WHAT ARE CANNAS?
Canna are exotic tropical-looking plants which have are often seen in municipal and public gardens where they form spectacular bedding displays and centerpieces.
BOTANY AND TAXONOMY
Botanically, they are members of the family Cannaceae, in which there is only one genus, Canna. This means that they are very different to anything else in the plant kingdom. There is no other plant in nature which is like a canna. But because there is only 1 genus, they are very much like each other.
Within the Canna genus, there are either 9 or 19 species (ie truly wild cannas). It may seem odd that botanists haven't been more specific about the number of species, and the reason is that there are 2 expert taxonomists who have studied cannas in the wild, the japanese botanist Nobuyuki Tanaka, and the Dutch botanist Hilga Maas. Tanaka came to the conclusion that there were 19 species, and Maas decided that there were only 10 species. The difference is mainly that C. indica is a very variable species, and where C. indica ends and another species begins is arguable. The closest relations to cannas are the gingers (Zingiberaceae), bananas (Musaceae), strelitzieas (Strelitzeaceae), heliconias (Heliconiaceae), and marantas (Maranteaceae).
All cannas originate in the tropical and sub-tropical Americas from Florida down to northern Chile and northern Argentina. These wild cannas mostly have small flowers. Colonialists took samples to Europe, particularly France, where large-flowered hybrids were bred. These were exported around the world and now are widely naturalised in hot countries.
There are many types of canna. Some are giants growing 2 meters tall or more (up to 8 ft). Some are very short and remain about 0.5m (18 inches). Some are mainly grown mainly for the foliage and have small flowers, and some for large flowers. The foliage can be in shades of green and bronze, or striped. Flowers are generally shades of orange, red, yellow, pink, cream. They are equally at home being grown in the garden border or in pots and planters (but they do not like to be pot-bound).
They have some very desirable qualities as garden plants. They produce a lot of flowers and have a very long flowering season, from the end of June until mid October. They can tolerate any kind of soil: acid, alkaline, sandy, clay, (but it must be rich). They are happy in full sun or shade. They can tolerate extended periods of drought, yet they are happy growing immersed in water. They are trouble free and so not need staking or spraying. They are not much troubled by insect pests or slugs. In case this sounds too good to be true, there do have some disadvantages. They are vulnerable to a type of virus disease which has decimated cannas during the past 10 years, and so you need to be very careful where you acquire your stock.
Canna are very easy to grow. A rhizome started into growth in February/March in a cold 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.
Cannas can be grown from rhizomes, from small plants, and a few can be grown from seeds. There are pro's and con's to each method. 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. Plants are the easiest to grow, you simply plant then in the garden, and they can be inspected for virus disease at the time of purchase, 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 most cannas are sterile and do not produce seeds. Where they do produce seeds they usually don't come true.
Rhizomes are usually started off in greenhouse between February and April. The earlier they are started into growth the earlier they will flower. 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.
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 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.
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.
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 beconing more popular, and some of these can be grown from seeds. If grown well (rich soil, not pot bound) they can flower in 100 days from sowing the seeds.
When you receive the rhizomes, 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, until the growing season starts.
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.
A few cannas can be grown froms seeds. These are the Tropical Series, produced in Japan, in colours red, pink, yellow, white and orange. They are very short and flower around 3 to 4 months from sowing the seed, if grown well (ie warmth, water, light, and good compost)
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.
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 disfigue the leaf for the rest of the year. 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 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 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.
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.
Deer eat cannas, but rabbits do not.
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 should 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 canna virus
© Copyright Cannifer Ltd 2010