Golden wattle

Appearance

"Acacia pycnantha" generally grows as a small tree to between 3 and 8 m in height, though trees of up to 12 m high have been reported in Morocco. The bark is generally dark brown to grey—smooth in younger plants though it can be furrowed and rough in older plants. Branchlets may be bare and smooth or covered with a white bloom. The mature trees do not have true leaves but have phyllodes—flat and widened leaf stems—that hang down from the branches. Shiny and dark green, these are between 9 and 15 cm long, 1–3.5 cm wide and falcate to oblanceolate in shape. New growth has a bronze coloration. Field observations at Hale Conservation Park show the bulk of new growth to take place over spring and summer from October to January.

Floral buds are produced year-round on the tips of new growth, but only those initiated between November and May go on to flower several months later. Flowering usually takes place from July to November in the golden wattle's native range; because the later buds develop faster, flowering peaks over July and August. The bright yellow inflorescences occur in groups of 40 to 80 on 2.5–9 cm -long racemes that arise from axillary buds. Each inflorescence is a ball-like structure that is covered by 40 to 100 small flowers that have five tiny petals and long erect stamens, which give the flower head a fluffy appearance.

Developing after flowering has finished, the seed pods are flattish, straight or slightly curved, 5–14 cm long and 5–8 mm wide. They are initially bright green, maturing to dark brown and have slight constrictions between the seeds, which are arranged in a line in the pod. The oblong seeds themselves are 5.5 to 6 mm long, black and shiny, with a clavate aril. They are released in December and January, when the pods are fully ripe.

Species similar in appearance include mountain hickory wattle, coast golden wattle and golden wreath wattle. "Acacia obliquinervia" has grey-green phyllodes, fewer flowers in its flower heads, and broader -wide) seed pods. "A. leiophylla" has paler phyllodes. "A. saligna" has longer, narrower phyllodes.

Distribution

Golden wattle occurs in south-eastern Australia from South Australia's southern Eyre Peninsula and Flinders Ranges across Victoria and northwards into inland areas of southern New South Wales and the Australian Capital Territory. It is found in the understorey of open eucalypt forests on dry, shallow soils.

The species has become naturalised beyond its original range in Australia. In New South Wales it is especially prevalent around Sydney and the Central Coast region. In Tasmania it has spread in the east of the state and become weedy in bushland near Hobart. In Western Australia, it is found in the Darling Range and western wheatbelt as well as Esperance and Kalgoorlie.

Outside Australia it has become naturalised in South Africa, Tanzania, Italy, Portugal, Sardinia, India, Indonesia and New Zealand. It is present in California as a garden escapee, but is not considered to be naturalised there. In South Africa, where it had been introduced between 1858 and 1865 for dune stabilization and tannin production, it had spread along waterways into forest, mountain and lowland fynbos, and borderline areas between fynbos and karoo. The gall-forming wasp "Trichilogaster signiventris" has been introduced in South Africa for biological control and has reduced the capacity of trees to reproduce throughout their range. The eggs are laid by adult wasps into buds of flower heads in the summer, before hatching in May and June when the larvae induce the formation of the grape-like galls and prevent flower development. The galls can be so heavy that branches break under their weight. In addition, the introduction in 2001 of the acacia seed weevil "Melanterius compactus" has also proved effective.