Why don’t cannas flower?
Over the years the common problems have changed a bit. There are still lots of questions about roses, clematis and camellias which I think are because they are so common rather than because they are difficult to grow, but there are lots of questions about ‘new’ plants. I think this is because we are all getting more adventurous, either by design or because some plants are more readily available.
Last year I had more questions about cannas than ever before. Just this week I had one about when to cut them back but more often it is about why they have not bloomed, or at least bloomed as much as expected. A better understanding of the plant will often help avoid such issues.
Cannas are frost-tender plants with underground, horizontal rhizomes. Frost will kill any part of the plant it touches. The creeping rhizomes spread almost indefinitely. From these arise ‘stems’ with large, paddle-shaped leaves and, eventually, a branched cluster of flowers. The flowers can be produced within a few months of the ‘stems’ being produced, or much longer, depending on the rate of growth. Once all the flowers on the stem have opened, and perhaps been followed by round, rough pods, the stem will die.
The key to lots of growth, and lots of flowers, is to keep them growing as long as possible. They are native to warm, frost-free climates and there they are evergreen and can flower all year. There you can cut out the flowered stems and tidy the old leaves all year round. But that is not applicable in cold climes.
It is normal to buy cannas as either dried rhizomes in spring, buy plants in bloom in summer or to grow from seed. Dried rhizomes are the most traditional way to get cannas. They need to be started in warmth and they can be slow to get going. It is not unheard of for the plants to fail to flower the first year if they are started late. I would start them, in as much heat as possible (being sensible) in pots, watering sparingly at first. The rhizomes are long and thin and it can sometimes be tricky to fit them into pots – they need to be planted horizontally and about 1cm deep. Another trouble is that most canna stocks are riddled with virus so I am reluctant to buy them this way any more. Virus causes mottling of the leaf colour – the green blotches in the plant below.
If you buy plants in growth you can see if there is virus. Most potted plants are seed-raised, probably the quite good ‘Cannova’ strain. You can also buy seeds of these and they can flower the first year from seed.
Once you have a plant, you need to keep it warm and sunny and give it loads of water and feed. few plants reward a rich diet more than cannas. In my climate I can’t put out cannas till late May. They have to be kept free from frost. They can be grown in the border or in pots but, if in pots, never let them go short of water and fertiliser. They are tough plants and will struggle on in a dry pot of multipurpose compost. But they will never look good.
In winter there is the choice of lifting them or leaving them in the ground. Here in southeast Ireland they usually do survive outside – the rhizomes are deep enough so that frost does not reach them. But they come into growth so late in spring that they do not always bloom before the first frost of autumn.
Lifted plants should be cut back, all the flowered stems being cut back, and then you can either dry them off or keep them ‘ticking over’ in frost-free conditions. If dried off, they are easier to store but they will be slow to start again in spring. If you can keep them light and in a frost-free spot they can be started more easily in spring and will perform much better.
So lack of flowers may be because of a short growing season or it can be because the plants are not warm enough, do not have enough sun or are short of water and fertiliser.
Cannas are so variable, and SO difficult to identify. I grow only one known cultivar, but have no idea what the others are. I believe that some are Canna musifolia, and two others are Canna edulis. However, some sourced deny the existence of one or both of these species. Anyway, CAnna musifolia is grown more foliage than for bloom. Although mine bloom quite nicely, I would not miss bloom if they failed to bloom once in a while.