for lifting canna rhizomes is dictated by the weather and signals the plant’s natural shift toward dormancy. Gardeners should wait for the first light frost to naturally blacken and collapse the above-ground foliage. This initial frost triggers the plant to cease growth and send its stored energy and nutrients down into the underground rhizomes. Waiting for this natural signal provides the longest possible growing season, allowing the plant to store maximum food reserves.
It is important to lift the rhizomes before a deep, hard freeze occurs, as this can penetrate the soil and destroy the tender tissues. Before digging, the remaining plant stalks should be cut back, leaving a short stem section approximately two to six inches tall attached to the rhizome. This short stem provides a handle for lifting and helps mark the position of the rhizome clump.
Lifting and Curing the Rhizomes
The physical process of lifting the rhizomes requires a gentle approach to prevent unnecessary bruising or breaking of the storage structures. Using a garden fork or shovel, carefully loosen the soil in a wide circle around the base of the cut stem before gently prying the clump from the earth. Damage to the rhizomes creates open wounds that make them susceptible to fungal pathogens and rot during the long storage period.
Once lifted, the bulk of the excess soil should be carefully shaken or brushed off the rhizome clump. Avoiding washing with water is generally preferred, as introducing moisture at this stage can promote the development of mold or decay. Any small, diseased, or damaged sections of the rhizome should be cleanly trimmed away using disinfected shears to prevent the spread of infection.
Following the initial cleaning, the rhizomes must undergo a “curing” period before being placed into long-term storage. This involves setting the rhizomes in a protected, well-ventilated area that is shaded and warm, often between 70 and 80 degrees Fahrenheit. Curing allows the outer skin to dry and harden, which helps seal any minor cuts or abrasions incurred during the lifting process, typically taking about one week.
Establishing the Ideal Winter Storage Environment
Successful overwintering relies on maintaining a consistently appropriate storage environment to keep the plant in a dormant state. The ideal temperature range for dormant canna rhizomes is generally between 35 and 50 degrees Fahrenheit (2 to 10 degrees Celsius). Temperatures above this range may encourage premature sprouting, which depletes the rhizome’s energy reserves.
The storage area must also balance humidity to prevent two opposing problems: desiccation and mold growth. A moderate humidity level, often around 40 to 50 percent, is necessary to prevent the rhizomes from drying out and shriveling over the months. Good air circulation is also important to prevent stagnant air, which can exacerbate moisture issues and encourage the growth of pathogens on the stored material.
To ensure the rhizomes maintain stable moisture levels and are protected, they should be nested within a suitable packing medium. Materials like dry peat moss, vermiculite, sawdust, shredded newspaper, or coir (shredded coconut husk fiber) work well. These materials provide insulation and absorb excess moisture while allowing air to circulate.
Appropriate containers include cardboard boxes, wooden crates, or sturdy paper bags, all of which permit air exchange. Plastic containers or sealed bags should be avoided because they trap moisture and inhibit ventilation, which can lead to rapid decay. The rhizomes should be spaced so they are not touching, and the container should be labeled with the variety name and date for easy identification in the spring.
Mid-Winter Inspection and Spring Preparation
Periodic inspection throughout the winter storage period is necessary to monitor the health of the dormant rhizomes. A check should be performed approximately once a month to look for signs of either dehydration or excessive moisture. Dehydration appears as shriveling and wrinkling, while excessive moisture manifests as soft spots, mold, or decay.
If a small section of rot is discovered, it should be cleanly trimmed away using a sterile knife, and the cut surface can be dusted with sulfur powder to prevent further fungal spread. For rhizomes that appear too dry, a light misting of the surrounding packing material with a spray bottle can prevent further desiccation. If the packing medium is visibly damp, it should be replaced immediately with completely dry material before repacking.
As spring approaches, typically four to six weeks before the last expected frost date, the rhizomes can be prepared for replanting by initiating the pre-sprouting process. This involves bringing the clumps into a warmer, brightly lit indoor area and potting them in lightly moistened soil or peat. The appearance of small shoots indicates that the rhizomes are successfully breaking dormancy and are ready for transplanting outdoors once the danger of frost has completely passed.