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 nectariferous 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-
The character most commonly associated with Banksia is the flower spike, an elongated inflorescence consisting of a woody axis covered in tightly-
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-
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-
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-
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-
Most occur in heathlands or low woodlands; of the eastern species, B. integrifolia and B. marginata occur in forests; many south-
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 -
Studies of the south-