| Canna | |
|---|---|
| Canna hybrid flowers and foliage | |
| Scientific classification | |
| Kingdom: | Plantae |
| (unranked): | Angiosperms |
| (unranked): | Monocots |
| (unranked): | Commelinids |
| Order: | Zingiberales |
| Family: | Cannaceae |
| Genus: | Canna L. |
| Species | |
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19 classified species, see List of Canna species |
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Canna (or Canna lily, although not a true lily) is a genus of nineteen species of flowering plants.[1][2][3] The closest living relations to cannas are the other plant families of the order Zingiberales, that is the gingers, bananas, marantas, heliconias, strelitzias, etc.[4]
Canna is the only genus in the family Cannaceae. Such a family has almost universally been recognized by taxonomists. The APG II system of 2003 (unchanged from the APG system, 1998) also recognizes the family, and assigns it to the order Zingiberales in the clade commelinids, in the monocots.
The species have large, attractive foliage and horticulturists have turned it into a large-flowered and bright garden plant. In addition, it is one of the world's richest starch sources, and is an agricultural plant.[4]
Although a plant of the tropics, most cultivars have been developed in temperate climates and are easy to grow in most countries of the world as long as they can enjoy at least 6–8 hours average sunlight during the summer. See the Canna cultivar gallery for photographs of Canna cultivars.
The name Canna originates from the Celtic word for a cane or reed.[5][6]
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The plants are large tropical and subtropical perennial herbs with a rhizomatous rootstock. The broad, flat, alternate leaves, that are such a feature of this plant, grow out of a stem in a long narrow roll and then unfurl. The leaves are typically solid green but some cultivars have glaucose, brownish, maroon, or even variegated leaves.[4]
The flowers are composed of three sepals and three petals that are seldom noticed by people, they are small and hidden under extravagant stamens. What appear to be petals are the highly modified stamens or staminodes. The staminodes number (1–) 3 (–4) (with at least one staminodal member called the labellum, always being present. A specialized staminode, the stamen, bears pollen from a half-anther. A somewhat narrower, 'petal' is the pistil which is connected down to a three-chambered ovary.[4]
The flowers are typically red, orange, or yellow or any combination of those colours, and are aggregated in inflorescences that are spikes or panicles (thyrses). Although gardeners enjoy these odd flowers, nature really intended them to attract pollinators collecting nectar and pollen, such as bees, hummingbirds and bats. The pollination mechanism is conspicuously specialized. Pollen is shed on the style while still in the bud, and in the species and early hybrids some is also found on the stigma because of the high position of the anther, which means that they are self-pollinating. Later cultivars have a lower anther, and rely on pollinators alighting on the labellum and touching first the terminal stigma, and then the pollen.[4]
The wild species often grow to at least 2–3 m (6.6–9.8 ft) in height but there is a wide variation in size among cultivated plants; numerous cultivars have been selected for smaller stature.[4]
Canna grow from swollen underground stems, correctly known as rhizomes, which store starch, and this is the main attraction of the plant to agriculture, having the largest starch particles of all plant life.[4]
Canna is the only member of the Liliopsida Class (monocot group) in which hibernation of seed is known to occur, due to its hard, impenetrable seed covering.[7][8]
The genus is native to tropical and subtropical regions of the New World, from the southern United States (southern South Carolina west to southern Texas) and south to northern Argentina.[4]
Although all cannas are native to the New World, they have followed mankind's journeys of discovery and some species are cultivated and naturalized in most tropical and sub-tropical regions.[4]
Canna cultivars are grown in most countries, even those with territory above the Arctic Circle, which have short summers but long days, and the rapid growth rate of Cannas makes them a feasible gardening plant, as long as they receive 6–8 hours of sunlight each day during the growing season and are protected from the cold of winter.
The first Cannas introduced to Europe were C. indica L., 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 indicates this origin, and states that it was given the name of 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 Western Indies.[9]
Much later, in 1658, Pison made reference[10] to another species which 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 Canna angustifolia L., (later reclassified as C. glauca L. by taxonomists).[1]
Without exception, all Canna species that have been introduced into Europe can be traced back to the Americas, and it can be asserted with confidence that Canna 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. Canna is an American genus, as pointed out by Lamarck[11] where he argues that "Cannas were unknown to the ancients, and that it is only after the discovery of the New World, that they made their appearance in Europe; Since Canna have very hard and durable seed coverings,[7][8] it is likely that seed remains would have survived in the right conditions and found by archaeologists in the Old World. If the soils of India or Africa had produced some of them, they would have been imported before the 1860s into European gardens.[6]
Although most Cannas these days are cultivars (see below), there are approximately 20 known species 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.[1] 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, however, this only covers a subset of the species range.[18]
See List of Canna species for full species information and descriptions.
See the List of Canna cultivars for photographs of Canna cultivars.
Cannas became very popular in Victorian times as a garden plant and were grown widely in France, Germany, Hungary, India, Italy, the United Kingdom, and the USA.[4][19]
As tender perennials in northern climates, they suffered severe setbacks when two world wars sent the young gardening staff off to war. It took many years for the frugalities of war and its rationing subsequences to change to the more prosperous times of the late 20th century. The Canna genus has recently experienced a renewed interest and revival in popularity.[2]
There were once hundreds of cultivars but many of these are now extinct. In 1910, Árpäd Mühle, from Hungary, published his Canna book, written in higher 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 re-surfacing without viable names, with the increase in popularity from the 1960s onwards. Research has accumulated over 2,800 Canna cultivar names, however, many of these are simply synonyms.[20]
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[21] and C. × orchiodes[22]) 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).[4][23] The definition was based on the genotype, rather than the phenotype, of the two cultivar groups.[19] Inevitably, over time those two floriferous groups were interbred, the distinctions became blurred and overlapped, and the Bailey species names became redundant.[2] 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).[24]
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 seed are also edible, and achira was once a staple foodcrop in Peru and Ecuador. [4] Trials in Ecuador using a wide range of varieties have shown that achira can yield on average 56 tons of rhizomes and 7.8 of extractable starch per hectare. However the crop needs 9-12 months to mature to full productivity [25].
Many more traditional varieties exist worldwide, they have all involved human selection and so are classified as agricultural cultivars. Traditionally, Canna 'edulis' has been reputed to be the variety grown for food in South America, but there is no scientific evidence to substantiate the name. It is probable that edulis is simply a synonym of C. discolor, which is also grown for agricultural purposes throughout Asia.</ref>[26][27][28][29][30]
Cannas grow best in full sun with moderate water in well-drained rich or sandy soil. Cannas grow from perennial rhizomes but are frequently grown as annuals in temperate zones for an exotic or tropical look in the garden.[2] In arid regions, cannas are often grown in the water garden, with the lower inch of pot submerged. In all areas, high winds tear the leaves so shelter is advised.
The rhizomes are frost tender and will rot if left unprotected in freezing conditions. In areas which go below about −10 °C (14 °F) in the winter (< USDA Zone 8), the rhizomes can be dug up before freezing and stored in a protected area (above 7 °C / 45 °F) for replanting in the spring. Otherwise, it is recommended that Cannas are protected by a thick layer of mulch overwinter.[2]
Cannas are largely free of pests but in the USA plants sometimes fall victim to the Canna Leaf Roller and the resultant leaf damage, while not fatal to the plant, can be most distressing to a keen gardener.[2]
Slugs and snails are fond of cannas and can leave large holes in the leaves, preferring the tender young leaves that have not yet unfurled. red spider mite can also be a problem for cannas grown indoors or during a very hot, long summer outdoors.[2] Japanese beetles can also ravage the leaves if left uncontrolled.[31]
Canna are remarkably free of disease, compared to many genera. However, they may fall victim to canna rust, a fungus resulting in orange spots on the plant's leaves, caused by over moist soil. Cannas are also susceptible to certain plant viruses, some of which are Canna specific viruses, 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.[19]
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.
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.[3] 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.[4]
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.[32]
The species are capable of self-pollination, but most cultivars require an outside pollinator. All cannas produce nectar and therefore attract nectar consuming insects, bats and hummingbirds that act as the transfer agent, spreading pollen between stamens and stigmas, on the same or different inflorescence.[4]
Since genetic recombination has occurred a cultivar grown from seed will have different characteristics from its parent(s) and thus should never be given a parent’s name. The wild species have evolved in the absence of other Canna genes and are deemed to be ‘true to type’ when the parents are of the same species. In the latter case there is still a degree of variance, producing various varieties or minor forms (forma). In particular, the species C. indica is an aggregate species, having many different and extreme varieties and forma ranging from the giant to miniature, from large foliage to small foliage, both green and dark foliage and many different coloured blooms, red, orange, pink, and yellow and combinations of those colours.[4]
Outside of a laboratory, the only asexual propagation method that is effective is rhizome division. This is done by using material from a single parent, and as there is no exchange of genetic material such vegetative propagation methods almost always produce plants that are identical to the parent. After a summer’s growth the horticultural Canna 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.[4]
Micropropagation, or tissue culture as it is also known, is the practice of rapidly multiplying stock plant material to produce a large number of progeny plants. Micropropagation using in vitro methods that produce plants by taking small sections of plants and moving them into a sterile environment, where they first produce proliferations that are then separated from each other and then rooted or allowed to grow new stem tissue. The process of plant growth is regulated by different ratios of plant growth regulators or PGRs, that promote cell growth. Many commercial organizations have attempted to produce Canna 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 of being difficult micropropagation specimens.[2]
Micropropagation techniques can be employed on specimens infected with Canna virus and used to dis-infest plants of the virus, it is possible to use a growing shoot tip as the explant, the growing tip is induced into rapid growth, which results in rapid cell division that has not 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, since it has not yet had contact with the sap of the plant which moves the virus within the plant. In this way, healthy stock can be reclaimed from virus contaminated plants.