Canna lilies are dramatic, rhizomatous perennials prized by gardeners for their tropical, showy foliage and vibrant flowers. Though often mistakenly called a bulb, this herbaceous plant grows from a fleshy, underground stem called a rhizome, which functions as a food storage organ. Originating in subtropical and tropical regions, the plant is naturally adapted to high heat and humidity, making it an exceptional summer performer. However, this tropical heritage makes the canna lily highly susceptible to freezing temperatures and prolonged cold. Gardeners must take specific measures to protect the underground rhizomes to ensure the plant returns with the same vigor the following season.
Critical Temperatures for Canna Lily Survival
The canna lily’s ability to survive the winter is directly tied to the severity and duration of freezing temperatures in its environment. The plant is considered winter-hardy only in USDA hardiness zones 8 through 10, where the soil rarely freezes deeply. A light frost, typically occurring when temperatures dip between 32°F and 35°F, will cause immediate and visible damage to the above-ground foliage. The large leaves will quickly blacken, wilt, and become mushy, signaling the start of the plant’s natural dormancy period.
While this initial light frost kills the leaves, it usually leaves the underground rhizome unharmed, provided the soil temperature remains above freezing. The real threat to the plant’s survival is a hard freeze, where temperatures fall below 28°F and remain there long enough for the cold to penetrate the soil deeply. A sustained hard freeze is lethal, causing the tender, water-filled rhizome tissue to freeze, rupture its cellular structure, and ultimately rot. This deep freeze is the point at which the entire plant is lost, making cold protection a necessity for gardeners in colder regions.
The visible dieback of the foliage after a light frost serves a beneficial purpose for the plant’s survival. This process signals the canna to withdraw energy and nutrients from the dying leaves and stems back into the rhizome. Allowing the plant to experience this initial frost helps maximize the food reserves stored in the rhizomes, which are relied upon for robust growth the following spring. Gardeners must gauge their local conditions to determine if they can risk waiting for this natural signal or if they must proactively protect the plant before the first frost arrives.
Preparing Canna Lilies for Winter in Mild Climates
Gardeners in mild regions, generally USDA Zones 7 and above, can often successfully overwinter canna lilies directly in the ground. The goal in these zones is not to prevent the cold entirely, but to insulate the rhizomes from deep soil freezing and sudden temperature fluctuations. Once the foliage is killed by the first light frost, the dead leaves and stems should be cut down to four to six inches above the soil line. Leaving a short stub helps mark the rhizome location and provides a natural seal against moisture entering the stem.
Following the trimming, a substantial layer of insulating mulch must be applied over the planting area. This protective layer should be six to eight inches thick, using materials like straw, shredded leaves, or wood chips. The mulch acts as an insulating blanket, trapping residual soil heat and slowing the rate at which cold temperatures can penetrate the ground to reach the rhizomes. This practice is particularly helpful in Zone 7, a transitional area where winter temperatures can occasionally dip low enough to be damaging without the extra cover.
Proper drainage is another factor for in-ground survival, even in warmer climates, because dormant canna rhizomes are susceptible to rot from excessive moisture. Soil that is perpetually soggy in winter will lead to the decay of the rhizome, regardless of the temperature. Gardeners should ensure the planting location drains well throughout the winter season, as the combination of cold, wet soil is frequently more destructive than cold alone. The thick layer of mulch also helps shed excess surface water, contributing to a drier environment around the dormant plant.
Lifting and Storing Canna Rhizomes in Cold Climates
In colder regions, specifically USDA Zone 6 and below, in-ground survival is generally not possible, making it necessary to lift the rhizomes for indoor winter storage. The best time to dig is shortly after the foliage dies back from the first light frost, but crucially, before the ground freezes solid. Waiting for this initial frost allows the plant to fully enter dormancy and transfer maximum energy reserves to the rhizome for next year’s growth.
Once the rhizomes are dug, the remaining stalks should be trimmed back to two to three inches, and most of the soil must be gently removed. The rhizomes should then be placed in a warm, well-ventilated area, ideally between 70°F and 80°F, for a curing period of several days up to a week. This drying process allows surface wounds to heal and the outer skin to toughen, which significantly reduces the risk of fungal disease and rot during storage.
For long-term storage, the cured rhizomes require a cool, dark, and slightly humid environment to prevent them from drying out or rotting. The ideal storage temperature range is narrow, hovering between 40°F and 50°F; temperatures above 50°F risk encouraging premature sprouting. The rhizomes should be packed in a breathable medium such as peat moss, vermiculite, or shredded newspaper, ensuring that no two rhizomes touch to prevent the spread of decay.
A cardboard box, plastic crate, or paper bag is preferred over a sealed container because it allows for air circulation, minimizing the buildup of moisture. Throughout the winter, stored rhizomes should be checked monthly for signs of shriveling, indicating they are too dry, or mold, signaling excessive moisture. If shriveled, the packing medium can be lightly misted with water before repacking; if molding, they must be removed, allowed to air-dry, and placed in a drier location.