Much like a pine forest can perfume the air with a distinctive scent, so too do large woodlands filled with eucalyptus trees and shrubs (Eucalyptus spp.). Aussies generally call them gum trees since some exude gooey gum from wounds on the trunk and branches. Providing welcome shade from the intense summertime sunshine, grow this group of plants in U.S. Department of Agriculture winter hardiness zones 7 and warmer, depending on the species.
Members of the myrtle family, Myrtaceae, Eucalyptus is a botanical genus comprising nearly 500 species of evergreen trees and shrubs. Recently, taxonomists removed roughly 100 species of trees and reclassified them into the genus Corymbia.
The vast majority of Eucalyptus hail from the Australian mainland and island of Tasmania, although a few species naturally occur in New Guinea, Indonesia and the Philippines. Today these plants are grown worldwide and in some regions have become invasive plants because of their ability to re-seed and outpace the growth of native plants.
Eucalyptus plants develop both juvenile and adult foliage. When a young sapling, trees often develop much larger or rounder leaves that are arranged in opposite pairs on the stems. Once the tree matures, it displays its adult foliage that is arrange oppositely on the branch twigs. Often tree bark is attractive, whether flaking off in strips on those called gum trees, or nearly smooth and skin-like while the rough, persistent bark occurs on the species called ironbarks. Shrubby species are typically called mallees. The flowers are borne in clusters, usually in spring and summer, consisting of many top- or cup-shaped flowers with lots of hair-like stamens. The seed capsules that form are hard and full of many tiny seeds.
The large number of gum trees affords their use as both shade and decorative flowering trees in garden landscapes in mild winter climates. Honey is made from their flowers and eucalyptus oil is made from crushing stems and foliage. Some flowers, leaf shapes and colors are particularly attractive and used in fresh and dried flower arrangements. The leaves may supply food and water to indigenous Australia wildflife such as the koala.
Fast growing, some species of Eucalyptus have rather brittle branches that will break in windstorms, and some trees that grow in wet soil environments are shallow-rooted and topple in wind, too. The oils in the cell tissues of these plants is extremely flammable and can exacerbate the intensity and spread of wildfire. Moreover, these oils contain cyanogenic glycoside, according to North Carolina State University, and can cause both skin irritation and poisoning if eaten. Diarrhea and stomach upset occurs if leaves are eaten but is deadly only if huge amounts are consumed. The concentrated eucalyptus oil sold in markets is extremely toxic if taken internally.