Plant 364

Eucalyptus species Myrtaceae)

Gum trees

There are over 750 species in the genus Eucalyptus occurring naturally in Australian landscapes as evergreen trees, shrubs or mallees. There are also sixteen species collectively native to the Philippines, New Guinea and Indonesia. Eucalypts are very familiar plants as they are cultivated worldwide. The wood is useful as it is hard and durable, enabling it to be used in construction industries for timber, transmission poles, railway sleepers, furniture, firewood and pulpwood production.

Many eucalypts have been established in cultivation, but most are not hardy enough to survive the frosts of a severe British winter, apart for southern parts of the country. The first native Australian tree to be cultivated in Britain was cider gum (Eucalyptus gunnii) in 1846. In some countries, such as South Africa, eucalypts have become invasive, having escaped from plantations. Eucalypts can grow rapidly to become very tall trees. The tallest known flowering plant is mountain ash (Eucalyptus regnans); a Tasmanian specimen, measured as 100.5 metres in 2018, has been named ‘Centurion’.

The flowers, which all arise from the one point on the stem, are arranged in umbellasters. In bud, the floral parts are covered by a cap, the calyptra, hence the generic name. This falls off to reveal numerous stamens. Copious nectar may be produced, making them plants favoured by apiarists. Fruits are woody capsules. Different Eucalyptus species are characterised by their bark, the arrangements of their flowers and their dimorphic leaves.

Essential oil, which has antiseptic properties, is extracted from the leaves. Eucalyptus oil is not only used medicinally but also in cosmetics and perfumes. The oil helps protect the trees from insect attack but also makes the leaves highly flammable, and therefore a fire hazard. Some Eucalyptus species have developed woody underground tubers which help the trees regenerate after fire.

The evergreen leaves of several Eucalyptus species are favoured as food by koalas. Another animal living in the forests of eastern Australia, the yellow-bellied glider, an arboreal marsupial, is dependent on eating the sap of the red stringybark (Eucalyptus resinifera). These gliders also live in hollows in the bark of old stringybark trees.

Using X-ray fluorescence, scientists have discovered a method of detecting gold deposits in Australia by analysing Eucalyptus leaf-discs. Traces of gold have been found in the leaves of Eucalyptus trees growing around Kimberley, Western Australia; ions of buried gold having been transported through the vascular system of these deep-rooted plants.

Further reading

Grimshaw J and Bayton R 2009. New trees. Recent introductions to cultivation. Kew Publishing, pp. 324-363.

Litern M et al. 2013. Natural gold particles in Eucalyptus leaves and their relevance to exploration of buried gold deposits. Nature Communications 4: 2614.

Serena Marner