Cannas once again take centre stage in the summer garden

Pink Cannas at Longwood Gardens Conservatory. Image by Glenn Marsch see his Flickr pagePink Cannas at Longwood Gardens Conservatory. Image by Glenn Marsch see his Flickr pageThere is nothing subtle about cannas with their voluptuous leaves, impressive height and exotic blooms, and these flashy extroverts just love being the centre of attention, adding a tropical ambiance wherever they are planted. They are fun and easy to grow so read more below about cultivating and using them in modern gardens.

Bold, tropical-looking and summer blooming cannas have once again stepped into the limelight from a post WWII low, as they have done many times before during their long history, and today they are approaching the heights of their popularity in the Victorian era. Modern breeders continue to release wonderful new cultivars with bold foliage and flowers in both warm and pastel colours, and today gardeners worldwide have more than 2000 cultivars to choose from.

The Canna genus has approximately 50 species native to subtropical and tropical parts of North and South America, extending from North Carolina south to Argentina, and including the Caribbean islands. In their native habitat they thrive in damp, shady locations and favour the margins of rivers and lakes. Canna species vary in height from 75 to 150cm, but a few like Canna iridiflora may reach heights of 3m or more. Click here to see Google images of this magnificent species.

Canna Foliage. Picture by mjimages from PixabayCanna Foliage. Picture by mjimages from PixabayThe large banana-like foliage of canna species may be green, bronze-burgundy, or variegated in a striped or marbled pattern, and the flowers are typically red, orange, or yellow, or any combination of these colours. Canna are the only genus in the family Cannaceae in the order Zingibales, and is thus distantly related to Banana (Musa), Bird-of-Paradise (Strelitzia), Heliconia, Maranta, and Ginger (Zingiber).

Some canna florets open in the morning and look at their best during the daytime, while others are night bloomers whose beauty is waning by the next morning. In their native range the flowers are pollinated by a variety of organisms. Day-flowering cannas ones are pollinated by bees or hummingbirds, and night-flowering ones are pollinated by moths or bats.

The thick seed coating allows canna seed to survive for a very long time, and in 1969 Canna indica seed was found in a 550 year old archaeological dig in Argentina, and was successfully germinated. The reason that the seed coat is so thick is that it protects the seed until conditions are perfect for germination. In the wild, canna seed germinates best in places burned by fire, which not only weakens the seed coat, but also destroys any competition for the emerging canna seedling.

Cannas grow from swollen underground stems, correctly known as rhizomes, which store starch, and because they are known to have the largest starch grains of all plant life, canna is still used today in agriculture as a source of starch for both human and animal consumption. The wild canna (Canna indica) which is commonly called “achira” in Latin America, has been cultivated by Native Americans in tropical America for thousands of years, and was one of the earliest domesticated plants in the Americas.

Although cannas may have arrived in Europe from the Americas as early as Columbus's 1492 travels, they were not well-known to European botanists until the 1500’s. They are first mentioned in the book, The Vienna Codex (1536-1566), and by 1576 cannas were cultivated in gardens in several European countries. However, it was only during the Victorian era in the mid to late 1800's that cannas became wildly popular garden and glasshouse plants. During this period cannas had a particularly large following in France, Hungary, England, Italy, Germany, America and India, and hundreds of cultivars with more compact growth and novel flower forms and colours were created between the 1860's and the 1910's, but unfortunately most of these cultivars were lost because gardeners stopped growing cannas during the tumultuous times from World War I through World War II.

In the first half of the 20th century garden fashions also changed, adding to the demise of the popularity of the canna. Garden designers, such as Gertrude Jekyll, replaced formal looking Victorian gardens with informal designs using mixed perennial borders to achieve a more relaxed garden feel.  

Gratefully, by the early 1950’s cannas slowly started making a comeback, and today they are once again approaching the heights of their popularity during the Victorian era, albeit these tropical beauties are now being used in very unusual and creative ways in modern gardens.

The most prominent botanist of the 20th century doing research on canna genetics and breeding was Triloki Nath Khoshoo of the National Botanic Gardens of Lucknow in India. He performed in-depth studies of canna history, breeding and genetics during the 1960s and 1970s.

Breeders continue to release wonderful new cultivars, from dwarf compact types to large voluptuous specimens, and even those with improved cold hardiness. Some cultivars have even been bred to grow partially submerged in shallow water, as well as in saturated soils. Cannas are now so versatile, and suitable for all types of gardens, even the smallest ones.

Of the approximately 50 canna species, only Canna indica was spread worldwide by humans as a food crop, and it has naturalised itself in eastern and south-eastern Australia, New Zealand, southern USA, Hawaii, and several other Pacific islands, as well as in southern and eastern Africa, in parts of Kenya, Uganda and Tanzania. In these regions it is commonly found in swampland and wetland edges, streambanks, and other moist areas, and is still sometimes found growing in old gardens, disturbed sites and waste areas.

Canna indica. Image by moonietunes from PixabayCanna indica. Image by moonietunes from PixabayIn South Africa the wild canna lily (Canna indica) has been declared as an alien invader plant which cannot be grow and should be removed from gardens. This weed species is tall and vigorous with narrow green or bronze leaves. It produces small but copious, multi-coloured yellow and orange flowers with narrow petals, but single-coloured white, yellow, orange or red flowers are also found. Spiny green fruits follow the flowers and turn brown before splitting open and releasing multitudes of black, pea-like seeds with a hard outer coating. In the past children used these seeds as ‘ammunition’ for peashooters. This tendency to produce vast amounts of viable seeds has seen many a garden totally overrun by the wild canna, along with many wilderness areas, and it is for this reason Canna indica was declared to be a noxious weed in South Africa.

Thankfully, hybrid canna cultivars are not considered invasive and gardeners continue to use them for their lush, tropical foliage and blooms. In older gardens you may find gorgeous, but long-forgotten garden hybrids, and garden centres stock modern hybrids which produce their tropical blooms on stately stems. Canna flowers may be solid single colours, or they can be striped, streaked, spotted or splotched with contrasting colours, and they come in amazing colours ranging from cream to yellow, pink, orange and red, and all shades in between, including delightful pastel shades of pink, primrose yellow, and pale orange.

 A few cultivars are marketed as being white, but that is not strictly true. The so called "white" canna usually emerges a very pale yellow and matures to a cream colour, and there are no true white cannas in cultivation. It is believed that some of the Victorian era’s cannas were pure white, but they have been lost to history and we have no way of verifying these claims. There are also no blue or purple canna flowers.

Canna florets tend to be short-lived, lasting only a day or two, but because new florets open constantly they can provide a continual bloom throughout the year in subtropical and tropical gardens. In temperate gardens, flowering usually begins in midsummer and will last until the first frost.

The start date and duration of flowering varies by cultivar, and breeders continue to strive to increase the length of time that an individual canna floret lives, and to increase the number of cultivars whose florets open during the day instead of at night. Improvements in both of these traits would result in plants that have more florets open at the same time, thus appearing to be more floriferous. There is also the potential to improve the scent of canna flowers but flower scent is not the focus of many breeders.

Canna hybridization has crossed many of the wild species in a very complex manner. Many epithets have been used in canna breeding programs leading to names such as Canna x hortensis, Canna x hybrida, and Canna x orchiodes. These have all been abandoned for the sake of simplicity and today all ornamental hybrids of canna are called Canna x generalis, although breeders do not always mention the epithet "x generalis" when they write the name.

Canna x generalis Cannova F1 Mango. Picture courtesy Ball StraathofCanna x generalis Cannova F1 Mango. Picture courtesy Ball StraathofUses:

Cannas are valuable as a food source because their rhizomes contain a high quality starch, and the primary species used for food production is Canna indica. Canna starch has the largest starch granule size in the plant kingdom and is still used today as an industrial starch in processed foods, because it has a low viscosity and does not break down during cooking and freezing.

In the Huila region of Colombia, a wild type of canna called “Achira” is grown to produce achira flour which is used instead of wheat flour to make traditional biscuits called “bizcochos de achira.”​ These delicious biscuits melt slightly in your mouth and have a crispy, soft and gritty texture, with a taste of milk. Achira flour is gluten-free with high protein content, and provides minerals such as calcium, iron, phosphorus, and sodium. It is also used in Asian cuisine, especially in China, Vietnam, Taiwan, and Thailand, where the starch is used to make noodles, and as a thickener in soups, sauces, and condiments.

In the modern era of agriculture, canna is only rarely used as a primary food source, as it has been replaced by more nutritious and higher yielding crops such as potatoes and corn.

In addition to food, Cannas have a variety of other uses, and “Indian Shot” is another common name for them which refers to the very hard, pea-like seeds that have been used as shotgun pellets in India. The seeds are also used as beads for jewellery and rosaries, and have been incorporated into baby rattles and musical instruments. A purple dye can be extracted from the seed and fibres extracted from the stem of the plant are used to make jute and paper.

Canna has also been used for phytoremediation, a plant-based approach which involves the use of plants to extract and remove elemental pollutants, or lower their bioavailability in soil and groundwater. Phytoremediation removes toxic heavy metals such as copper and zinc from pig waste, and excess fertilisers and insecticides from greenhouse runoff.

In the Garden:

Cannas really hit their stride in late summer, when most flower gardens are beginning to fade, and there are so many ways to incorporate these showy, tropical plants into your landscape. Today they are used as much for their bold foliage as their brightly coloured flowers, and if used discreetly and wisely these hassle-free plants will add body and substance modern gardens.

Make the most of their impressive size by using them to hide an unwanted view or to define an outdoor room. You can use cannas to dress up an outbuilding or to soften the lines of a fence, and you could even enclose your patio with cannas for a bit of privacy.

While cannas always look great on their own, consider pairing them with other big and assertive plants such as New Zealand flax, elephant ears, dahlias, or sunflowers. Clumps of cannas interspersed amongst shrubs and other perennials contrast strikingly with their companion plants, and they look best in mixed borders when the colours are grouped together.

The canna’s dramatic foliage and exotic flowers take pots and planters to new heights, and you can plant cannas in their own pot or in mixed plantings where they are often used as tall focal points.

Cannas can be paired with an array of other summer flowering shrubs, perennials and annuals, including: alternathera, caladiums, coleus, dahlias, euphorbia, rudbeckia, gaillardia, lantana and hibiscus. To accentuate the drama of dark-leaved varieties, pair them with the equally dark foliage of coleus or alternathera, highlighted with a splash of lime, creamy-white or yellow. Cannas come in a range of heights, short, medium and tall, so choose a variety that fits your garden or container size.

Image by hartona subagia from PixabayImage by hartona subagia from PixabayCultivation/Propagation:

Cannas can be grown throughout South Africa, and plants grown in nursery bags can be planted out at any time but the rhizomes are planted out in early spring.

In the warm, frost free regions of South Africa cannas can be coaxed into continuous growth by watering and feeding throughout the year. However, these tropical plants will not take heavy frost, and in colder climates they go completely dormant during winter, emerging with new spring growth when temperatures rise again. In cold regions where the rhizomes can be kept relatively dry throughout winter the soil can simply be mulched to keep the rhizomes from freezing, and new growth will quickly emerge in spring. In extremely cold regions the rhizomes should be lifted before the first frosts, and stored. 

In their native habitat, cannas grow in shaded locations, and in the humid subtropical regions of South Africa they will also enjoy some shade. This also applies to the arid and semi-arid regions of the country, where the lower humidity and soil moisture may cause foliar burning, and the intense sun can bleach the blooms and foliage. Cannas grow well in coastal gardens if they are protected from the ocean by a wall or another structure.

In temperate Highveld gardens and the mist belts, cannas need full sun to flower well, the more sun the better. Remember, off-white flowering varieties prefer dappled shade, but the brightly coloured red or purple leaved types will lose their colours in too much shade and will not grow as profusely. 

Cannas will adapt to a very wide range of garden soils but prefer rich, loamy, water-retentive yet well-drained soils that are high in organic matter, and a pH around 6.5 which is just about right for most home gardens, since most plants thrive in the 6.0 to 7.0 range, which is slightly acidic to neutral. If your soil is sandy add copious amounts of well-rotted animal manure or compost, and for clay soils, prepare the beds well and do not plant the rhizomes too deep. 

Like their distant cousins bananas, cannas are heavy feeders and without adequate fertility or moisture, cannas can look quite ugly. So, if your canna looks ‘ratty’ during the summer it’s a sure sign that an extra shovel or two of manure is required, and an application of an organic fertiliser will also go a long way. As long as you are using organics, it is almost impossible to over-fertilise a canna.

An annual mulch of well-rotted animal manure or compost in early spring will encourage lush, healthy foliage and masses of blooms right through the flowering season. Fertilise with any balanced organic fertilisers for flowering plants throughout summer in cold regions, and all year round in warm regions.

Although their fleshy rhizomes enable them to survive some drought, cannas definitely grow best in the garden when watered lavishly, at least once a week. Some cultivars have even been bred to grow partially submerged in shallow water, as well as in saturated soils.

In areas where frosts are moderate the rhizomes can be left in garden beds which are well mulched and which are kept relatively dry over winter. Where heavy frost occurs the rhizomes should be lifted and stored until all danger of frost is past. 

When lifting canna rhizomes you may have to use a garden fork, but take care not to damage them as much as possible, and especially those cultivars that have long narrow rhizomes like Canna 'Stuttgart'. Shake off the excess soil and select only undamaged and healthy, plump rhizomes with between 4 and 7 eyes to store. Layer them in ventilated boxes or baskets, using only slightly moist sawdust, peat moss, vermiculite, or palm peat between the layers of rhizomes. Do not add too much moisture, or you will promote rotting. Dusting the rhizomes with sulphur is not required but will help to prevent fungi and bacteria growth.

Store indoors in a cool and dry spot, where the temperatures are not freezing, but where they reach no higher than 10°C. Make sure that the layering between the rhizomes does not dry out too much during the winter, and if it does, give the medium a light misting with water. Prior to planting in the spring, wet the medium and allow the rhizomes to plump up before planting them out.

Pruning of cannas is not necessary, but spent flower stems are generally cut back to ground level immediately after flowering to keep the plants looking neat and to encourage the formation of new flowering stems.

If your Canna is looking ragged due to severe hailstorms, or for whatever reason, you can cut the entire plant right down to the ground, even in midsummer. Add some fertiliser, water the plants regularly and they will quickly recover.

Due to centuries of breeding, most of the commercial canna plants are sterile and don't produce seed. However, it is easy to propagate cannas by simply dividing up the rhizomes. The best time to divide is when the rhizome is actively growing in spring, so that the new buds are easily seen. This task should be carried out at least every second year to rejuvenate the plants.

Cut all the foliage stems off level with the ground and lift the rhizomes carefully with a garden fork, shaking off the excess soil. Cut the fleshy rhizomatous stems into sections with a sharp, sterilised knife or secateurs, ensuring that each section is about 10 to 15cm long, and has at least 3 prominent red buds, called “eyes”. Single-eye divisions may survive but will take longer to produce a vigorous new plant.

Prepare the soil for replanting by digging it over well and incorporating lots of mature compost or manure, along with a balanced general purpose fertiliser like 2:3:2. Replant the rhizomes at a depth of 8 to 10cm and water well. In clay soils the rhizomes are planted at a depth of  5 to 6cm.

Canna x generalis Cannova F1 Yellow. Picture courtesy Ball StraathofCanna x generalis Cannova F1 Yellow. Picture courtesy Ball StraathofGrowing Cannas in Pots:

If you're growing your canna in a container, remember, most cannas are fairly large plants and therefore need a large pot. Any good quality potting soil which holds moisture well will work fine. Potted specimens require regular feeding to look their best, and adding a slow release fertiliser to the potting soil will save you plenty of time on feeding, and water retention granules will go a long way in keeping your plant happy. Container-grown Cannas will need even more frequent watering than those growing in the soil, so check them daily and water liberally throughout summer.  Unlike many plants, during hot weather cannas can even stand in a drip tray filled with water.

The plants will lose vigour as they become pot-bound, so regular lifting and dividing is required. Lift the root-ball out, gently divide the rhizomes and replant only the plumpest and healthiest ones.

Problems, Pests & Diseases:

Cannas rarely have serious issues with pests, and in the garden slugs, snails, caterpillars and beetles are the most common culprits.

Aphids, spider mites, or whiteflies will rarely attack cannas in the garden, but can sometimes be a problem if the plants are grown in a small, enclosed garden, or under a roof or overhang, and especially in greenhouses.

Cannas aren’t prone to diseases, but rust, fungal leaf spot and bacterial blight may occur when the plants are kept too wet and overcrowded.

In hot, humid climates cannas can develop a fungal problem called “canna rust”. It forms rusty-orange coloured pustules on the leaves, which eventually turn black and die. Canna rust is difficult to control but there are fungicidal sprays that can prevent it from starting.

Canna viruses are the most serious cultivation problem, causing spotted or streaked leaves, stunted growth, and distorted blooms. Viruses are easily passed from plant to plant by sucking insects such as aphids, and are also spread by unsanitary division techniques.

Most canna cultivars tolerate a certain amount of viral load and will grow and thrive despite being infected. Low levels will not kill the plant but they may reduce its vigour, and in most cases, low levels are unnoticeable, except during cool weather. High virus loads, on the other hand, can render cannas so unattractive, they must be discarded.

Because the plants are often carelessly divided, viruses can easily spread and multiply, and cannas are also one of the few plants in which viruses can also be transmitted by seed. While many of the seed strains are fairly clean, this is not a guarantee of a virus-free plant. There are several viruses that can infect canna, including: bean yellow mosaic virus, hippeastrum mosaic virus, tomato aspermy virus, cucumber mosaic virus, canna yellow streak virus, and most seriously canna yellow mottle virus.

Warning:

Canna x generalis is listed as non-toxic to humans, cats, dogs and horses, but it is always advised to supervise small children and pets in the garden, and to discourage them from chewing on plants.