The she-oak family has the scientific name of Casuarinaceae. In 1982, the Casuarina genus was divided into four. The true Casuarina now has about 17 species, six of which are confined to Australia.
She-oaks are tough and hardy. The genus Casuarina is a shapely, evergreen tree. They have distinctive, grey- or dark-green branchlets which are modified to function as leaves. The real leaves appear as a ring of tiny teeth at regular intervals along the wiry, slender branchlets.
Casuarina equisetifolia is also known as the Coastal She-oak or Horsetail She-oak. It is a native of subtropical and tropical coastal regions of Queensland, New South Wales and the Northern Territory. It is also found on the Malay Archipelago and Pacific Islands.
It may grow to 20m and is a graceful, spreading tree with an open branching crown, drooping branches and fine foliage. The flowers are relatively insignificant. It is best suited to tropical and subtropical regions and will withstand salt winds. It is widely used in Australia for stabilising coastal dunes, and for sand erosion control. It is frost-hardy. The subspecies C.e.incana is very attractive with weeping silvery green branchlets.
Another she-oak which is used for binding soil is the Swamp She-oak (Casuarina glauca). This species is native to eastern coastal Australia. It is an upright medium to tall tree with weeping, dark green branchlets. The branchlets are covered in a waxy substance. It often forms dense thickets in brackish swamps. It withstands dry conditions and is also useful as a windbreak.
Casuarina cunninghamiana (River Oak or River She-oak) is often seen on riverbanks in eastern Australia. It is a stately tree with fine, pendulous foliage. Light brownish male flower spikes are produced in December and January. It is a quick growing tree which prefers an open moist position but will withstand seasonal flooding.
She-oaks are also found in arid conditions. One such species is Casuarina pauper (Belah or Black Oak). This is a valuable shade and shelter tree in dry areas. It adapts to most soil types and withstands harsh conditions. It is a stout, rough-barked tree which grows from 6 to 15 metres.
Casuarina cristata is another she-oak which is native to the drier areas of eastern Australia. It too is sometimes called Belah as it is similar to C.pauper. The bark is hard, dark and scaly and the branchlets grey-green and pendulous. It is a good choice for heavy soils.
Casuarinas are fast growers. Grouped together, they are ideal as providers of shade and shelter. They are also suited to act as screening plants. They must have well-drained soil and will withstand harsh winds.