Cannas (Canna spp.) are tropical-looking perennials grown from thick, fleshy rhizomes, prized as much for their bold, sometimes strikingly striped foliage as for the bright flower spikes that rise above it through summer. A single planting can anchor a border with the lush, jungle feel more typically associated with the tropics, even though the plants themselves are easy enough for a first-time gardener to manage. Cannas are not hardy in Kentucky's zone 6b winters; left in the ground, the rhizomes freeze and rot, so growers here either dig and store them each fall or treat the plant as a disposable annual. Given a full season of heat and sun, though, cannas reward very little fuss with months of color and dramatic leaves.
Sowing and planting
Cannas aren't grown from seed in most home gardens; nearly everyone starts with a rhizome, either held over from the previous year or bought fresh in spring. Wait until after the last spring frost and the soil has warmed before planting outdoors; in zone 6b that generally means early to mid-May. Rhizomes set into cold soil sit and sulk, or rot outright, so there's no advantage to rushing them into the ground.
Plant rhizomes 3 to 4 inches (7 to 10 cm) deep with the growing points, or "eyes," facing up, in soil enriched with compost. Space plants 18 inches (45 cm) apart to give the clumps room to fill out over the season. For an earlier start on bloom, pot rhizomes indoors in a loose potting mix 4 to 6 weeks before the last frost, keep them in a warm, bright spot, and transplant outdoors once the danger of frost has passed. Water well immediately after planting, whether direct in the garden or from pots.
Growing on
Keep the soil consistently moist through the season; cannas want more water than many border perennials, especially during a hot, dry stretch, and mulching around the base helps hold that moisture. Established plants tolerate a brief dry spell, but repeated wilting cuts both leaf size and flowering noticeably. Feed monthly with a balanced fertilizer through the growing season for the best bloom and the largest leaves. Tall varieties in an exposed or windy spot may need a stake to keep flower spikes upright; otherwise cannas are self-supporting. As clumps thicken over a season, divide the rhizomes at planting time in future years so the planting doesn't become congested and bloom less.
Plan for the end of the season well before frost arrives. Once the first fall frost blackens the foliage, usually by mid- to late October in zone 6b, cut the stems back to 4 to 6 inches above the ground. Dig the clumps carefully with a garden fork, working from well outside the base so you don't pierce the rhizomes, then shake or rinse off the excess soil. Let the clumps cure in a sheltered, frost-free spot for several days to a week, until the surface is dry to the touch, then pack them in slightly damp peat moss or vermiculite inside a box or paper bag. Store the bag somewhere cool but frost-free, around 40 to 50°F (4 to 10°C), such as a basement or garage, and check on it occasionally over winter to make sure the rhizomes are neither drying out nor rotting. Replant outdoors the following spring once the danger of frost has passed. Gardeners who would rather skip the digging and storage step can treat cannas as an annual and buy fresh rhizomes each spring, or grow them in a large container and simply carry the whole pot indoors before frost.
Harvesting
Cut flower spikes for indoor arrangements once the lowest one or two blooms on the spike have opened and the rest are still in bud. Cut in the cool of morning, strip any leaves that would sit below the waterline, and set the stems in water right away; a cut spike will keep opening new flowers over several days even though each individual bloom is short-lived. Because the flowers are too small and numerous to deadhead one at a time, prolong bloom instead by cutting the whole spent flower stalk back to its base once every flower on it has faded. This keeps the plant from putting energy into seed pods and pushes it to send up fresh flowering stems through summer and into fall.
Problems and pests
Leaves rolled, folded, or shredded
Canna leafroller caterpillars web young leaves shut before they unfurl, then feed inside the fold, leaving ragged holes and dark frass once the leaf opens. Unroll and crush affected leaves by hand where practical, and remove badly damaged foliage; a heavy infestation may call for an insecticide labeled for caterpillars on ornamentals.
Orange-brown spots or pustules on leaves
Canna rust shows up as small raised pustules, mostly on the undersides of older leaves, and is worse in warm, humid weather. Strip and discard infected leaves as you see them, avoid wetting the foliage when you water, and space plants so air can move through the clump.
Ragged holes chewed in flowers and leaves
Japanese beetles feed heavily on canna blooms and foliage in midsummer, skeletonizing leaves between the veins. Handpicking beetles into soapy water each morning controls light numbers; row covers or a labeled insecticide are worth considering where beetles are heavy.
New shoots chewed off near the ground
Slugs and snails target tender new growth, especially after a wet spring. Scatter iron phosphate bait around emerging shoots or set out shallow traps; clearing nearby mulch and debris removes the damp daytime hiding spots the pests need.
Rhizomes soft or rotted after winter storage
Rhizomes that go into storage with cuts, excess moisture, or too little curing time often turn soft and mushy by midwinter. Let dug rhizomes dry fully before packing them away, check on stored rhizomes periodically and remove any that show rot before it spreads, and keep the storage medium just barely damp rather than wet.
Varieties worth growing
- Tropicanna (Phasion) – Broad leaves striped orange, pink, yellow, and green, topped with bright orange flowers; one of the showiest foliage cannas available.
- Australia – Deep bronze-purple leaves that hold their color all season, paired with vivid red flower spikes.
- Bengal Tiger – Green leaves boldly striped in yellow, with orange blooms held above the foliage.
- President – A compact, reliable variety with plain green leaves and large, clear red flowers; a good choice where bold striped foliage isn't the goal.
- Cannova series – Seed-grown, more compact, and more weather-tolerant than many older varieties, making it a good fit for containers and smaller beds.