The Australian tree that became a global wanderer and a South African weed

Gardening · Published 3 October 2026 Swipe more like this →

In short: Acacia decurrens, a tree native to New South Wales, has traveled far beyond its subtropical home. While valued for its edible flowers, medicinal properties, and tannin-rich bark, this fast-growing species has earned the nickname green cancer in South Africa, where it has naturalized and become an environmental nuisance.

Known by many names, including Sydney green wattle and early black wattle, Acacia decurrens is a resilient tree that thrives in forest, woodland, and heath environments. It is characterized by its dark green, bipinnate leaves and striking golden-yellow flowers. The specific epithet decurrens, meaning running down, refers to the distinctive narrow ridges on its leaves that continue along the branchlets. These features help it adapt to various conditions, allowing it to reach heights of up to 25 meters.

The tree's utility has driven its global spread. Beyond its native Australian range, it has been introduced to countries across Africa, Asia, and the Americas, including India, China, Colombia, and the United States. It serves multiple human needs, from providing fuel and food—such as edible flowers used in fritters and a gum substitute for jelly—to environmental management and chemical production. Its bark is particularly notable for containing 37 to 40 percent tannin, a substance historically valued for industrial applications.

Despite its benefits, the species is an aggressive colonizer. It reproduces primarily through seeds, which can persist in the soil and germinate rapidly following bushfires or in disturbed landscapes. In South Africa, this adaptability has turned it into a significant weed. While it supports local wildlife in Australia—providing food for various butterfly caterpillars and jewel beetle larvae—its invasive potential in other regions highlights the complex ecological impact of introducing non-native flora into new biomes.

Taxonomically, the tree has a storied history. First described by Johann Christoph Wendland in 1798 as Mimosa decurrens, it was later reclassified by Carl Ludwig Willdenow. Even in recent decades, botanists have debated its placement, with some proposing the genus Racosperma. Today, it remains a subject of study for its polyphyletic section, Botrycephalae, as researchers continue to map its genetic relationships and ecological footprint across the globe.

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