Well – 10 things I didn’t know. I have just been reading a book on Cannas by Ian Cooke and thought that I would like to share with you some of the main points.
1) Cannas come from tropical areas of South America. They have been grown for over 5000 years for food, and are still used for that. In Peru and Colombia they are baked before being eaten, and in Vietnam they are used as a source of flour for making noodles. In Australia they are used on an industrial scale for the production of starch. However the book this information came from was published in 2001 before the extensive spread of Canna virus. Things may have changed since.
2) Cannas can be cultivated in a similar way to Dahlias. But they are grown from rhizomes (swollen underground stems), and not bulbs or tubers (like Dahlias) as some sources quote. There are two important differences between bulbs and rhizomes. Bulbs can be allowed to dry out totally when dormant and still remain viable. Rhizomes will die if they dry out completely, so must always be kept very slightly moist. Garden centres and shops are often too warm and dry to keep Canna rhizomes properly, and many that are bought are desiccated and already dead or on the point of death. They can sometimes be coaxed back into life by gentle rehydration, but may develop slowly afterwards or simply refuse to grow. So when you buy Cannas make sure the rhizomes are plump and healthy, rather than shrivelled. The second difference is in respect of propagation discussed below.
3) Cannas naturally grow in marshy places. And one species C. glauca is found in swamps. Hybrids of C. glauca have been developed and are known as Water Canna as they can be grown in standing water. But nearly all Canna can be treated in the summer as water marginals. This means that they like a lot of water and must not be allowed to dry out. So it can be useful to stand pots of Canna in large saucers of water. But this should only be done in the summer. In spring, they won’t have developed the root system or foliage to handle a lot of water, and in autumn the rhizomes should start to dry out in preparation for winter storage.
Cannas have grown up where part of the year becomes very dry and they have developed the ability to go dormant to survive this dry period. Outside their native areas they have adapted this dormancy to cope with the cold in our winters as well. But cold dormancy must be accompanied by dry as well. I think this is a similar mechanism to Dahlias that has been discussed elsewhere on the forum.
4) There is a small number of Canna species (ie varieties that occur in the wild), but these have small flowers and the Cannas that are grown in gardens are complex hybrids (ie hybrids of hybrids etc) whose parentage has generally been lost. Cannas can flower for anything up to 10 months of the year in their native habitat where the temperature and light levels are right.
5) Cannas only have three narrow insignificant petals, these you can see on the outside coming from the base. The big blowsy coloured bits that we think are petals are really sterile stamen called staminoides, growing inside the petals. One or more staminoides will not be sterile and will produce pollen in areas along the edge of what seems to be a petal.
6) Canna rhizomes should be careful lifted in autumn after the first frost, cleaned off and stored frost free, and never allowed to completely dry out. They must be stored between 0C and 10C. If they are kept above 10C they will continue to grow. In theory you could keep them growing inside the house over the winter, but in practice there wouldn’t be enough light to support this.
The rhizomes are on the borderline of hardiness and it is possible to let them overwinter outside in the ground, but they need a thick insulating mulch to have a chance of surviving. And even then may suffer from excess winter wet.
7) Although it is possible to plant dormant Cannas directly into the ground outside in spring, it is generally advisable to start them off in a pot under glass. The rhizomes need a decent temperature to start growing and pots will be warmer than the ground. Don’t put them out until after the last frosts.
8) Later on they can be planted out into the garden, but if conditions are cool there is no point in rushing. Quite late plantings tend to catch up if conditions are right, whereas an early planting in cold soil can make the Canna sulk. They only need to be planted about 2 inches deep. Any deeper and the Canna shoots have to waste energy in reaching the surface. The soil should always be damp, rich and with an open structure. Lots of manure will help, and grit will help to keep the texture open. And of course plenty of water.
Cannas can be grown successfully in pots, but make them large as the roots can easily get potbound.
9) You can’t take cuttings as such, so division is the main method of propagation. Some gardeners do this when the rhizome is totally dormant, but there are distinct advantages in doing this once young growth has started. The rhizome is cut into pieces in a way that gives each piece at least one bud and preferably 2 or 3. Once growth had started you can easily identify these growing points, and any damage done to the rhizome will be much more quickly repaired that if the Canna were dormant.
The young divisions need to be grown on in pots, ideally with bottom heat of 15C to 20C at the roots and an air temperature of 10C. But I suspect that they are pretty robust and will grow under a range of conditions. You need loamless, open compost with sharp sand or grit to help drainage. Initial watering should be light until the roots have developed.
1O) You can also propagate by seed, but it won’t come true. Even if the young plant looks the same as the parent, its genetic make up will still be different. Cannas from seed are capable of flowering in the first year. I sowed some Canna ‘Tropicana’ (from Jungle Seeds) at the end of January and they are in flower now – picture above. First year Cannas will make rhizomes, but they may be small so extra care is needed to make sure they don’t dry out.