Banksia plagiocarpa

Hinchinbrook Banksia, Blue Banksia, Dallachy’s Banksia

Family: Proteaceae

A shrub, usually to about 5 m tall. It is restricted to Hinchinbrook Island in northern Queensland and the adjoining mainland close to the coast (between Townsville and Cairns). It was re-discovered after a long interval in the 1980s. On Hinchinbrook Island, it grows on the slopes of mountains in open woodland or grassland between 200 and 800 metres on rock or clay soils. However, on the mainland, it grows on lower altitudes on sand.

The leaves are alternate to whorled, somewhat stiff and leathery, elliptic to obovate, to 20 cm long and 1.5 cm wide with lobed to toothed to entire margins which are recurved. Dark green to blue-green on upper surface and white on under surface. New growth is covered in dense hairs and is red.

The inflorescences of banksias are generally referred to as spikes or a spike-like structure which consists of fused racemes of paired flowers (hence it gets referred to as a conflorescence). There can be 100s of flowers in each spike. The flowers of banksias have 4 tepals (petals or sepals) in a tubular formation, 4 anthers hidden inside and an elongated carpel (female part) where the style and extends outwards forming the edge of the spike (somewhat similar to individual grevillea and hakea flowers).

In this species, the spikes are generally to 15 cm long and up to 6 cm wide, with each flower thin but up to 3 cm long, bluish-grey in bud, opening to pale yellow.

The spike then turns into a cone-like structure of follicles; a fruit which splits open on one side. Each follicle has one or two winged seeds which is actually a fruit in itself called a samara. The follicles can take a long time to mature and open, usually needing a fire or a drying out period. Each “cone” can produce a fair amount of seed with this species having follicles to 15 mm long with up to 60 follicles per cone.

The follicles have a strong wedge-shape.

In the garden

Not as popular as other banksias, but known to be cultivated and is reported to be hardy (see references). It provides a nice architectural form with stiff leathery leaves. The inflorescence colour is unusual, being described as blue-grey to green-purple in bud and then pale yellow when flowers begin to open.

Plant in a sunny spot / dappled sun and allow for drainage. Many banksias benefit from growing on a slight slope. Do not mulch too heavily and try to keep grass away from the trunk. Very hardy once established. It is excellent for attracting honeyeater and other birds. It is tolerant to high humidity. Not overly frost tolerant.

Will grow consistently if happy and will form a very nice shrub, to 3 m tall, but can be kept much more compact if pruned. May get sparse and leggy if not pruned. Very suitable for sandy gardens. Suitable for small gardens.

Do not apply a high phosphorus fertiliser as Proteaceae are generally sensitive.

Prune after flowering or harvest cut flowers. Can be shaped into a denser bush if pruned early. Susceptible to Phytophthora and any other root rotting fungus.

Propagation

Propagation from seed is very reliable and not too difficult.

Other information

This species can regenerate after fire from the seed bank.

Banksia – named in honour of Joseph Banks (1743 – 1820), famous naturalist and botanist on the Endeavour and other expeditions, and President of the Royal Society for over 40 years. The genus was named in his honour by Linnaeus filius (Carl von Lynne – the Younger, son of the famous Carl von Lynne (Linneaus)).

plagiocarpa – Greek – plagio (πλάγιο) meaning “oblique” and -carpa (καρπός / καρπa) meaning “fruit” referring to the up-turned oblique or wedge-shaped follicles of the species.

Not considered to be at risk in the wild. However, is considered rare and confined.

https://www.gardeningwithangus.com.au/banksia-plagiocarpa-hinchinbrook-banksia/
http://anpsa.org.au/b-pla.html
https://en.wikipedia.org/wiki/Banksia_plagiocarpa

By Dan Clarke