Canna Care Through Summer: Water, Feed and Cutting Out Spent Canes

Water generously and never let it check

Canna moves an enormous amount of water. A mature clump of a tall type carries several square feet of leaf surface, and on a hot day it transpires faster than almost anything else in the border. Give it 1 to 2 in (2.5 to 5 cm) of water a week in the ground, delivered as one or two long soakings rather than a daily sprinkle, so the water reaches the full root depth.

The plant tells you when it is short before it wilts. Leaves that were flat and horizontal begin to roll inward along the midrib and stand more upright, which is the plant reducing the surface facing the sun. Rolled leaves in the middle of a hot afternoon that flatten again by evening are normal. Rolled leaves still rolled at dawn mean the root zone is dry.

The damage from drying out is permanent for that leaf. Whichever leaves were expanding during the dry spell emerge with brown crisped edges or torn ends, and they stay that way until the cane is cut out. This is why a canna that has been allowed to dry once in a container looks scruffy for the rest of the season even after watering resumes.

Once the shoots are up, canna tolerates ground far wetter than most plants and will grow at a pond margin in 2 to 6 in (5 to 15 cm) of standing water. The exception is a dormant rhizome in spring, which rots in saturated cold soil.

Feed steadily, but not with nitrogen alone

A canna clump builds several feet of stem and a stack of large leaves in a single season, then flowers on top of it. That takes a continuous nutrient supply, not one feed in spring. Start feeding once the plant reaches 12 in (30 cm) and continue every two to three weeks until roughly six weeks before your usual first frost.

Use a balanced feed, or one slightly higher in potassium, rather than a high nitrogen lawn type. Excess nitrogen gives you a magnificent stand of foliage with almost no flower spikes, soft stems that flop, and leaves that are more attractive to leaf roller caterpillars. If a clump is all leaf and no bud, look at what you have been feeding it before you look at anything else.

Containers need the same schedule at higher frequency, because every heavy watering flushes nutrients out of the drainage holes. A liquid feed every two weeks through the growing season is the minimum for a pot canna, and every week in high summer is not excessive for a large tall type in full growth.

Cut each finished cane down at the base

This is the single most important piece of canna maintenance and it is not deadheading. Each canna stem produces one flowering head, works through its succession of buds, and is then finished for good. It will never flower again. Snapping off the spent flower head leaves a green cane standing that is doing nothing except taking light, water and feed from the shoots coming up behind it.

So once the last bud on a head has opened and faded, cut that entire cane right down to soil level, or to an inch (2.5 cm) above it. Removing the whole cane pushes the rhizome to send up replacement shoots, and it is that succession of new canes, not any treatment of the flower head, that keeps a clump in flower from midsummer until frost.

You can pick off individual faded flowers in between if you want the display tidy, and it does stop the plant setting the knobbly green seed pods that follow. But the tidying is cosmetic. Once the whole head is over, the cane goes, and a clump worked over this way every week or two through the season carries flowers at all times while an untouched clump stops after its first flush.

1. Walk the clump weekly and find the finished canes A finished cane has no unopened buds left at the tip of its head, only spent flowers or forming seed pods. Look at the top of the spike rather than at the leaves. If there is a single fat green bud still to come, leave the cane.
2. Cut the whole cane at ground level Use pruners or a knife and cut cleanly within 1 in (2.5 cm) of the soil. Do not leave a 12 in (30 cm) stub, which stays in the way, shades the new shoots and gives leaf roller larvae somewhere to shelter.
3. Pull the cut cane clear of the clump The cut cane is usually wedged between neighboring leaves. Twist it free and take it away rather than dropping it into the clump, where it holds moisture at the crown and hides pests.
4. Check the new shoots at the base You should see fresh spears at the crown within a week or two of removing a cane. If nothing comes, the clump is either short of feed and water or so congested that it needs dividing next spring.

Wind, support and tidying the leaves

Canna stems are strong enough that most clumps need no staking, and the plants support each other when grown in a group. Where a tall type is grown singly in an exposed spot, put a stake in at planting and tie the cane loosely at 24 in (60 cm) and again at 48 in (120 cm). A stake driven in later goes through the rhizome.

The leaves are the plant's main asset for most of the year, so keep them clean. Remove any leaf that is more than about half browned, cutting it off at the sheath rather than tearing it, which strips a ribbon of green tissue down the stem. Torn or wind shredded leaves do not repair and can be taken off once the plant has replacements.

Keep a mulch of 2 in (5 cm) of compost or bark over the root area from early summer. It halves the watering in hot weather, keeps the shallow rhizomes cool, and suppresses the weeds that otherwise grow up through the middle of the clump where they cannot be reached.

Dividing a congested clump, and what to record

A canna clump doubles in size every year or two. By the third or fourth season the center is a solid mass of rhizome, the shoots come up thin and crowded, and flowering falls away even with good feeding. Divide at that point, in spring as growth begins or at lifting time in autumn, cutting the mass into pieces with three to five growing points each.

Discard the oldest woody sections from the middle of the clump. They have few live growing points, they store poorly, and keeping them is how a collection slowly fills with unproductive rhizome. Replant only firm outer pieces with visible points.

Through the season, the notes worth keeping are the date each clump produced its first open flower, the number of canes you cut out and when, and any week the plants went short of water. Cane counts across the summer show at a glance whether a clump is still building or has stalled, and the first flower date compared with the planting date is the number that tells you next spring whether to plant earlier or wait for warmer soil.