How to Plant Canna Rhizomes: Depth, Spacing and Warm Soil

Plant on soil temperature, not on the calendar

A canna rhizome carries no roots when it goes into the ground. It lives on its own starch until a growing point pushes a shoot up, and only then does it start drinking. In soil at 50F (10C) that wait drags on for a month or more, and a month of a dormant rhizome sitting in cold wet ground is an invitation to soft rot at the cut faces.

The signal to plant is soil that has reached 60F (15C) at planting depth and stayed there, with the last frost past. Canna foliage is killed outright at 32F (0C), so nothing is gained by being early. In warm soil the first spear breaks the surface in two to three weeks and growth is fast and unbroken from that point.

Where spring is long and cool, start the rhizomes under cover instead. Pot them into 1 gallon (4 liter) containers about six weeks ahead, keep them at 65 to 70F (18 to 21C), water sparingly until a shoot shows, and move the rooted plants outdoors once frost risk has gone. Plants started this way flower two to three weeks earlier than rhizomes planted straight into cold beds.

Do not water a freshly planted rhizome in the ground. It has no roots to take the water up, and saturated soil around a dormant rhizome is the single most common cause of a canna that never appears.

Depth, orientation and spacing

Dig a hole wide enough that the rhizome lies flat rather than being bent to fit, and set it horizontally with the growing points, the pale pink or white cones at the ends and along the top, facing upward. Cover with 3 to 4 in (8 to 10 cm) of soil in a bed, or 2 in (5 cm) in a container. Deeper than 6 in (15 cm) in heavy soil simply delays emergence.

Spacing decides how the clump looks in its second and third year, because canna spreads sideways by rhizome and a planting hole that fits this year will be full by the end of the season. Give tall types 18 to 24 in (45 to 60 cm) between plants and dwarf types 12 to 15 in (30 to 38 cm). Set them in groups of three or more rather than in a single line; the leaves are the point of the plant and they read better massed.

Choose the sunniest open spot you have. Canna needs at least six hours of direct sun to flower properly, and eight is better. It will grow in half shade, but it makes leaf instead of flower spikes and the stems stretch and lean. The one thing to avoid is a windy corner, because the large paddle leaves shred along the veins in strong wind and stay shredded for the rest of the season.

Load the bed before you plant, not after

Canna is one of the hungriest ornamentals you can grow and it feeds hardest in its first six weeks, when it is building the stems that will carry the flowers. Compost and slow release fertilizer worked into the bed before planting reach the roots at the right moment. The same materials scattered on the surface in midsummer mostly feed the weeds.

Work 2 to 3 in (5 to 8 cm) of compost or rotted manure into the top 12 in (30 cm) of the bed, and add a balanced slow release feed at the rate on the label. Avoid piling on high nitrogen material such as fresh grass clippings or a lawn fertilizer, which produces enormous leaves and very few spikes.

Backfill firmly, level the surface, and mark the position of each rhizome with a short cane. Canna takes two to three weeks to appear and an unmarked bed gets hoed, trodden on or planted over in the meantime. The marker also tells you later, when you lift the clumps, where each one started, which is the only way to see how far a clump has traveled in a season.

1. Check the rhizome before it goes in A sound rhizome is firm, heavy for its size, and has at least two or three plump growing points. Discard any piece that is soft, hollow, moldy at the cut end or shriveled to a husk. A piece with no visible growing point will never sprout, however large it is.
2. Dig wide and shallow, not narrow and deep Open a hole 12 in (30 cm) across and 6 in (15 cm) deep, loosen the base, and mix a shovel of compost into the soil you removed. Canna roots run sideways in the top layer of soil, so width matters more than depth.
3. Lay the rhizome flat with the points up Set it horizontally on the loosened base. If the growing points face sideways the shoots will still find their way up, but they emerge a few days later and often at an angle. Cover with 3 to 4 in (8 to 10 cm) of the enriched soil and firm gently.
4. Mark it and leave it dry Push in a labeled cane at the rhizome position. Give no water at all until a shoot shows, unless the bed is bone dry through a heat wave, in which case one light watering is enough. Then begin normal watering.

Containers, pond margins and wet ground

Canna is unusual among summer bulbs in that once it is growing it does not object to wet feet. Established plants will stand in 2 to 6 in (5 to 15 cm) of standing water at a pond edge, and some types are grown deliberately as marginals. That tolerance begins after the shoots are up. A dormant rhizome in the same conditions rots within days.

In containers use a large pot and a heavy mix. A tall type needs at least 10 gallons (38 liters) or it dries out every afternoon and blows over in the first storm. Use a loam based compost rather than a light peat mix, because the weight keeps the pot upright and the loam holds nutrients through a long season of heavy watering.

Container plants need more feeding than bedded ones. Start a liquid feed every two weeks once the plant is 12 in (30 cm) tall, and expect to water daily in high summer. A pot canna that dries out badly once will show it for the rest of the year as a band of brown crisping down the edge of the leaves that were expanding that week.

What to write down on planting day

Planting day is the only time you can see the rhizome clearly, and a few notes made now save guesswork for years. Record the date you planted, the soil temperature you measured, the depth, and the number of growing points on each piece. When one clump comes up thin next spring you will be able to see whether it was planted too deep, too early, or was simply a weak piece.

Note the exact position of each rhizome, ideally as a sketch with measurements from a fixed point such as a path edge or a fence post. Canna clumps migrate sideways by several inches a year, and the sketch tells you which way and how fast, which decides when the clump needs dividing.

Finally record the date the first shoot broke the surface. Compared against the planting date, that gap is the clearest single measure of whether you planted into warm enough soil. Two to three weeks means you got it right; five or six weeks means wait longer next spring.