Canna

Notes on Canna Cultivation

Canna are grown from rhizomes, and a rhizome started in the spring will flower all summer and autumn. The earlier a rhizome is started into growth, the sooner it will flower. In the south of England, rhizomes will start to grow of their own accord in a cool greenhouse in March, no heat being required except to protect against frost (they can tolerate a mild frost), and they will then be ready to plant out when the danger of frost is over in late May or early June. They will then soon start to flower. Flowering will be continuous until they are lifted in early October, or until they are cut down by winter frosts. In the milder parts of the country - central London and the coastal areas of the south, canna were still tolerably healthy and showing buds in December 1999. Plants for indoor display can be started into growth at any time, and will continue to grow and flower so long as they get adequate food, warmth and light.

Individual rhizomes may be planted in 2 litre pots, using a rich compost. Alternatively, 3 rhizomes may be planted in a 6 litre pot. Take great care with emerging shoots, which are extremely fragile. Plants remain in the 2 litre or 6 litre pots until planted out, or potted-on for pot cultivation..

Preparing the planting site is as essential as if you were growing tomatoes or potatoes. Canna are equally vigorous. The soil needs to be dug, the deeper the better, and compost/manure/fertiliser added. It is difficult to overfeed cannas. If you simply scrape a hole in hard soil beneath a tree, and expect a canna to thrive, then forget it. Canna are best planted in groups, typically 3 in each location. The plant within a group should be typically 6" apart, and at least 18" from a neighbouring clump. For a block effect, more cannas can be planted in a group, the more the better, and when grown this way they will grow taller and generally make a more impressive display.

Canna are very strong and sturdy plants, and even the tall varieties require no staking. It is very rare to see a canna which has been blown over, whatever its location. However, in an open and exposed site site they will look somewhat windswept and tattered. The ideal site for canna is a warm quiet sultry corner, protected by an adjacent wall or building. They will grow in damp places, even waterlogged places, and can also withstand dry conditions (though they may curl their leaves for protection if it gets too bad - a sign that they need watering. They will grow well in sandy soil and also in heavy clay. They are amazingly tolerant and gutsy plants. As with many other gutsy creatures, they have a good appetite, and what they do not like is poor soil. If they are starved of nourishment, then they will become spindly, but still they will flower.

In England they are at the northern edge of their range, and do not grow as tall or flower so well as in warmer countries, but even so do very well in the southern counties, and are often encountered making a valiant effort much further north. A customer wrote to tell us that he grows them with great success at Lytham-St-Annes, Lancashire. They grow better in sheltered locations where they can benefit from reflected warmth, and where their large leaves will not get battered by incessant winds.

Rhizomes should be lifted in the autumn, typically early October, and stored in a dry frost free place. Preferably leave the lifted plant as an undivided solid clump until the following spring when it may be divided. If left in cold wet soil they may well rot even if kept frost-free. A single rhizome planted in the spring will have multiplied by the year-end to give typically 4 to 8 rhizomes which can be saved for the following year, when they will grow and flower, completing the cycle.

Canna are ideal for pot culture, and will amaze and delight visitors to your garden/patio/conservatory. All canna varieties can be grown in pots/tubs. The bigger varieties are truly spectacular when in flower, but it should be remembered that taller cannas need to be taller before they begin to flower, and so need a longer growing period before they begin to flower. The dwarf varieties begin to flower very early when as short as 1 ft, yet continue flowering all year and get bushier and bushier with more and more flowers. Dwarf patio cannas are very popular on the continent (France, Germany, Holland) for these reasons, but have yet to be discovered by most gardeners in the UK who insist on planting the taller varieties. We recollect visiting a garden open day last summer where the hosts glumly welcomed guests to their patio where they had lots of pots of Wyoming - very nice foliage and quite tall, but not a flower in sight. Dwarf cannas, eg Primrose Yellow, Delibab, Aranyalon, even Lucifer, would have been a blaze of glory at that height. Tall varieties need to be planted in 12 litre pots minimum, and preferably in tubs. Dwarf cannas will remain happy in 5 litre pots or planters. As to compost, we recommend rich peat based compost, and you may care to add something extra. We know of one large botanical garden in the midlands who begin by half filling their planter tubs with pure rotted manure. At the end of the season, let the pots dry out, and store in a frost free place. Divide and repot in the spring.

Take great care with unwrapping - new shoots at this stage are extremely fragile, and a shoot broken off represents a lost flowering shoot.

The rhizomes should be immediately potted up, irrespective of the time of year. Use a peat/coir based potting compost, and place the pots in a light airy frost-free place. A cool greenhouse with heat only when frost threatens is ideal. Keep the compost slightly moist (not dry, not over-wet), until the growing season starts.

If immediate potting is impractical, the rhizomes should be covered in damp peat. Canna rhizomes do not normally enter a totally dormant stage, and if they are thoroughly dried out, then some rhizomes will be lost. (This is perhaps one reason why cannas rhizomes are not often sold in garden centers - they do not appreciate being kept for long periods in a handful of dry sawdust in pre-packs - many are lost leading to complaints).

If the canna are intended for indoor cultivation, then they may be potted up immediately and grown on under heat.

As with many rhizomatous plants, not every rhizome will grow (canna growers are happy with an 80% success rate), although some rhizomes will throw up 2 or 3 shoots.

In the spring, newly emerging shoots should be sprayed for aphids, though aphids are not often a problem with grown plants.

Young plants should be protected from slugs and snails which ignore the open leaves but have a preference for the newly unrolling leaves. A single nibble at this stage by a slug will cause a disfiguring row of holes as the leaf unfurls that will remain with the plant for several months. Older plants are not often troubled by slugs and snails.

Red Spider Mite can occasionally infest indoor canna. The symptoms are dry-looking leaves which turn uniformly brown. When examined closely on the underside, such leaves show traces of a white powder (which is the dried egg-cases) particularly near the central leaf rib, and myriads of extremely tiny creatures all running around. You really need a magnifying glass to see them. To answer a common query, Red Spider Mites are not often red in colour. Red Spider Mite is immune to most if not all proprietary preparations available to the amateur. Soap-based insecticides combined with a powerful spray can dislodge and/or suffocate them, and minimise the problem to an acceptable level.

Canna virus disease is more widespread than is commonly appreciated. It is initially recognised by pale coloured spots and streaks in leaves, and by distorted or "crinkly" leaves. Later, badly affected plants show dead rust-coloured streaks in the leaves, throughout the plant, and the growth is badly stunted. The plant may still flower, but the flowers may have a distorted shape with white patches. Little is known about canna virus - some plants are badly affected and may die of their own accord, or remain badly stunted, but other plants show only a mild infection of one or two leaves and seem to be able to recover. There is no cure for virus disease, and plants that are obviously diseased should be dug up and destroyed.