Canna lilies, often mistakenly referred to as “bulbs,” are actually grown from thick, fleshy underground stems called rhizomes. These vibrant plants are native to tropical and subtropical regions, meaning their successful growth is entirely dependent on warm temperatures. Timing the planting correctly is the single most important factor for ensuring a spectacular display of their dramatic foliage and brilliant summer blooms.
Determining the Ideal Planting Window
Planting canna rhizomes directly into the garden must be delayed until all risk of the last spring frost has passed. Because of their tropical nature, freezing temperatures will destroy the rhizomes. Beyond the frost date, the temperature of the soil is the most critical factor for successful outdoor planting.
Canna rhizomes require a consistently warm environment to sprout, making a soil temperature of 60°F or higher ideal for planting. Planting into cold, wet soil, even if the air temperature is mild, dramatically increases the risk of rot. Gardeners in warmer climates (USDA Zones 7 through 10) can often leave their cannas in the ground year-round. Those in cooler zones must wait until late spring to early summer for the soil to naturally warm.
Jump-Starting Canna Growth Indoors
For gardeners in northern regions with shorter growing seasons, starting cannas indoors provides a substantial head start and guarantees earlier summer flowers. This technique involves pre-sprouting the rhizomes in containers approximately four to six weeks before the last anticipated frost date. Starting them too early can result in leggy, root-bound plants that struggle when transplanted outdoors.
Plant the rhizomes horizontally in pots filled with moist, well-draining potting mix, covering them with about two inches of soil. The pots should be kept in a warm location, maintaining a soil temperature between 70°F and 75°F, and provided with at least six hours of bright light daily. Once the transplants are actively growing and the outdoor soil has warmed, they must be gradually exposed to outdoor conditions over about 10 days (hardening off) before being permanently moved to the garden.
Essential Steps for Outdoor Planting
Once the threat of frost is gone and the soil temperature is stable, cannas perform best in a location that receives full sunlight, though they will tolerate partial shade in hotter climates. The planting site should feature organically rich, moist, and well-draining soil, which can be achieved by amending the area with compost before planting.
Dig a shallow hole and position the rhizome two to three inches deep with any visible growth points, or “eyes,” facing upwards. Place rhizomes 1 to 3 feet apart to allow for the plant’s mature size, though spacing depends on the variety. After covering the rhizome with soil and tamping it gently, water the area thoroughly. Keep the soil on the drier side until the shoots break the surface, as excessive moisture before sprouting can lead to rot.
Post-Planting Requirements and Overwintering
After the cannas have emerged, they demand significant amounts of water throughout the summer to support their robust growth. They benefit from consistent moisture, especially during the hottest parts of the season. Regular fertilization with a balanced, slow-release formula will encourage continuous blooming and lush foliage development during the active growth period.
In regions colder than USDA Zone 8, the rhizomes cannot survive the winter and must be dug up for storage. Wait until the foliage is killed by the first light frost, signaling the plant to enter dormancy. After cutting the stems back, carefully lift the rhizomes, remove excess soil, and allow them to cure in a warm, dry area for several days. Store the dormant rhizomes in a cool, dark, and dry location (40°F to 50°F) until the following spring to prevent premature sprouting or rotting.