Acacia decurrens, commonly known as black wattle or early green wattle, is a perennial tree or shrub native to eastern New South Wales, including Sydney, the Greater Blue Mountains Area, the Hunter Region, and southwest to the Australian Capital Territory.

Cultivated throughout Australia and in many other countries, Acacia decurrens has naturalised in most Australian states and in Africa, the Americas, Europe, New Zealand and the Pacific, the Indian Ocean area, and Japan.

The bark is brown to dark grey colour and smooth to deeply fissured longitudinally with conspicuous intermodal flange marks.

They are straight, parallel-sided, with a pointed tip, tapering base, shiny and hairless or rarely sparsely hairy.

German botanist Johann Christoph Wendland first described this species as Mimosa decurrens in 1798,[3] before his countryman Carl Ludwig Willdenow redescribed it in the genus Acacia in 1919.

However, as Donn's description was a nomen nudum, the proper citation is Acacia decurrens Willd.

[7][8] Maiden noted that it was called Wat-tah by the indigenous people of Cumberland (Parramatta) and Camden districts.

[citation needed] Along with other bipinnate wattles, it is classified in the section Botrycephalae within the subgenus Phyllodineae in the genus Acacia.

An analysis of genomic and chloroplast DNA along with morphological characters found that the section is polyphyletic, though the close relationships of A. decurrens and many other species were unable to be resolved.

It prefers high rainfall areas with 600–1,400 mm (24–55 in) per year, and is otherwise tolerant of a wide range of conditions.

In woodlands and dry sclerophyll forests in New South Wales, it grows with trees such as grey gum (Eucalyptus punctata) and narrow-leaved ironbark (E.

[14] The wood serves as food for larvae of the jewel beetle species Agrilus australasiae, Cisseis cupripennis and C.

[8] The flowering of A. decurrens was used as a seasonal indicator of the ceasing of cold winds and the beginning of a period of gentle rain.

[11] Cultivation of A. decurrens can be started by soaking the seeds in warm water and sowing them outdoors.

[20] Fieldwork conducted in the Southern Highlands found that the presence of bipinnate wattles (either as understory or tree) was related to reduced numbers of noisy miners, an aggressive species of bird that drives off small birds from gardens and bushland, and hence recommended the use of these plants in establishing green corridors and revegetation projects.