Mimosa in Australia

In Australia, Mimosa pigra has been declared a noxious weed or given similar status under various weed or quarantine Acts. It has been ranked as the tenth most problematic weed and is listed on the Weeds of National Significance. It is currently restricted to the Northern Territory where it infests approximately 80,000 hectares of coastal floodplain.

Mimosa pigra invades sedgeland and grassland communities on open floodplains, particularly in areas where feral buffalo have removed the vegetation. It forms dense, practically monospecific tall shrubland in which the ground flora is sparse to non-existent. Similarly, it invades the paperbark (Melaleuca spp.) swamp forests fringing the floodplains, where it forms a dense understorey, and shades out native tree seedlings.

The common name is mimosa or giant sensitive plant. Other common names include: bashful plant, catclaw mimosa, black mimosa.Mimosa pigra can also be confused with Leucaena leucocephala (coffee bush), Aeschynomeme species and Sesbania species, but can be distinguished from this plants by its sensitive leaves, prickles and mauve flowers.

Mimosa pigra was probably introduced in Australia at the Darwin Botanic Gardens in the 20 years prior to 1891, either accidentally in seed samples or as a curiosity, because of its sensitive leaves. Its spread around Darwin over the next 60 years was not particularly conspicuous, until a large infestation was discovered in 1952 at Adelaide River, 100 km south of Darwin. By 1968, it had spread downstream on the Adelaide River to the Marrakai Crossing, and by 1975 had reached the Arnhem Highway bridge. The plant was then well placed to take over the vast floodplains of the Adelaide River. Being low in palatability, it was assisted in establishment by high densities of feral water buffalo, which were heavily overgrazing the floodplains. Moreover, its seed can float, which aided its rapid spread. The population increased dramatically. During the early to mid 1980s, other rivers were colonized, but the rate of establishment has slowed since the late 1980s.

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