Canna (plant)

Canna (Crozy Group) 'Alberich', Pfitzer 1949

Canna indica, 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. The starchy root is edible.[11]

The first species of Canna introduced to Europe was C. indica, which was imported from the East Indies, though the species originated from the Americas. Charles de l'Ecluse, who first described and sketched C. indica, indicated this origin, and stated that it was given the name indica, not because the plant is from India, in Asia, but because this species was originally transported from America: Quia ex America primum delata sit; and at that time, one described the tropical areas of that part of the globe as the West Indies.[12]

Much later, in 1658, Willem Piso made reference[13] to another species that he documented under the vulgar or common name of 'Albara' and 'Pacivira', which resided, he said, in the "shaded and damp places, between the tropics"; this species is C. angustifolia L. (later reclassified as C. glauca L. by taxonomists).[3]

Without exception, all Canna species that have been introduced into Europe can be traced back to the Americas, and Canna definitely is solely an American genus. If Asia and Africa provided some of the early introductions, they were only varieties resulting from C. indica and C. glauca cultivars that have been grown for a long time in India and Africa, with both species imported from Central and South America.[14] Since cannas have very hard and durable seed coverings,[8][9] seed remains likely would survive in the right conditions and been found by archaeologists in the Old World if Canna had been grown there from antiquity. If the soils of India or Africa had produced some of them, they would have been imported before the 1860s into European gardens.[15]

Although most cannas grown these days are cultivars (see below), about 20 known species are of the wild form, and in the last three decades of the 20th century, Canna species have been categorized by two different taxonomists, Paul Maas, from the Netherlands[17] and Nobuyuki Tanaka from Japan.[3] Both reduced the number of species from the 50-100 accepted previously, assigning most as synonyms.

This reduction in species is also confirmed by work done by Kress and Prince at the Smithsonian Institution, but this only covers a subset of the species range.[18]

Cannas are largely free of pests, but in the US, plants sometimes fall victim to the canna leaf roller moth, and the resultant leaf damage, while not fatal to the plant, can be most distressing to a keen gardener.[4]

Slugs and snails are fond of cannas, and can leave large holes in the leaves, preferring the tender, young, unfurled leaves. Red spider mites can also be a problem for cannas grown indoors or during a very hot, long summer outdoors.[4] Japanese beetles can also ravage the leaves if left uncontrolled.[20]

Cannas are remarkably free of diseases, compared to many genera. However, they may fall victim to canna rust, a fungal disease resulting in orange spots on the plant's leaves, caused by over-moist soil. They are also susceptible to certain plant viruses, some of which are Canna-specific, which may result in spotted or streaked leaves, in a mild form, but can finally result in stunted growth and twisted and distorted blooms and foliage.[21]

The flowers are sometimes affected by a grey, fuzzy mold called botrytis. Under humid conditions, it is often found growing on the older flowers. Treatment is to simply remove the old flowers, so the mold does not spread to the new flowers.

Canna 'Golden Gate'

Canna (Crozy group) 'Theresa Blakey'

Cannas became very popular in Victorian times as garden plants, and were grown widely in France, Germany, Hungary, India, Italy, the United Kingdom, and the United States.[6][21] Some cultivars from this time, including a sterile hybrid, usually referred to as Canna × ehemannii, are still commercially available.[27] C. × ehemannii is tall and green-leafed with terminal drooping panicles of hot pink iris-like flowers, looking somewhat like a cross between a banana and a fuchsia.[28]

As tender perennials in northern climates, they suffered severe setbacks when two world wars sent the young gardening staff off to war. The genus Canna has recently experienced a renewed interest and revival in popularity.[4] Once, hundreds of cultivars existed, but many are now extinct. In 1910, Árpäd Mühle, from Hungary, published his Canna book, written in German. It contained descriptions of over 500 cultivars.

In recent years, many new cultivars have been created, but the genus suffers severely from having many synonyms for many popular ones. Most of the synonyms were created by old varieties resurfacing without viable names, with the increase in popularity from the 1960s onwards. Research has accumulated over 2,800 Canna cultivar names, but many of these are simply synonyms.[29] See List of Canna hybridists for details of the people and firms that created the current Canna legacy.

In the early 20th century, Professor Liberty Hyde Bailey defined, in detail, two "garden species" (C. × generalis[30] and C. × orchiodes[31]) to categorise the floriferous cannas being grown at that time, namely the Crozy hybrids and the orchid-like hybrids introduced by Carl Ludwig Sprenger in Italy and Luther Burbank in the USA, at about the same time (1894).[6][32] The definition was based on the genotype, rather than the phenotype, of the two cultivar groups.[21] Inevitably over time, those two floriferous groups were interbred, the distinctions became blurred and overlapped, and the Bailey species names became redundant.[4] Pseudo-species names are now deprecated by the International Code of Nomenclature for Cultivated Plants which, instead, provides Cultivar Groups for categorising cultivars (see groups at List of Canna cultivars).[33]

These canna cultivars have gained the Royal Horticultural Society's Award of Garden Merit:

Canna (Agriculture Group) 'Edulis Dark'

The Canna Agriculture Group contains all of the varieties of Canna grown in agriculture. "Canna achira" is a generic term used in South America to describe the cannas that have been selectively bred for agricultural purposes, normally derived from C. discolor. It is grown especially for its edible rootstock from which starch is obtained, but the leaves and young seeds are also edible, and achira was once a staple food crop in Peru and Ecuador.[6] Trials in Ecuador using a wide range of varieties have shown that achira can yield on average 56 tons of rhizomes and 7.8 tons of extractable starch per hectare. However, the crop needs 9–12 months to mature to full productivity.[49]

Many more traditional kinds exist worldwide; they have all involved human selection, so are classified as agricultural cultivars. Traditionally, Canna edulis Ker Gawl. has been reputed to be the species grown for food in South America, but C. edulis probably is simply a synonym of C. discolor, which is also grown for agricultural purposes throughout Asia.[50][51][52][53][54]

Canna fruit (green) and ripe seed pods

Canna fruits

Seeds are produced from sexual reproduction, involving the transfer of pollen from the stamen of the pollen parent onto the stigma of the seed parent.[5] In the case of Canna, the same plant can usually play the roles of both pollen and seed parents, technically referred to as a hermaphrodite. However, the cultivars of the Italian group and triploids are almost always seed sterile, and their pollen has a low fertility level. Mutations are almost always totally sterile.[6]

Canna seeds have a very hard seed coat, which contributes to their dormancy. Germination is facilitated by scarification of the seed coat, which can be accomplished by several techniques.[55]

Pollination

The species are capable of self-pollination, but most cultivars require an outside pollinator. All cannas produce nectar, so attract nectar-consuming insects, bats, and hummingbirds, that act as the transfer agent, spreading pollen between stamens and stigmas on the same or different flowers.[6]

Genetic changes

Since genetic recombination has occurred, a cultivar grown from seed will have different characteristics from its parent(s), thus should never be given a parent's name. The wild species have evolved in the absence of other Canna genes and are usually true to type when the parents are of the same species, but a degree of variance still occurs. The species C. indica is an aggregate species, having many different and extreme forms ranging from the giant to miniature, from large foliage to small foliage, both green and dark foliage, and many differently coloured blooms of red, orange, pink, or yellow, and combinations of those colours.[6]

In vitro culture of plants in a controlled, sterile environment

Division of plant parts

Outside of a laboratory, the only effective asexual propagation method is rhizome division. This uses material from a single parent, and as no exchange of genetic material occurs, it almost always produces plants that are identical to the parent. After a summer's growth, the horticultural cultivars can be separated into typically four or five separate smaller rhizomes, each with a growing nodal point (growing eye). Without the growing point, which is composed of meristem material, the rhizome will not grow.[6]

Micropropagation

Micropropagation, also known as tissue culture, is the practice of rapidly multiplying stock plant material to produce a large number of progeny plants. Micropropagation uses in vitro division of small pieces in a sterile environment, where they first produce proliferations of tissue, which are then separated into small pieces that are treated differently so that they produce roots and new stem tissue. The steps in the process are regulated by different ratios of plant growth regulators. Many commercial organizations have produced cannas this way, and specifically the "Island Series" of cannas was introduced by means of mass-produced plants using this technique. However, cannas have a reputation for being difficult micropropagation candidates.[4]

Micropropagation techniques can be employed to disinfest plants of a virus. In the growing tip of a plant, cell division is so rapid that the younger cells may not have had time to be infected with the virus. The rapidly growing region of meristem cells producing the shoot tip is cut off and placed in vitro, with a very high probability of being uncontaminated by virus.