Golden Wattle

Acacia pycnantha generally grows as a small tree to between 3 and 8 metres (10 and 30 feet) in height, though trees of up to 12 m (40 ft) 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 (3.5 and 5.9 in) long, 1–3.5 cm (0.4–1 in) wide and falcate (sickle-shaped) to oblanceolate in shape. New growth has a bronze colouration. Field observations at Hale Conservation Park show the bulk of new growth occurs 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 (late winter to early summer) in the golden wattle's native range, and 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 (1–4 in)-long racemes that arise from axillary buds. …

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 where it is considered an invasive alien plant and is uprooted to prevent water depletion and protect local flora, 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. …

Though plants are usually killed by a severe fire, mature specimens are able to resprout. Seeds are able to persist in the soil for more than five years, germinating after fire. Like other wattles, Acacia pycnantha fixes nitrogen from the atmosphere. It hosts bacteria known as rhizobia that form root nodules, where they make nitrogen available in organic form and thus help the plant grow in poor soils. A field study across Australia and South Africa found that the microbes are genetically diverse, belonging to various strains of the species Bradyrhizobium japonicum and genus Burkholderia in both countries. It is unclear whether the golden wattle was accompanied by the bacteria to the African continent or encountered new populations there. Self-incompatible, A. pycnantha cannot fertilise itself and requires cross-pollination between plants to set seed. Birds facilitate this and field experiments keeping birds away from flowers greatly reduces seed production. Nectaries are located on phyllodes; those near open flowers become active, producing nectar that birds feed upon just before or during flowering. While feeding, birds brush against the flower heads and dislodge pollen and often visit multiple trees. …

Golden wattle has been grown in temperate regions around the world for the tannin in its bark, as it provides the highest yield of all wattles. Trees can be harvested for tannin from seven to ten years of age. Commercial use of its timber is limited by the small size of trees, but it has high value as a fuel wood. The scented flowers have been used for perfume making, and honey production in humid areas. However, the pollen is too dry to be collected by bees in dry climates. In southern Europe, it is one of several species grown for the cut-flower trade and sold as "mimosa". Like many other species of wattle, Acacia pycnantha exudes gum when stressed. Eaten by indigenous Australians, the gum has been investigated as a possible alternative to gum arabic, commonly used in the food industry.

Golden wattle is cultivated in Australia and was introduced to the northern hemisphere in the mid-1800s. Although it has a relatively short lifespan of 15 to 30 years, it is widely grown for its bright yellow, fragrant flowers. As well as being an ornamental plant, it has been used as a windbreak or in controlling erosion. Trees are sometimes planted with the taller sugar gum (Eucalyptus cladocalyx) to make a two-layered windbreak. One form widely cultivated was originally collected on Mount Arapiles in western Victoria. It is floriferous, with fragrant flowers appearing from April to July. The species has a degree of frost tolerance and is adaptable to a wide range of soil conditions, but it prefers good drainage. It tolerates heavy soils in dry climates, as well as mild soil salinity. It can suffer yellowing (chlorosis) in limestone-based (alkaline) soils. Highly drought-tolerant, it needs 370–550 mm (10–20 in) winter rainfall for cultivation. It is vulnerable to gall attack in cultivation. Propagation is from seed which has been pre-soaked in hot water to soften the hard seed coating.

Wattles, and in particular the golden wattle, have been an informal floral emblem of Australia for many years (for instance, it represented Australia on the Coronation gown of Queen Elizabeth II in 1953). While some advocates forcefully argued for the adoption of the waratah, during Australia's bicentenary in 1988 the golden wattle was formally adopted as the floral emblem of Australia. This was proclaimed by governor-general Sir Ninian Stephen (on the advice of the Hawke government) in the Commonwealth gazette published on 1 September. The day was marked by a ceremony at the Australian National Botanic Gardens, which included the planting of a golden wattle by Hazel Hawke, the prime minister's wife. In 1992, 1 September was formally declared "National Wattle Day". The Australian Coat of Arms includes a wreath of wattle; this does not, however, accurately represent a golden wattle. Similarly, the green and gold colours used by Australian international sporting teams were inspired by the colours of wattles in general, rather than the golden wattle specifically. The species was depicted on a stamp captioned "wattle" as part of a 1959–60 Australian stamp set featuring Australian native flowers. In 1970, a 5c stamp labelled "Golden Wattle" was issued to complement an earlier set depicting the floral emblems of Australia. …