Acacia polystachya

polystachya wattle


Acacia polystachya

Benth. 1842

pronounced: uh-KAY-shuh poh-lee-STAK-yuh

(Mimosaceae — the wattle family)

synonym — Racosperma polystachyum

(Benth.) Pedley 1984

pronounced: rak-ow-SPER-muh poh-lee-STAK-yum

common names: polystachya wattle

native 4Acacia s from the Greek ακις (akis), a thorn or spike, while polystachya is from πολυς (polys), much, many, great, and σταχυς (stachys), a spike, ear of corn. In the synonym, Racosperma is from 'ρακος (rhakos), rough, ragged, and σπερμα (sperma), a seed.

This wattle, growing by the side of a road in Nelly Bay, has been planted as a street tree. Betsy Jackes records the species as found between Picnic and Cockle Bays. It has also been recorded on Palm Island. Its natural habitat is usually described as being from Banks Island in the Torres Strait along the Queensland east coast to just south of Cairns. It is recognizable by its wonderfully curly seed pods. Some confusion may arise, when the pods are not on the tree, about the width of its leaves: it would appear that this species hybridizes with Acacia mangium , which has much broader leaves. In its natural state, Acacia polystachya occurs in alluvial soils, often near watercourses in semi-deciduous mesophyll vine thicket and deciduous vine thicket, or along beaches. It flowers between April and July.

Depending on the growing conditions, this tree will be anything from 3 to 25 m tall. The bark is smooth to corrugated or coarsely plaited, grey or dark brown. The branchlets are angular or flattened, grey, brown or purplish, smooth or pimply. Leaf-bearing twigs are more-or-less triangular in cross-section.

The phyllodes are very narrowly elliptic to narrowly elliptic, sometime asymmetrical, straight to falcate, 7.5 – 25 cm long, 1 – 3.2 cm wide, thin and papery, glabrous, with 2 or 3 (rarely 4) prominent nerves that run together and often join some distance above the phyllode base and remain separate from the lower margin.

The spikes are about 5 – 8 cm long, loosely arranged, pale yellow (almost white), sparse, on peduncles about 5 mm long. The calyx is rather membranous, broad, just under 1 cm long, glabrous, the lobes fringed, about 2.5 mm long. The corolla is deeply lobed, glabrous, about 1.5 – 2 mm long, a little more than twice as long as the calyx; the stamens are about 3 – 4 mm long, and the ovary densely pubescent.

The fruit is a more-or-less flat pod, usually twisted, curved or coiled, up to about 10 cm long by 6 – 8 mm wide. The seeds are about 4 mm long, disk-shaped, black, minutely pitted, and longitudinally orientated in the pod; the stalk is red or reddish, encircling the seed and thickened to form an aril-like structure beneath the seed.

This is a host plant for the caterpillars of Prosotas dubiosa (the Small Purple Line Blue butterfly).

Photographs taken in Nelly Bay, 2010
Page last updated 24th September 2018