There are about 60 species of Allocasuarina, all of which are endemic to Australia. They are typical of nutrient-impoverished soils and are most abundant in southern Australia. Allocasuarina species are shrubs or trees with distinctive articulated stems and reduced tooth-like leaves. The leaves (teeth) occur in whorls and the portion of branchlet between each whorl is termed an ‘article’. The articles have several longitudinal ridges (‘phyllichnia’) which are separated by deep furrows. The number of ridges matches the number of teeth. Branchlets may be persistent or deciduous. The leaves are in whorls of 4–14. Allocasuarina species are monoecious or dioecious, with separate staminate and pistillate inflorescences. Inflorescences are composed of alternating whorls of tooth-like bracts, each of which bears a single flower with two bracteoles. The staminate inflorescences resemble catkins, with the flowers arranged along the branchlets, while the pistillate inflorescences are globose, with the flowers clustered at the tips of lateral branches. The pistillate inflorescences may also be sessile. The individual flowers are insignificant. Fruits are borne inside the pistillate inflorescence, which becomes woody at maturity and is often described as a ‘cone’. The floral bracteoles extend only slightly from the cone and typically have a divided or spiny dorsal protuberance (absent in Casuarina, in which the bracteoles extend well beyond the cone). The fruit is a shiny, reddish brown or black samara (Wilson & Johnson 1989, 1990). Allocasuarina, together with Gymnostoma L.A.S. Johnson and Ceuthostoma L.A.S. Johnson, was split from the genus Casuarina (Johnson 1980, 1982, 1988).
As with the more familiar Casuarina, the members of Allocasuarina have fine, equisetoid growth that gives the trees a very distinctive appearance. Allocasuarina littoralis (Salisb.) L.A.S. Johnson and A. verticillata (Lam.) L.A.S. Johnson are not infrequently cultivated in Australia, where they are recommended as suitable small trees for gardens and landscapes, being tolerant of a range of ‘difficult’ conditions ranging from hot, dry sites to clay soil and the shoreline (Metro Trees 2007). Senior (2004) mentions that a few species of ‘Casuarina’ have been attempted in Cornwall in recent years, including Allocasuarina littoralis, but gives no indication of their success, while Johnson (2007) mentions that A. littoralis, A. torulosa (Ait.) L.A.S. Johnson and A. verticillata are in cultivation in the British Isles, the latter two recorded from Tresco Abbey, Isles of Scilly.