Cannas are vibrant, tropical perennials that grow from thick, underground stems called rhizomes. They provide lush foliage and bright flowers throughout the summer, but their native warmth means they cannot tolerate freezing temperatures. Successfully keeping cannas for the next season requires protecting these rhizomes from frost. For container-grown plants, this is easily accomplished by bringing the pot indoors. The specific winter storage method depends on the climate and the gardener’s available indoor space.
Determining the Overwintering Method Based on Climate
Overwintering potted cannas relies heavily on local winter temperatures, categorized by USDA Hardiness Zones. Gardeners in Zones 9 through 11 generally do not need to bring cannas inside, as the soil rarely freezes deep enough to damage the rhizomes. In Zone 8, and sometimes Zone 7, a heavy layer of mulch may protect plants left in the ground.
Potted cannas are more vulnerable to cold than those planted directly in the soil because their roots are exposed to chilling temperatures on all sides. Therefore, cannas in containers must be brought indoors for protection in Zone 7 and all colder regions. The expected low temperatures determine whether the plant is forced into dormancy or maintained in active growth.
Method 1 Dormant Storage Indoors
Dormant storage is the most practical and common overwintering method for gardeners in colder climates, requiring minimal light and maintenance. The process begins when the first light frost hits the foliage, signaling the plant to enter dormancy. Waiting for the leaves to blacken ensures maximum energy transfer from the leaves down to the rhizomes for storage.
After the foliage is killed by frost, cut the stems back, leaving a stub of one to two inches above the soil line. The pot must then dry out almost completely, as excessive moisture causes rhizome rot during storage. Keep the soil barely moist to prevent the rhizomes from desiccating, but dry enough to inhibit fungal growth.
Move the container to a cool, dark, and dry location where the temperature remains consistently above freezing, ideally between 40°F and 50°F. Good storage locations include an unheated garage, cool basement, or root cellar. Maintaining this temperature is important because warmer conditions cause premature sprouting, while temperatures near or below 32°F kill the plant tissue.
During dormancy, the canna should not be watered. Only mist minimally if the soil seems bone dry. The rhizomes rely on stored energy. Periodically inspect the pots for signs of mold, rot, or premature sprouting until early spring.
Method 2 Continuous Growth Indoors
An alternative approach is to treat the canna as a houseplant, maintaining continuous growth throughout the winter. Bring the plant inside well before the first frost, before the foliage is damaged. The canna needs a location that provides warmth, typically above 60°F, and significant light to continue photosynthesis.
A south-facing window or supplemental grow lights are necessary to prevent the plant from becoming long and spindly, a condition known as etiolation. Since the plant is actively growing, the maintenance routine contrasts sharply with dormant storage.
The canna requires consistent, though reduced, watering compared to the summer months. Allow the soil to dry slightly between waterings. Apply a diluted, balanced liquid fertilizer occasionally to support slow winter growth. This method is better suited for smaller container cannas or if a greenhouse or sunroom provides the necessary high light and temperature conditions.
Preparing Cannas for Spring Revival
A transition period is necessary to prepare cannas for the outdoor season, regardless of whether they were stored dormant or kept actively growing. For dormant pots, begin the revival process four to six weeks before the anticipated last frost date. Move the pot to a warmer location and gradually increase watering to signal the end of dormancy.
If the rhizomes are overcrowded, which is common after a full growing season, divide them before new growth begins. Carefully remove, separate, and replant the rhizomes into fresh potting mix. Each division must contain at least one healthy “eye,” the growth point where a new shoot will emerge.
For actively growing cannas, cut back the spindly winter growth to encourage bushier, stronger new shoots. Both dormant and active plants must be hardened off once the danger of frost has passed. This involves gradually exposing the plants to increasing amounts of outdoor sun and wind over one to two weeks before placing them in their permanent summer location.