Cannas are tropical plants that thrive in summer heat but cannot take a hard winter freeze. For most gardeners the real question is cold, not heat, and the answer depends on where you live. What survives or dies over winter is the underground part, so winter care is about protecting the roots rather than the leaves.
How Cold Canna Rhizomes Can Get
Cannas grow from rhizomes, thick underground stems that store food for the next season. The foliage dies back at the first frost, but the plant lives on if the rhizome survives. That’s the catch: rhizomes are frost-sensitive and rot if left unprotected in freezing ground.1Wikipedia. Canna (plant)
In freezing tests, a canna variety lost half its tissue survival at temperatures just below -3°C (27°F).2Acta Horticulturae. Evaluation of Freezing Hardiness and Low Temperature Exotherm of Ornamental Geophytes That puts cannas with freesia and agapanthus among the bulbs that cannot take a hard freeze, unlike grape hyacinth, which survives below -9°C (16°F).3Acta Horticulturae. Evaluation of Freezing Hardiness and Low Temperature Exotherm of Ornamental Geophytes Some bulbs grow hardier as autumn cools, but cannas did not get any more cold-tolerant from October to December.4Acta Horticulturae. Evaluation of Freezing Hardiness and Low Temperature Exotherm of Ornamental Geophytes
Air temperature and soil temperature are not the same thing. Soil stays warmer than the air on frosty nights, so a brief air frost rarely reaches the rhizomes. What matters is whether the ground itself freezes at rhizome depth, and a few cold nights are very different from weeks of frozen ground. Bare, snow-free soil loses heat faster than ground under snow or a layer of leaves, so an exposed bed can freeze deeper than a covered one in the same yard. Pots freeze faster than garden soil because cold reaches them from all sides. To check the soil, push a trowel or garden fork a few inches down. If it goes in easily, the ground probably isn’t frozen at that depth.
A healthy rhizome feels firm and heavy, like a fresh potato, with plump pinkish or whitish buds (eyes). One killed by freezing turns soft, mushy, or waterlogged, and often smells sour or rotten. A dried-out, hollow one with no eyes is unlikely to sprout. Rhizomes left in the ground may not show their fate until spring, so wait several weeks into warm weather before giving up on a bed that looks empty.
Chilly weather above freezing is a different matter. Rhizomes held at 0 to 10°C (32 to 50°F) for up to 8 days survived with no effect on survival.5Acta Horticulturae. Influence of Short-Term Storage Temperature and Duration of Canna Rhizomes on Forcing After 8 days of that cold, though, they sprouted 5 to 10 days later and grew fewer shoots.6Acta Horticulturae. Influence of Short-Term Storage Temperature and Duration of Canna Rhizomes on Forcing A cold snap in spring storage or after planting slows them down but won’t kill them.
Can You Leave Cannas in the Ground Over Winter
Cannas are generally hardy in USDA Zones 8 through 11, where the ground doesn’t freeze deep enough to harm rhizomes.7Longfield Gardens. When to Dig Canna Lily Bulbs: A Guide for Winter Care Some growers say cannas survive in Zones 7 to 10 and grow larger each year.8American Meadows. How To Overwinter Cannas Zone 7 is the borderline, where results vary by site and winter. To find your zone, look it up by zip code, and treat it as a starting point rather than a guarantee.
Where winters drop below -10°C (14°F), roughly colder than Zone 8b, dig the rhizomes before the ground freezes.9Wikipedia. Canna (plant) In borderline areas, a thick layer of mulch can protect rhizomes left in place.10Wikipedia. Canna (plant)
To mulch, wait until frost has blackened the foliage, then cut the stems down to a few inches so the mulch sits on the crown rather than on dead leaves that hold moisture. Pile on several inches of straw, shredded leaves, or bark to keep frost from penetrating as deeply. The spot should drain well, since cold, soggy soil rots rhizomes even when it never freezes hard. In spring, pull the mulch back once the worst frost is past so the soil can warm and shoots can emerge.
A bed against a south-facing wall or a raised bed tends to run warmer than open lawn, which can make a borderline zone workable. Low spots where cold air and water collect are riskier than slopes, even in a mild zone. If losing a prized or hard-to-replace variety would upset you, dig it up anyway, because rhizomes are cheap to store and costly to lose.
Containers are another option. Cannas grow well in large pots, and in winter the pot can move into a garage or basement during dormancy.11American Meadows. How To Overwinter Cannas Keep it from freezing and water very sparingly.
Storing Dug Rhizomes at the Right Temperature
Store dug rhizomes above 7°C (45°F) until spring replanting.12Wikipedia. Canna (plant) Commercial growers store them 4 to 6 months at 6 to 8°C (43 to 46°F).13Elsevier. Effect of six pre-storage rhizome treatments on rhizome vitality and seasonal growth characteristics of three Canna × generalis cultivars Rhizomes often lose 20 to 30% of their weight in storage,14Elsevier. Effect of six pre-storage rhizome treatments on rhizome vitality and seasonal growth characteristics of three Canna × generalis cultivars so expect some shrinking.
Dig after frost has blackened the leaves but before the ground freezes, so the rhizomes have finished bulking up yet aren’t damaged. Cut the stems back to a few inches and loosen the clump with a garden fork, working wide of the plant so you don’t spear the rhizomes. Shake off loose soil rather than washing them, since extra moisture raises the chance of rot. Let the clumps dry for a few days in a sheltered spot out of the rain and away from freezing air.
Then pack them in a box or crate with peat, sawdust, or vermiculite, which slows drying without soaking them. Don’t pack them so tightly or so wet that air can’t move, because trapped moisture breeds mold. An unheated basement, cellar, or attached garage that stays cool but never freezes suits the storage range above.
Check the rhizomes about once a month. Toss any soft or moldy pieces so rot doesn’t spread, and lightly mist badly shriveled ones. If the spot is too warm, the rhizomes may sprout early.
How Much Heat Cannas Can Handle
Cannas hold up in rising summer temperatures and bloom from early summer through fall.15Home & Garden Information Center. Cannas Can Take the Heat! In a hot, dry climate averaging 27 to 33°C (80 to 92°F) in summer, they grew vigorously above ground.16BIO Web of Conferences. Biomorphological features of varieties Canna x generalis Bailey when introduced under arid conditions Once temperatures fell to about 20°C (67°F) in autumn, growth shifted to the underground rhizomes.17BIO Web of Conferences. Biomorphological features of varieties Canna x generalis Bailey when introduced under arid conditions
They need at least 6 hours of direct sun daily and bloom less in partial shade.18Nature Hills Nursery. Growing Canna Lilies for Tropical Summer Color Still, extreme midsummer afternoon heat can scorch leaf edges brown.19Longfield Gardens. Can Canna Lily Grow in Shade? Essential Light Level Guide Filtered afternoon shade keeps leaves fresh in very hot spots.20Longfield Gardens. Can Canna Lily Grow in Shade? Essential Light Level Guide
Scorch appears as crispy brown edges and tips, most often on leaves facing the afternoon sun. Scorch is usually cosmetic and doesn’t harm the rhizome, so the plant carries on blooming. If damaged leaves bother you, trim the worst ones for looks, and new leaves will replace them.
Big leaves and fast summer growth mean cannas drink heavily, so a drooping plant in hot weather is more often short of water than damaged by heat. Push a finger into the soil: if the top inch is dry, water. Water deeply and less often so moisture reaches the roots instead of just wetting the surface. A layer of mulch keeps the soil cooler and moister around the roots. Pots dry out fastest, since they heat up and hold little soil, and in a hot spell may need water every day.
Keeping Cannas Growing Indoors in Winter
Kept above 10°C (50°F) in a greenhouse or conservatory, cannas stay in growth all winter and flower occasionally.21Royal Horticultural Society. Canna Plant Care Give them full light, but shade them from hot sun under glass.22Royal Horticultural Society. Canna Plant Care
Growing a canna on through winter needs a bright window, a sunroom, or a conservatory, and a big canna takes up a lot of room. In the weak light of winter, indoor plants often stretch and turn leggy. Dry indoor air encourages spider mites, so check the undersides of the leaves for fine webbing and speckling.
In spring, plant out only after frost danger has passed and the soil has warmed, since a young canna is as frost-tender as a mature one. A plant that spent winter indoors needs hardening off: put it outside in shade for a few hours at first, and lengthen the time over about a week. For earlier blooms, start stored rhizomes in pots indoors a few weeks before the last frost, then plant out the growing shoots when the weather is safe.