Every northern gardener knows that cannas (Canna x generalis) are summer bulbs. With their green, purple or variegated leaves reminiscent of those of a banana plant and their flowers in mostly bright colors like red, orange, yellow, pink, or white, not to mention bicolors, the canna is a plant built to impress. You plant them in the spring and they sprout, grow and bloom in the summer, then you dig up their thick rhizomes in the fall and store them dry in the winter in a frost-free spot. But how many gardeners know you can use cannas as houseplants and see them bloom all year long on your windowsill?
Dormancy is Optional
Obviously, you do have to bring cannas indoors for the winter in a cold or even moderate climate: they are simply not frost tolerant. But you don’t have to put them into dormancy. It is perfectly possible to bring a canna in leaf or bloom indoors, just like you would bring back indoors any houseplant you’d put outside for the summer, and keep it growing all winter. And if you have the right conditions, it will bloom over and over, all winter long!
This is possible because of a detail rarely mentioned in descriptions of how to grow cannas: that their dormancy is optional. In their native South and Central America, cannas mostly grow in swamps, ponds, and other poorly drained sites. But these wetlands are subject to periodic droughts, occasionally drying up completely. Sometimes this is an annual occurrence, but in others, the swamp can remain wet for years in a row before a drought hits.
Cannas have adapted to this by going dormant only when their soil dries out, sleeping underground in their thick rhizomes while waiting for the rains to return. Otherwise, as long as the soil remains moist, the plant will keep growing and blooming all year long. Unlike hardy bulbs like tulips and narcissus, where dormancy is mandatory (they will go dormant in the summer no matter what conditions you offer them), cannas don’t need a dormant period to grow and bloom. And no, blooming again and again with no period of dormancy doesn’t “tire them out”. As long as you continue to water your cannas and protect them from frost, they will continue to bloom on and on, even in winter.
The Right Conditions
It’s not at all that difficult to keep a canna growing and blooming on a windowsill indoors.
Start by bringing in a plant in full growth early in the fall, before the nights cool off too much. Indoors, offer as much sun as you can, such as a spot in front of a large south-facing window. The more sun, the better the bloom! Also, the room should be heated adequately, to 60°F (15°C) or above if possible. Most homes can easily offer that temperature range. Anything cooler will reduce or even eliminate flowering.
All that you have to do is to water the plant regularly, just like you would any houseplant. And for once, you can even leave the saucer filled with water: cannas are a swamp denizens, after all. If ever you do let your canna dry out too long, however, it will go dormant.
Each stem produces only 2 or 3 flower clusters. So when a stem has stopped blooming, cut it off at the base. Yep, the whole stem, leaves and all. This will give more space for the sprouts that appear regularly at the plant’s base to flower in their turn.
Since your canna is now growing and flowering all year, you’ll have to fertilize all year as well: use an all-purpose fertilizer at a quarter of the recommended rate in fall and winter and and at one-half the recommended rate in spring and summer.
You can keep your canna growing for years, even decades, indoors all year if you like or you can put it outdoors for the summer, which tends to stimulate even greater bloom, because sunlight is more intense outdoors and not filtered by passing through glass as it would be on a window sill.
A canna indoors? Why not? It works for me!