Cannas are very easy to grow. A canna started into growth in February/March in a cool 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 cut down by winter frosts when they are dug up and stored for the winter. It will then be seen that the original rhizome or plant has multiplied many-fold.
Cannas can be grown from rhizomes, by division of plants, and a few can be grown from seeds. There are pro's and con's to each method.
Pros and cons: rhizomes, plants, seeds
- Rhizomes require more work, and also 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 coud be grown from seeds, but the problem is that many cannas are sterile and do not produce seeds. Where they do produce seeds they usually don't come true. You may get something interesting, but it probably won't look like the parent
Cultivation by Rhizomes
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.
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.
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 they are lifted in early October, or until they are cut down by winter frosts.
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. Otherwise their treatment is as for rhizomes.
Cultivation from seeds.
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. If you wish to try growing cannas from seeds produced by some floral hybrids, then if you sow the seeds in February, they should be flowering by August.
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 (30C), and very wet.
Floral varieties not known to produce seeds include: Durban, Pretoria, Wyoming, General Eisenhower, Ehemannii, Stuttgart, Black Knight, Assaut, Panache, President, Rosemond Coles.
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". Red spider mite can be a problem.
Rhizomes should be lifted in the autumn, typically mid October, and stored in a frost free place. A single rhizome planted in the spring will have multiplied by the year-end to give typically 4 to 6 rhizomes which can be saved for the following year. Rhizomes need to be stored damp, not dry. We always keep the rhizomes covered in compost, which needs to be watered to prevent the rhizomes becoming dry. 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. If canna rhizomes dry out in the winter, then many will be lost.
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 is quite effective. Soap-based insecticides combined with a powerful spray can dislodge and/or suffocate them, and minimise the problem to an acceptable level.
Deer eat cannas but rarely cause much damage, even in deer infested areas. Apparently rabbits do not eat cannas.
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.
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.
- The rule of 7. A canna usually produces 7 leaves and then it flowers. It seems to be 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 are vigourously trying 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 may 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.
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