Canna lilies, known for their dramatic foliage and vibrant, showy flowers, are popular additions to summer gardens. These striking plants are technically perennial, meaning they can live for more than two years. Whether they return annually depends almost entirely on the winter temperatures of your location. In colder climates, human intervention is necessary to ensure these heat-loving plants survive the winter. Their survival is contingent upon protecting their underground storage structures, called rhizomes, from freezing.
Canna Lilies and Hardiness Zones
The survival of Canna lilies in the ground is determined by the USDA Plant Hardiness Zones, which map the average minimum winter temperatures. Canna lilies are reliably perennial in Zones 8 through 11, where the soil does not freeze deeply enough to damage the rhizomes. Gardeners in these warmer climates can leave their plants in the ground year-round, sometimes applying insulating mulch after the foliage dies back. The rhizomes remain dormant through winter and naturally sprout new growth when the soil warms in the spring.
In Zones 7 and below, the ground freezes solid for extended periods, which is lethal to the fleshy rhizomes. In these regions, Canna lilies must be treated as annuals or the rhizomes must be manually lifted and stored indoors to prevent freezing. If left in the ground in colder zones, the rhizomes will turn to mush and cannot regenerate the next season.
The Process of Overwintering
When the first light frost arrives, gardeners in colder regions must initiate the overwintering process. The ideal time to dig is shortly after the foliage is blackened by the frost, but before a deep freeze sets into the soil. This initial frost triggers the plant’s natural dormancy, which is necessary for successful storage. Cut the stems and foliage back to two to six inches above the soil line before carefully digging up the rhizomes.
Once lifted, gently clean the rhizome clumps to remove most of the soil. They must then be allowed to dry, or “cure,” for a few days in a warm, dry location away from direct sunlight to prevent rot during storage. After curing, store the rhizomes in a well-ventilated container, such as a cardboard box or a plastic bin with the lid ajar. Layer the rhizomes with a dry medium like peat moss, sawdust, or shredded newspaper to maintain a consistent, slightly humid environment and prevent desiccation.
The storage location should be dark, cool, and frost-free, ideally between 40 and 50 degrees Fahrenheit. Temperatures above this range can cause premature sprouting, while freezing temperatures destroy the tissue. Check the stored rhizomes periodically throughout the winter for signs of shriveling (too dry) or mold and rot (too much moisture). If they appear too dry, a light misting with water can rehydrate them, but avoid excessive moisture to prevent fungal issues.
Waking Up Rhizomes for Spring
To ensure an early season of flowers, bring the overwintered rhizomes out of storage four to six weeks before the last expected frost date. This pre-sprouting process gives the plants a head start, which is beneficial in regions with shorter summers. The firm rhizomes, which should have visible “eyes” or growth points, are planted horizontally in containers filled with moist potting mix. Place them in a warm area, such as under grow lights or on a heat mat, to encourage new shoot development.
Once shoots emerge and the indoor plants have developed a few leaves, they must be gradually introduced to outdoor conditions, a process called hardening off. This involves moving the potted cannas outdoors for increasingly longer periods over a week or two to acclimate them to sunlight, wind, and cooler temperatures. After all danger of frost has passed and the soil temperature has warmed to at least 50 degrees Fahrenheit, the pre-sprouted plants can be safely transplanted into the garden. The new growth from these stored rhizomes will then continue the cycle, producing vibrant tropical displays throughout the summer.