Cannas, commonly known as Canna lilies, are perennial plants originating from tropical and subtropical regions of the Americas. Their fleshy underground storage organs, called rhizomes, are sensitive to freezing temperatures. Gardeners outside of the warmest climates must prepare Cannas for winter to ensure survival and rebloom. This preparation involves timing the cut-back and carefully storing the rhizomes.
Determining the Time to Cut Back
Cutting back Cannas should only begin after the growing season is over, signaled by the first hard frost. A hard frost occurs when temperatures drop below 32°F for several hours, causing the above-ground foliage to blacken and wilt.
Waiting for this natural die-back is beneficial because the foliage continues resource allocation until killed by the cold. During this time, leaves photosynthesize and transport sugars to the underground rhizomes, storing energy for next year’s growth. Removing green foliage prematurely prevents this final energy transfer, potentially weakening the plant. Once the foliage is visibly blackened, the transfer is complete, and the plant is dormant.
Cutting Back the Foliage
Once the foliage is killed by frost, cutting back the plant can begin. Use sharp, clean bypass pruners or garden shears to make clean cuts through the thick stalks.
The stalks should be cut down to four to six inches above the soil line. This remaining stub serves as a handle if the rhizomes are dug up, and it marks the plant’s location. Cutting the stalk too close to the ground allows water to seep down to the rhizome, increasing the risk of rot. Dispose of the spent foliage away from the garden, especially if disease or pests were present, to prevent overwintering pathogens.
To Dig or Not to Dig
The decision to dig up Cannas depends on the geographical location and the potential for prolonged freezing temperatures. Cannas are perennial in USDA Hardiness Zones 8 and warmer, where the soil does not freeze deeply. In these regions, rhizomes can be left in the ground if covered with an insulating layer of mulch.
Gardeners in Zones 7 and colder must lift the rhizomes to prevent freezing. Since rhizomes are mostly water, freezing temperatures cause the water to expand, bursting the plant cells and leading to rot. While Zone 7 gardeners may attempt in-ground overwintering using six to eight inches of organic mulch, digging remains the safest option in Zone 7 and all colder zones.
Preparing and Storing Rhizomes
For those in colder climates, the steps following the cut-back involve carefully lifting the rhizomes and preparing them for indoor dormancy. Using a garden fork, gently loosen the soil in a wide circle around the remaining stalks to avoid puncturing the rhizomes. Once loosened, the entire clump can be carefully lifted from the ground, using the remaining stalk for leverage.
After lifting, gently shake off the majority of the clinging soil, or lightly rinse the clumps with water to remove heavy dirt. The rhizomes should then be cured or dried in a well-ventilated, protected area for a few days to allow surface moisture to evaporate. This drying process, which should occur at a temperature between 60°F and 70°F, helps to seal minor cuts or bruises, reducing the risk of fungal rot during storage.
Once cured, the rhizomes must be stored in a cool, dark, and moderately humid location that maintains a temperature between 40°F and 50°F. Basements, root cellars, or unheated crawl spaces that remain above freezing are suitable environments.
The rhizomes should be packed in a storage medium that provides ventilation and absorbs excess moisture, such as:
- Peat moss
- Dry sand
- Sawdust
- Vermiculite
Place the rhizomes in a cardboard box or a ventilated plastic tote, ensuring they are not touching each other to prevent the spread of decay. Check them periodically throughout the winter for signs of shriveling or mold.