Casuarina is a genus of 17 species in the family Casuarinaceae, native to Australasia, southeastern Asia, and islands of the western Pacific Ocean. It was once treated as the sole genus in the family, but has been split into three genera (see Casuarinaceae).
They are evergreen shrubs and trees growing to 35 m tall. The foliage consists of slender, much-branched green to grey-green twigs bearing minute scale-leaves in whorls of 5–20. The flowers are produced in small catkin-like inflorescences; the male flowers in simple spikes, the female flowers on short peduncles. Most species are dioecious, but a few are monoecious. The fruit is a woody, oval structure superficially resembling a conifer cone made up of numerous carpels each containing a single seed with a small wing.
Casuarina species are a food source of the larvae of hepialid moths; members of the genus Aenetus, including A. lewinii and A. splendens, burrow horizontally into the trunk then vertically down. Endoclita malabaricus also feeds on Casuarina. The noctuid Turnip Moth is also recorded feeding on Casuarina.
C. equisetifolia is a common tropical seashore tree known as Common Ironwood, Beefwood, Bull-oak, or Whistling-pine and is often planted as a windbreak. The wood of this tree is used for shingles, fencing, and is said to make excellent, hot burning firewood.
C. oligodon has been planted in New Guinea in an ancient (more than 3,000 years) silviculture by highland gardeners practicing an intensive traditional permaculture. The wood of this tree is used for building-timber, furniture and tools and makes excellent firewood. The tree's root nodules are known to fix nitrogen, and it is traditionally prized for its ability to increase the soil's fertility. Its abundant leaf-fall is high in nitrogen and traditionally prized for mulch.
The gum exuded from some casuarinas is edible and was a food source for Aboriginal people.
C. equisetifolia is widespread in the Hawaiian Islands where it is growing both on the seashore in dry, salty, calcareous soils and up in the mountains in high rainfall areas on volcanic soils. It is also an introduced, invasive plant in Bermuda. It was introduced to replace the Juniperus bermudiana windbreaks killed by juniper blight in the 1940s. Now they are growing on cliffs and sandy slopes strangling all surrounding plants, or covering them in needles, they also erode the cliffs by digging their roots deep into them and splitting them apart. The plants are strongly suspected of having allelopathic properties, as evidenced by the near absence of understory once a mat of litter develops around the plants.