Overview of the Eucalyptus Genus of Australian Trees

There are around 800 eucalypt species, most of them endemic to Australia. The genus Eucalyptus is part of the myrtle (Myrtaceae) family. Most are evergreen trees. The leaves are aromatic and dotted with oil glands. Most species have two types of foliage with the juvenile leaves being carried opposite each other and adult leaves being alternate. One of the features which unite the various species is the arrangement of the flowers.

The flowers have numerous fluffy stamens which are contained in the flower bud. The bud is covered with a cap called the operculum. The operculum is composed of fused sepals and/or petals. As the enclosed stamens expand, the operculum is forced off and splits away from the cup-like base of the flower. The fruits are woody capsules.

Many bear the name ‘gum’ as part of their common name. One such is the poplar gum or white gum (Eucalyptus alba). This tree is found over most of tropical Australia and also in parts of New Guinea and eastern Indonesia. It grows to 18 metres and is usually deciduous. It has smooth, white bark. The large juvenile leaves are a bright green and may persist on mature trees. The flowering season is late winter to early spring when small clusters of creamy white flowers appear. This is a good choice for a tropical garden. The tree is also able to withstand seasonal waterlogging.

Another beautiful gum is the Tasmanian snow gum (Eucalyptus coccifera). This tree is endemic to Tasmania and has peeling white and grey bark. When the bark is shed the new bark underneath is a yellow-brown or pink. The young leaves are rounded and a bluish-green while adult leaves are lanceolate in shape and grey-green. Creamy white flowers are produced in summer.

Western Australia’s tallest tree is the karri or Eucalyptus diversicolor. It grows to 60 metres and sheds its bark in irregular blotches. The tree is highly valued for its hardwood timber and the nectar from the creamy white flowers produces a superior honey.

The red tingle (Eucalyptus jacksonii) is another of Western Australia’s ‘tall trees’. It grows to 60 metres with a trunk of up to 4.5 metres in diameter. It has brownish stringy bark and a dense canopy of glossy bright green mature leaves.

The Buxton gum or Eucalyptus crenulata is known in only one valley north-east of Melbourne, Victoria. It has conspicuously glaucous new beds and leaves. The mature leaves are very aromatic, usually heart-shaped, and grey-green with shallow-toothed margins. Small white flowers are produced in spring.

 The tallowwood (Eucalyptus microcorys) comes from the moist forests of subtropical eastern Australia. It has distinctive soft, fibrous, reddish brown bark and a dense, spreading crown. Showy clusters of white flowers appear in winter through to early spring.

Eucalypts have many functions. Some are used for timber for all manner of construction from wharves to fine furniture, some are harvested for oil or wood chips, some are regarded as superior food sources for bees producing commercial honey.

Most eucalypts are fast growing and long lived. Once established they should need little in the way of attention. In general, they are best suited to semi-arid or warm-temperate regions. Most species can be kept in shape by pruning or can be cut back heavily. Some Western Australian species will not tolerate the summer humidity which may be encountered on the east coast.