ALL CANNAS GREAT AND SMALL.
For the tropical lepidopterist, Canna is a most interesting and useful plant. Canna is one of the main plant groups used as a food-plant for larvae of the South American Owl butterflies (Caligo sp.) and has great advantages over other Caligo food-plants such as Musa (Banana) or Strelitzia (Bird of Paradise).
Cannas generally don't grow quite so big as their relatives - the bananas & plantains, although I have seen Canna grow as high as 15ft (4.6m) given the right (or wrong!) conditions. Generally though, Canna has the following advantages: It is compact in size yet can produce good quality food-plant leaf, it can easily be grown from seed (or over-wintered corms - see below) and it can make a very attractive bedding feature for the summer months outdoors. Seed has a long storage life (2 - 5 years). Banana can seldom be grown from seed and must be propagated from offshoots.
Canna seed should be sown as early as possible in the year if you have access to a propagator or heated greenhouse. If you are growing in a warm room, don't sow too early, unless you can provide heat and light to keep the young plants growing nicely through till May, when you will be able to transfer them to the greenhouse. Given hothouse conditions, I usually sow in February and by June, can have usable plants. If you can't get off to an early start, plan to rear your larvae a little later in the year and grow from corms (root pieces) planted in May or June. These are becoming easily available from many garden centres at prices ranging from 50p to £5.00 per corm, depending on the variety or cultivar. As always, growing from seed is the cheapest method, with seeds costing from 50p to £2.00 per pack.
Even though the hard shot-like seed has long viability, the freshest seed possible should always be used to guarantee a high germination rate. Any seed up to 15 months old should give good results. The key to success is to chip or nick the hard seed-coat to allow water penetration. Without this, seeds may take many months to germinate or may not germinate at all. So hard are these seeds and so impossible to germinate without chipping, I often wonder what mysterious process they undergo in the wild to germinate successfully. Shrivelled or pitted seeds should be discarded and the hard glossy seeds should be chipped or nicked slightly to expose the creamy-white inner. Various tools can be employed for this job, a sharp craft knife, small electrician's wire-cutters or a file, but watch out for your fingers! Chipped seeds should be soaked overnight in tepid water until they swell to around 50% bigger than the dry size. Seeds can be sown 4 to a pot, in 4" (100mm) pots using normal peat-based compost. Sow half an inch (12mm) deep. The seeds should be watered well and a temperature of 75-80ºF (24-27ºC) is needed for germination and bottom heat is beneficial. Germination takes 10 - 20 days and if the temperature is maintained and adequate bright light provided, growth of the young plants can be rapid.
As soon as the plants are about 6" (150mm) high and there is good root-growth, the plants can be potted on into 6" (15cm) pots using compost with a little added well-rotted manure or slow-release fertiliser granules. Cannas are very greedy plants and respond well to heavy feeding during the growth period. No further potting on should be necessary as long as the plants are well fed. Liquid feeds can be used. For larger plants, when pot-bound in the 6" pots, they can be transplanted into a greenhouse border with a rich soil or compost at least 12" (30cm) deep. All the advice so far is aimed at producing a lot of leaf growth to feed many hungry Caligo larvae. All the plants will flower, but the greater the feed, the taller the plants will be before flowering, so often a normal greenhouse is not high enough. For more compact flowering Cannas, cut down on the feed a little and reduce the nitrogen content of the feed. In West Africa, I have seen Cannas flowering nicely in barren sand, at 20 - 30" (50 - 70 cm) high, although the plants are often dry and dusty and the leaves are tough.
If you can obtain corms, these should be planted in May, in 6" pots of rich compost and watered well. Liquid feeds can be given as soon as the corms are sprouting well. A temperature of 70-75ºF (21-24ºC) is ideal. Choose healthy solid corms, each should have a piece of root and at least one growing bud. The more buds present, the better! Plant 3" (75mm) deep with the buds pointing upwards. By mid June there is usually enough growth to commence feeding Caligo larvae.