Vachellia farnesiana, also known as Acacia farnesiana, and previously Mimosa farnesiana, commonly known as sweet acacia, huisache or needle bush, is a species of shrub or small tree in the legume family, Fabaceae. It is deciduous over part of its range, but evergreen in most locales. The species grows to a height of 15–30 feet (4.6–9.1 m) and grows multiple trunks. The base of each leaf is accompanied by a pair of thorns on the branch.
It was first described by Europeans under the name Acacia Indica Farnesiana in 1625 by Tobias Aldini from plants grown in Rome in the Farnese Gardens from seed collected in Santo Domingo, in what is now the Dominican Republic, which germinated in 1611. Tobias Aldini included an illustration of the plant, which he contrasted with an illustration of the first known Acacia; Acacia nilotica. This first (European) illustration of the plant was later designated as the (lecto-)type. In 1753 Linnaeus used Aldini's work as basis for his taxon Mimosa farnesiana. In 1806 Carl Ludwig Willdenow moved this taxon to the genus Acacia.
In 1936 Cory moved Vachellia densiflora to Acacia densiflora, but as this name had already been used for another taxon and was therefore invalidated sensu Cory, in 1969 this taxon was renamed A. smallii by Isely. In 1948 F.J.Herm. synonymised Vachellia peninsularis and V. insularis under Acacia pinetorum.
Acacia smallii was used in the United States for the 'native' A. farnesiana growing in the drylands west of Louisiana, but at the same time the taxon A. farnesiana was recognised in the United States for purportedly imported non-native plants originally cultivated in the southeastern United States as ornamentals and later thought naturalised there. Additionally, in Florida, A. pinetorum was recognised as a rare endemic native.
Acacia farnesiana var. guanacastensis is primarily distinguished by larger leaflets. It remains controversial; most taxonomic authorities in Mexico and Central America recognise this taxon as a full species under either A. guanacastensis or Vachellia guanacastensis, the Arboles nativos e introduciados de El Salvador of 2009 subsumed it under Acacia farnesiana. It is recognised as present in Costa Rica, El Salvador, Guatemala, Nicaragua and the southern Gulf and southwestern regions of Mexico by the Kew World Checklist of Selected Plant Families (2018) but it is unclear if the taxon as recognised by the different authorities replaces Acacia farnesiana in Central America (but not the Caribbean or South America) or exists sympatrically. This has implications for the classification of the extra-American distribution of A. farnesiana as the populations growing in Australia and the Philippines have recently (2017) been shown to derive from ancient Central American origins.
It is considered a serious pest plant in parts of Australia, as it interferes with cattle ranching operations. It readily spreads in commercial grazing pastures, especially along creeks, which might affect ease of transport for farmers, complicates muster, and can damage farm machinery. The seeds are dispersed by cattle after they eat the nutritious pods, and growth is promoted by overgrazing. Numerous herbicides are used to control it on ranches, chemical control is the only way to kill it. The plant has been spread to many new locations as a result of human activity and it is considered a serious weed in Fiji, where locals call it Ellington's curse.
The flowers are processed through distillation to produce a perfume called Cassie which has been described as "delicious". It is widely used in the perfume industry in Europe. Flowers of the plant provide the perfume essence from which the biologically important sesquiterpenoid farnesol is named.