Canna Cultivation

Canna are very easy to grow. A canna starting to grown in the winter and spring in a cool (ie frost-free) greenhouse, and planted out at the beginning of June, will usually be flowering by July. It will then flower continuously all summer and autumn, each stem producing a succession of flower spikes, and new flowering stems growing from the ground. They will continue to flower until late autumn when they are dug up for the winter. It will then be seen that the original rhizome has multiplied many-fold.

Cannas can be grown from rhizomes, by division of plants, and a few can be grown from seeds. There are pros and cons to each method:

Pros and cons: rhizomes, plants, seeds

• Rhizomes are more unreliable, and need a greenhouse, and they may be infected with virus disease unless you are VERY careful where you obtain your stock.
• Cannas from small plants, either by division or purchased as plants, are the easiest to grow, you simply plant then in the garden when the frosts are over, and away they go. If you are buying them as plants, then they can be inspected for virus disease at the time of purchase (any naff-looking plants being discarded), but they are usually much more expensive than rhizomes.
• It would be nice if cannas could be grown from seeds, but the problem is that most canna varieties are sterile.

Cultivation by Rhizomes

In nature cannas grow throughout the year and don't have a dormant period because the rhizomes are always active and sending up new shoots. If cannas are forced into dormancy, which is how they are often sold in garden centres, then losses will occur. Try to make sure that the rhizomes you buy always have at least one actively growing shoot, otherwise your money may have been wasted. Rhizomes are usually started off in greenhouse between February and April. Individual rhizomes may be planted in 2 litre pots, using a rich compost preferably enriched with a little fertiliser. 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. The earlier they are started into growth the earlier they will flower. They are fairly hardy and heat is not essential except to protect against frost. They should 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 the cold weather of autumn, ususally mid October in much of the UK. They are then stored for the winter (see Winter Care below.

Cultivation by small plants/divisions.

Cannas are not fussy creatures. You can divide them at any time of the year, even when in full growth. Provided you give them a good watering after this trauma they will hardly notice. Small plants purchased in winter and spring should be grown on in a cool (frost-free) greenhouse and planted out when the danger of frost is over (late May to early June).

Cultivation from seeds.

Most floral varieties are sterile (they will produce seed pods but the seed pods will be empty). There is a genetic reason for this, which also occurs in other types of plants. Some plants have multiple copies of chromasomes in each cell (ie they are polyploid). This has the double effect of producing a plant with bigger flowers, but also makes the plant sterile. Some floral varieties do produce seeds, but there is a downside: where fertile seed is produced it usually doesn't "come true". You may get something interesting, but it probably won't look like the parent. The canna species do produce seed, and it does come true. This is the best way of growing cannas such as C. warszewiczii and C. patens which are copious producers of seed. More seed is produced if the flowers are hand pollinated. But there are increasingly floral varieties that now come true from seed (though these may be F1 hybrids which may rule out the own collection of seed). These include the 'Tropical' and 'Cannova' range of varieties, and others such as 'South Pacific'.

Canna seeds are glass-hard. They will not germinate unless scarified. If scarified, they will germinate in about 1 week. Scarifying canna seeds is an engineering job. We grip the seeds in pliers, and rub them on an engineers file, just enough intil the white shows theough at one point. The best conditions for germination is very hot (25 to 30C), and very humid (ie put the seed tray in a closed polythene bag.

Do not believe the colour-suppliment horticultural pundit who said that cannas grown from seeds take 2 years to flower. He was clearly not writing from experience. The seeds of the Tropical Series flower in 90 to 120 days from sowing the seeds, which is quicker than most cannas when grown from rhizomes. Seed from the species C. warszewiczii and C. patens are almost as quick. Other varieties will be flowering by August from seeds sown in January/February.

Choosing the site

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, on chalk, and also in heavy clay. They are amazingly tolerant and gutsy plants

Growing cannas in borders and flower beds

Preparing the planting site is as essential as if you were growing tomatoes or potatoes. Canna are voracious feeders, and if they are not well fed they do not flower well, and may not flower at all. The soil needs to be dug, and compost/manure/fertiliser added. It is difficult to overfeed cannas, and they will tolerate lots of FRESH manure! If you simply scrape a hole in barren soil beneath a tree, and expect a canna to thrive, then forget it.

Plant the cannas when the danger of frost is over, typically the start of June. It is best to plant cannas in clumps, At least 3 plants together. Then sit back and enjoy them until the time comes to dig them up at the end of the season.

Growing cannas in pots or planters

Canna are ideal for pot culture, and will amaze and delight visitors to your garden/patio. 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. Tall varieties need a 15 litre pot or planter. Dwarf varieties at least a 5 litre pot. Use a good quality potting mix enriched with a general purpose fertiliser. Stand the pot/planter in a tray of water. They will appreciate a top dressing now and then.

Growing cannas in a conservatory or greenhouse

Cannas need good overhead light otherwise they will become "leggy".

Winter care

Canna plants should be lifted in the autumn, typically mid October to November. A single rhizome or plant with a single stem will be seen to have multiplied by the year-end to give typically 4 to 6 rhizomes.

The gardener here has a choice. He/she can leave the frosts of autumn to do their work. A moderate frost will cut all green growth down to the ground, and all the gardener then has to do is rake up this debris and put it on the compost heap. The rhizomes, now devoid of all green growth, but with obvious new shoots, can be stored through the winter in a cool place where the growth is halted.

But it may be noted that where cannas grow in nature, ie the sub tropical americas, frost is not a feature of the climate. The process is that there is a continuous cycle of stems flowering and dying back to be replaced by new stems growing up from the base.

A gardener can replicate this cycle in our colder temperate climate. The stems that have been flowering during the summer are now finished and can be cut down to the ground. Rhizomes, many of which will have new shoots already growing, can be potted up and grown through the winter in a frost-free greenhouse. So new shoots already growing are not lost as they would be if only the rhizome was stored for the winter.

It should be noted that rhizomes need to be stored damp, not dry. If canna rhizomes dry out in the winter, then many will be lost. We always keep the rhizomes covered in compost, which occasionally needs to be watered. Rhizomes that are lifted out of the garden border are best kept through the winter as an undivided clump, still encased in the soil in which they were growing. They are then divided in the spring. Plants that have been grown in pots need to be kept in the pots through the winter, and and divided and repotted in the spring.

Some gardeners advocate leaving cannas in the ground throughout the winter. This is possible in milder part of the country. In our opinion this is a risky procedure, because if the ground in which the rhizomes are enbedded freezes, then the rhizomes will be killed. This possibility can be avoided if the ground is mulched. But even then the growth will start much later than if the plants were started under glass, and flowering will start later.

Pests and diseases

Young plants should be protected from slugs and snails which ignore full grown leaves but have a preference for the new shoots. A single nibble at this stage by a slug will cause a disfiguring row of holes as the leaf unfurls that will disfigure the leaf for the rest of the year. Older plants are not often troubled by slugs and snails.

Aphids are rarely seen on cannas, but because virus disease is spread by aphids, it is best to spray for aphids if any are seen, particularly if there are infected canna plants nearby. Aphids may be seen on the newly emerging shoots in spring.

Red Spider Mite can occasionally infest indoor canna. The symptoms are dry-looking leaves which turn 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 will be seen all running around. You really need a magnifying glass to see them. Red Spider Mite is immune to many proprietary preparations available to the amateur. Insecticides that contain Bifenthrin can be quite effective. Soap-based insecticides can suffocate them. There are predators for spider mite but once the infection becomes overwhelming that it is beyond the capacity of predators to cope. An effective treatment for individual plants is physically wiping the undersides of the leaves with a wet sponge, and/or dislodging the mites with a powerful water jet/spray.

Deer eat cannas but rarely cause much damage, even in deer infested areas. Apparently rabbits do not eat cannas.

Occasional damage can be seen, particularly in autumn, by various pests including hedge cricket, shield bug, and more.

In tropical countries the leaf roller caterpillar is a serious pest.

Canna virus disease

Canna virus disease is a recent problem which reached epidemic proportions in the early 2000's, and is still a very severe problem. Overseas growers are still producing large numbers of diseased cannas and exporting them to the UK. If it was a vegetable crop it would not be allowed, but because it is an "ornamental" there are no laws against it. It may be assumed that cannas purchased as rhizomes from hardware stores and garden centres will be infected.

To read more about canna virus disease, with photos, see
www.canna-virus.org.uk

Frequently Asked Questions (FAQ)
Anyone who exhibits at flower shows know that the same questions come up again and again.
The commonest questions that we are asked is "Why haven't my cannas flowered this year" and "What do you do with cannas in the winter".

• "Why haven't my cannas flowered this year".
  The usual reason is "they are hungry". Cannas are voraceous feeders. In poor soil, or soil that they have exhausted they stop growing. There are 2 other possible answers to this question. The cannas may be a tall variety planted late in the season, so they will flower given more time. Canna 'Musiolia Grand' rarely flowers because it is so big that it rarely reaches its full height. cannas are never "blind". They always flower when they they get big enough. See "Christine's Rule", which follows.
• "What is Christine's Rule?
  I have been growing cannas for many years, but it was my wife, Christine, who discovered that a canna plant usually produces 7 leaves and then it flowers. Among our canna friends this is now known as "Christine's Rule". It's true for all varieties short and tall. You may get exceptions, but it is amazing how often this rule holds. So if a canna hasn't flowered, and it has stopped growing at 4, 5, or 6 leaves then it is hungry.
• "What do you do with cannas in the winter".
  We always dig them up and store them until the worst of the winter is over. Other people leave them in the ground, but you are pushing your luck because if the ground freezes than the cannas will be killed. Also, cannas left in the ground are always late to grow in the spring. We recommend keeping them frost-free, cool, and damp. Unlike most bulbs tubers and rhizomes, canna rhizomes don't like to be dried out. They should be covered in damp compost, and watered occasionally if required. If grown in pots it is better to leave them in the pots through the winter. If dug out of the ground in a clump, they should be left in a clump through the winter.
• Do cannas rot if stored too wet?
  I've seen canna rhizomes left totally immersed in water in wintry weather for months. Yet when they are fished out they look totally healthy and ready to grow. In my opinion, wetness doesn't cause rhizomes to rot. Cannas die and rot if they get damaged by frost. They may die if they are badly diseased. They often die if their only growing shoot is broken off. A percentage of canna rhizomes die anyway, it is just in the nature of cannas.
• "Can canna virus be cured?"
  No, you must discard them and start again.
• "Does dead-heading cannas encourage them to flower"
  This is another topic where the magazine pundits often get it wrong. Dead heading can be positively harmful. Cannas always send up another flower shoot from within the dead flower head. They do this 3 or 4 times per flowering stem, which is why cannas have such a long flowering season. If you dead-head a canna, you may inadvertently be cutting off all the future flowers for the rest of the season!
• "Do Water Cannas need to be grown in water?"
  No, they grow just as well in ordinary soil. The strange thing is that ordinary cannas will also grow in water. The difference is that Water Cannas (and the species C. glauca and C. flaccida) will happily grow when the crown of the plant is submerged. Other cannas won't die, but neither are they very happy.
• Can we contact you for advice about canna cultivation?
  We prefer you not to. Please first try to find the information on our website. The reason is simply that we have lots of customers, and we just don't have enough time to spend on the phone or writing emails dealing with the same query over and over and over again. This is why we try to provide the answers to most questions on this website.