Canna lily, a genus of 10 species of plants in the Cannaceae family, aren’t true lilies, they are assigned to the Zingiberales order in the monocot group Commelinids, along with their relatives, spiral ginger, true ginger, bananas, heliconias, and arrowroots.
C. indica, called achira in Central America, was among the earliest plants grown by Native Americans for its edible starchy root. Although the species was originally from the Americas, the genus was first introduced in Europe from the islands of the East Indies. Without exception, all these species introduced to Europe track back to both Americas.
While some varieties developed from C. glauca and C. indica cultivars have long been cultivated in Africa and India, both species have been imported to the Americas as well. C. indica has come to be naturalized in several tropical areas in the world as it is invasive in some areas and is a difficult plant to banish because of its rapid rate of growth. These plants have large leaves, so horticulturists have selected and bred large-flowered plants for gardens. Although they are tropical, several varieties have been developed to grow under temperate climates, provided they get an average of 6 to 8 hours of sunlight in summer and are protected from cold in winter.
These perennials have a rhizomatous rootstock and can grow quite tall, with wild species often at 6.6–9.8 feet high, although cultivated varieties have been developed to grow smaller, between 3-5 feet tall. The broad, flat, leaves that feature in most of these plants emerge in long, narrow rolls and unfurl. The leaves are normally green, but some varieties have brown, maroon, and even variegated leaves.
The flowers are generally yellow or red or orange or combinations of these colors, and are grouped into spikes or clusters. While gardeners enjoy these colorful flowers, they naturally develop to attract pollinators who collect nectar and pollen, such as honeybees, sunbirds, hummingbirds, and bats.
These plants grow from starch-bearing enlarged underground rhizomes and are the main reason this plant is cultivated in agriculture. These rhizomes, since they have large grains of starch, are still cultivated for consumption by people in the Andes, southern China, and Vietnam. The leaves along with the rhizomes are grown and harvested for fodder for pigs and cattle in Africa and Hawaii.
These plants should receive 6 or 8 hours of bright light every day during their growing season for vibrant leaves and flowers, but they can also grow under partial shade.
Water the plants once or twice weekly. The soil must be kept damp but don’t over-water them or they will develop rot.
These plants can thrive under humid conditions as well as drier or temperate climates. If you grow them indoors, humidity can be increased by positioning a humidity tray under the container or close by or using a humidifier. If you place the container on the humidity tray, ensure that the pot isn’t sitting in water.
They are sensitive to frost and cold temperatures, although growth will slow down or stop and resume when it becomes warmer. They prefer growing in temperatures between 0-90°F.
In the cooler climates below zone 7, you can start them indoors in containers and move them outside in spring when they’re actively growing.
They can tolerate almost any soil with good draining capacity, although they prefer rich organic soils with a 6.5 pH range. However, they can deal with acidic and alkaline soil as well.
They are large plants, so select a container with at least a 16-inch diameter with suitable drainage holes. This gives the plants enough space to develop healthy root systems and will also prevent the container from falling over as the plants get older. To repot the plants, carefully lift the plants out of their old pot and place them gently into the newer one, without damaging their delicate roots. Fill the new container with soil to almost an inch under the container’s rim. Keep the soil consistently moist after repotting.
They are readily propagated by digging up and dividing the rhizomes.
In spring or fall, dig up the plant without causing damage to the rhizomes or roots of the plant. Trim the foliage down to an inch over the crown (where the stems connect to the rhizomes).
Remove excess soil around the rhizomes and cut the joints where the old rhizome connects with the new ones, ensuring that the separated rhizome has one or two eyes. If you do this in autumn, store the rhizomes during winter and replant them in spring.
Plant each new rhizome division in suitable soil, about four to six inches deep.
These plants need plenty of nutrients, so lots of compost or fertilizer is needed to keep them happy. You can’t over-fertilize these plants if you use organic compost. However, if you’re using regular fertilizer, feed the plants every month throughout their growing season using a balanced fertilizer.
They generally don’t need pruning, but faded flowers must be deadheaded to produce more blooms. If you value the foliage over the flowers, flower stalks can be pruned before they start to bloom to direct the plant’s energy towards the foliage.
Snails, slugs, and Japanese beetles enjoy munching on leaves and flowers but the worst one is the caterpillar of a specific moth called the canna leaf roller that lays eggs on the buds of emerging leaves. The caterpillars leave a sticky web that stops the leaves from unfurling. Cut off any leaf that’s unable to unfurl and spray the plants with insecticidal soap.
These plants are also vulnerable to canna mosaic virus, rust fungus, and aster yellows. Look for foliage that seems unhealthy and discolored. You can treat rust fungus by pruning away fungus-ridden leaves along with using a suitable fungicide for treating the plant. However, with aster yellows and canna mosaic viruses, infected plants have to be removed and destroyed as there isn’t any cure for these viruses.