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|About 170 species|
|Distribution of Banksia within Australia|
|About 170 species|
|Distribution of Banksia within Australia|
Banksia is a genus of around 170 species in the plant family Proteaceae. These Australian wildflowers and popular garden plants are easily recognised by their characteristic flower spikes and fruiting "cones" and heads. When it comes to size, banksias range from prostrate woody shrubs to trees up to 30 metres tall. They are generally found in a wide variety of landscapes; sclerophyll forest, (occasionally) rainforest, shrubland, and some more arid landscapes, though not in Australia's deserts.
Heavy producers of nectar, banksias form a vital part of the food chain in the Australian bush. They are an important food source for all sorts of nectarivorous animals, including birds, bats, rats, possums, stingless bees and a host of invertebrates. Furthermore, they are of economic importance to Australia's nursery and cut flower industries. However these plants are threatened by a number of processes including land clearing, frequent burning and disease, and a number of species are rare and endangered.
Banksias grow as trees or woody shrubs. Trees of the largest species, B. integrifolia (Coast Banksia) and B. seminuda (River Banksia), often grow over 15 metres tall, some even grow to standing 30 metres tall. Banksia species that grow as shrubs are usually erect, but there are several species that are prostrate, with branches that grow on or below the soil.
The leaves of Banksia vary greatly between species. Sizes vary from the narrow, 1–1½ centimetre long leaves of B. ericifolia (Heath-leaved Banksia), to the very large leaves of B. grandis (Bull Banksia), which may be up to 45 centimetres long. The leaves of most species have serrated edges, but a few, such as B. integrifolia, do not. Leaves are usually arranged along the branches in irregular spirals, but in some species they are crowded together in whorls. Many species have differing juvenile and adult leaves (e.g., Banksia integrifolia has large serrated juvenile leaves).
The character most commonly associated with Banksia is the flower spike, an elongated inflorescence consisting of a woody axis covered in tightly-packed pairs of flowers attached at right angles. A single flower spike generally contains hundreds or even thousands of flowers; the most recorded is around 6000 on inflorescences of B. grandis. Not all Banksia have an elongate flower spike, however: the members of the small Isostylis complex have long been recognised as Banksias in which the flower spike has been reduced to a head; and recently the large genus Dryandra has been found to have arisen from within the ranks of Banksia, and sunk into it as B. ser. Dryandra. Thus fewer than half of the currently accepted Banksia taxa possess the elongated flower spike long considered characteristic of the genus.
Banksia flowers are usually a shade of yellow, but orange, red, pink and even violet flowers also occur. The colour of the flowers is determined by the colour of the perianth parts and often the style. The style is much longer than the perianth, and is initially trapped by the upper perianth parts. These are gradually released over a period of days, either from top to bottom or from bottom to top. When the styles and perianth parts are different colours, the visual effect is of a colour change sweeping along the spike. This can be most spectacular in B. prionotes (Acorn Banksia) and related species, as the white inflorescence in bud becomes a brilliant orange. In most cases, the individual flowers are tall, thin saccate (sack-shaped) in shape.
As the flower spikes or heads age, the flower parts dry up and may turn shades of orange, tan or dark brown colour, before fading to grey over a period of years. In some species, old flower parts are lost, revealing the axis; in others, the old flower parts may persist for many years, giving the fruiting structure a hairy appearance. Old flower spikes are commonly referred to as "cones", although they are not: cones only occur in conifers and cycads.
Despite the large number of flowers per inflorescence, only a few of them ever develop fruit, and in some species a flower spike will set no fruit at all. The fruit of Banksia is a woody follicle embedded in the axis of the inflorescence. These consist of two horizontal valves that tightly enclose the seeds. The follicle opens to release the seed by splitting along the suture, and in some species each valve splits too. In some species the follicles open as soon as the seed is mature, but in most species most follicles open only after stimulated to do so by bushfire. Each follicle usually contains one or two small seeds, each with a wedge-shaped papery wing that causes it to spin as it falls to the ground.
The Banksia genus was first described and named by Carolus Linnaeus the Younger in his April 1782 publication Supplementum Plantarum; hence the full name for the genus is "Banksia L.f." The genus name honours the English botanist Sir Joseph Banks, who collected the first Banksia specimens in 1770, during James Cook's first expedition.
All but one of the living Banksia species are endemic to Australia. The exception is B. dentata (Tropical Banksia), which occurs throughout northern Australia, and on islands to the north including New Guinea and the Aru Islands. An extinct species, B. novae-zelandiae, was found in New Zealand. The other species occur in two distinct geographical regions: southwest Western Australia and eastern Australia. Southwest Western Australia is the main centre of biodiversity; over 90% of all Banksia species occur only there, from Exmouth in the north, south and east to beyond Esperance on the south coast. Eastern Australia has far fewer species, but these include some of best known and most widely distributed species, including B. integrifolia (Coast Banksia) and B. spinulosa (Hairpin Banksia). Here they occur from the Eyre Peninsula in South Australia right around the east coast up to Cape York in Queensland.
The vast majority of Banksia are found in sandy or gravelly soils, though some populations of B. marginata (Silver Banksia) and B. spinulosa do occur on heavier, more clay-like, soils. B. seminuda is exceptional for its preference for rich loams along watercourses.
Most occur in heathlands or low woodlands; of the eastern species, B. integrifolia and B. marginata occur in forests; many south-western species such as B. grandis, B. sphaerocarpa, B. sessilis, B. nobilis and B. dallanneyi grow as understorey plants in Jarrah (Eucalyptus marginata), Wandoo (E. wandoo) and Karri (E. diversicolor) forests, with B. seminuda being one of the forest trees in suitable habitat.
Most species do not grow well near the coast, notable exceptions being the southern Western Australian species B. speciosa, B. praemorsa and B. repens. Only a few species, such as B. rosserae and B. elderiana (Swordfish Banksia), occur in arid areas. Most of the eastern Australian species survive in uplands, but only a few of the Western Australian species native to the Stirling Ranges - B. solandri, B. oreophila, B. brownii and B. montana - survive at high altitudes.
Studies of the south-western species have found the distribution of Banksia species to be primarily constrained by rainfall. With the exception of B. rosserae, no species tolerates annual rainfall of less than 200 millimetres, despite many species surviving in areas that receive less than 400 millimetres. Banksia species are present throughout the region of suitable rainfall, with greatest speciation in cooler, wetter areas. Hotter, drier regions around the edges of the genus' range tend to have fewer species with larger distributions. The greatest species richness occurs in association with uplands, especially the Stirling Range.
Banksias are heavy producers of nectar, making them an important source of food for nectivorous animals, including honeyeaters and small mammals such as rodents, antechinus, honey possums, pygmy possums, gliders and bats. Many of these animals play a role in pollination of Banksia. Various studies have shown mammals and birds to be important pollinators. An interesting observation by Carpenter in 1978 was that some banksias had a stronger odour at night possibly to attract mammal pollinators. Other associated fauna include the larvae of moths (such as the Dryandra Moth) and weevils, which burrow into the "cones" to eat the seeds and pupate in the follicles; and birds such as cockatoos, who break off the "cones" to eat both the seeds and the insect larvae.
A number of Banksia species are considered rare or endangered. These include B. brownii (Feather-leaved Banksia), B. cuneata (Matchstick Banksia), B. goodii (Good's Banksia), B. oligantha (Wagin Banksia), B. tricuspis (Pine Banksia), and B. verticillata (Granite Banksia).
Banksia plants are naturally adapted to the presence of regular bushfires in the Australian landscape. About half of Banksia species are killed by bushfire, but these regenerate quickly from seed, as fire also stimulates the opening of seed-bearing follicles and the germination of seed in the ground. The remaining species usually survive bushfire, either because they have very thick bark that protects the trunk from fire, or because they have lignotubers from which they can resprout after fire. In Western Australia, banksias of the first group are known as 'seeders' and the second group as 'sprouters'.
Infrequent bushfires at expected intervals pose no threat, and are in fact beneficial for regeneration of banksia populations. However, too frequent bushfires can seriously reduce or even eliminate populations from certain areas, by killing seedlings and young plants before they reach fruiting age. Many fires near urban areas are caused by arson, and thus the frequency is often much higher than fires would have been prior to human habitation. Furthermore, residents who live in areas near bushland may pressure local councils to burn areas near homes more frequently, to reduce fuel-load in the bush and thus reduce ferocity of future fires. Unfortunately there are often discrepancies in agreed frequency between these groups and conservation groups.
Another threat to Banksia is the water mould Phytophthora cinnamomi, commonly known as "dieback". Dieback attacks the roots of plants, destroying the structure of the root tissues, "rotting" the root, and preventing the plant from absorbing water and nutrients. Banksia's proteoid roots, which help it to survive in low-nutrient soils, make it highly susceptible to this disease. All Western Australian species are vulnerable, although most eastern species are fairly resistant.
Vulnerable plants typically die within a few years of infection. In southwest Western Australia, where dieback infestation is widespread, infested areas of Banksia forest typically have less than 30% of the cover of uninfested areas. Plant deaths in such large proportions can have a profound influence on the makeup of plant communities. For example, in southwestern Australia Banksia often occurs as an understorey to forests of Jarrah (Eucalyptus marginata), another species highly vulnerable to dieback. Infestation kills both the Jarrah overstorey and the original Banksia understorey, and over time these may be replaced by a more open woodland consisting of an overstorey of the resistant Marri (Corymbia calophylla), and an understorey of the somewhat resistant Banksia sessilis (Parrotbush).
A number of species of Banksia are threatened by dieback. Nearly every known wild population of B. brownii shows some signs of dieback infection, which could possibly wipe it out within years. Other vulnerable species include B. cuneata, and B. verticillata.
Dieback is notoriously difficult to treat, although there has been some success with phosphite and phosphorous acid, which are currently used to inoculate wild B. brownii populations. However this is not without potential problems as it alters the soil composition by adding phosphorus. Some evidence suggests that phosphorous acid may inhibit proteoid root formation.
Because dieback thrives in moist soil conditions, it can be a severe problem for Banksias that are watered, such as in the cut flower industry and urban gardens.
Most of species are shrubs, only few of them can be found as trees and they are very popular because of their size, the tallest species are: B. integrifolia having its subspecies B. integrifolia subsp. monticola notable for reaching the biggest banksias and it is the most frost tolerant in this genus, B. seminuda, B. littoralis, B. serrata; species that can grow as small trees or big shrubs: B. grandis, B. prionotes, B. marginata, B. coccinea, B. speciosa and B. menziesii. Due to their size these species are popularly planted in parks, gardens and streets, the remaining species in this genus are only shrubs.
Banksias are popular garden plants in Australia because of their large, showy flower heads, and because the large amounts of nectar they produce attracts birds and small mammals. Popular garden species include B. spinulosa, B. ericifolia, B. aemula (Wallum Banksia ), B. serrata (Saw Banksia), Banksia media (Southern Plains Banksia) and the cultivar Banksia 'Giant Candles'. Banksia species are primarily propagated by seed in the home garden as cuttings can be difficult to strike. However commercial nurserymen extensively utilize the latter method (indeed, cultivars by nature must be vegetatively propagated by cuttings or grafting).
Over time, dwarf cultivars and prostrate species are becoming more popular as urban gardens grow ever smaller. These include miniature forms under 50 cm high of B. spinulosa and B. media, as well as prostrate species such as B. petiolaris and B. blechnifolia .
Banksias possibly require more TLC (i.e. maintenance) than other Australian natives, though are fairly hardy if the right conditions are provided (sunny aspect and well drained sandy soil). They may need extra water during dry spells until established, which can take up to two years. If fertilised, only slow-release, low-phosphorus fertilizer should be used, as the proteoid roots may be damaged by high nutrient levels in the soil. All respond well to some form of pruning.
Within the Australian horticultural community there is an active subculture of Banksia enthusiasts who seek out interesting flower variants, breed and propagate cultivars, exchange materials and undertake research into cultivation problems and challenges. The main forum for exchange of information within this group is ASGAP's Banksia Study Group.
With the exception of the nursery industry, Banksia have limited commercial use. Some species, principally B. coccinea (Scarlet Banksia), B. baxteri, B. hookeriana (Hooker's Banksia), B. sceptrum) (Sceptre Banksia), B. speciosa (Showy Banksia) and B. menziesii (Menzies' Banksia) are grown on farms in Western and Southern Australia, as well as Israel and Hawaii, and the flower heads harvested for the cut flower trade. Their nectar is also sought by beekeepers, not for the quality of the honey, which is often poor, but because the trees provide an abundant and reliable source of nectar at times when other sources provide little.
Banksia wood is reddish in color with an attractive grain but it is rarely used as it warps badly on drying. It is occasionally used for ornamental purposes in wood turning and cabinet paneling. It has also been used to make keels for small boats. Historically, the wood of certain species such as B. serrata was used for yokes and boat parts. The large "cones" or seed pods of B. grandis are used for woodturning projects. They are also sliced up and sold as drink coasters; these are generally marketed as souvenirs for international tourists. Woodturners throughout the world value Banksia pods for making ornamental objects.
The Indigenous people of south-western Australia would suck on the flower spikes to obtain the nectar, they also soaked the flower spikes in water to make a sweet drink. Banksia trees are a reliable source of insect larvae which are extracted as food.
A number of field guides and other semi-technical books on the genus have been published. These include:
Perhaps the best known cultural reference to Banksia is the "big bad Banksia men" of May Gibbs' children's book Snugglepot and Cuddlepie. Gibb's "Banksia men" are modelled on the appearance of aged Banksia "cones", with follicles for eyes and other facial features. There is some contention over which species actually provided the inspiration for the "Banksia men": the drawings most resemble the old cones of B. aemula, but B. attenuata (Slender Banksia) has also been cited, as this was the species that Gibbs saw as a child in Western Australia.
In 1989, the Banksia Environmental Foundation was created to support and recognise people and organizations that make a positive contribution to the environment. The Foundation launched the annual Banksia Environmental Awards in the same year.
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