Family: Myrtaceae
Common Name: Eucalypt, Gum Tree

Eucalypts are far and away the dominant trees in Australia, with over 600 species, as well as many hybrids. Only a few of the species are found in the wild outside Australia, in the Philippines, New Guinea and islands nearby. The genus was first described scientifically in 1788, though the trees had been noted by William Dampier about a century earlier. It is believed Australia's Governor Phillip was first to record the common name, 'gum tree'.

Eucalypts are found in a wide range of habi­tats in their native Australia, from the cold, windy highlands of the snow country to the semi-arid interior and the fertile soils of the forests. They also vary extensively in habit from twisted shrubs to forest trees and dwarf types, or mallees, from the almost rainless inland, which produce a number of slender stems, 1-6 m (3-20 ft) tall, from woody rootstocks known as lignotubers found underground or at the base of the trunks. While most eucalypts have lignotubers, only the mallees develop suchlarge rootstocks. In the more favorable condi­tions of the plains and open woodlands, the typical eucalypts (ironbarks and boxes) have short, straight trunks with rounded, spreading heads. In fertile, high rainfall areas, the eucalypt is very tall and straight, with a small head, at times reaching heights of 90 m (300 ft).

Many species have two distinct types of foliage, juvenile and adult, which differ in shape and colour, the younger leaves being broader and often covered with a waxy bloom, giving them a blueish hue. In some species, the juvenile type of foliage prevails. Eucalypt flow­ers do not have sepals and petals: these are fused together into a cap, which covers the sta­mens and is shed as the flower opens. The flow­ers have numerous stamens in either white, yellow, pink or red, and in some cases, they are very large and showy. When the flower dies, the floral tube or receptacle containing the seeds continues to grow, then changes colour, hardens and becomes a capsule, the 'gum-nut', which eventually opens to allow the seeds to disperse.

Eucalyptus species are extremely difficult to identify. The shape of the tree can alter markedly under different growing conditions. When attempting to identify a eucalypt, it is necessary to consider its leaves, both adult and juvenile, its bark, its flowers, particularly the shape of the cap, and its seed capsules. If it is not a cultivated specimen, but is growing naturally, its geograph­ical location may also be of assistance.

The ironbarks have dark, deeply furrowed, persistent bark on the trunk and main branches. It is very hard and cannot easily be stripped off. Ironbark's durable, strong timber is used for bridges and wharfs.

The stringybarks have persistent bark but it is strong and fibrous, and not so hard. The pep­permints also have fibrous, persistent bark, but the fibers are shorter and the bark paler. Their foliage is a rich source of fragrant oil which is used in perfumery and aromatherapy.

Similar to the peppermints in the appearance of their bark, but without the richness of their oil, are the boxes. The early settlers adopted the name 'boxes' because the timber was reminis­cent of the European box (Buxus semper­virens). The boxes provide some of the best hardwood for heavy construction work. Boxes and ironbarks hybridize freely with each other.

The ash group comprises a small number of species, among them some of the tallest hardwoods in the world, such as E. regnans (moun­tain ash), with persistent, more or less fibrous bark on varying amounts of the trunk, sometimes extending to the branches.

The bloodwoods are so named because of the large amount of 'kino' in their veins and timber. However, they have now been sepa­rated from Eucalyptus and reclassified as Corymbia.

Last of the groups is the gum, which usually has a smooth trunk, with a covering of rough bark at the base. The bark is shed, creating a variety of patterns, at different times. However, the mahoganies, included in this group, are covered in rough, persistent bark.

Commercially, the eucalypts provide a wide variety of high quality hardwood. The foliage yields essential oils, and tannins are extracted from the bark. Flowering eucalypts provide large quantities of high grade honey. Many eucalypts assist in preventing erosion by wind and water. They make effective windbreaks and have been used to reclaim swampy ground. Eucalypts have been grown in many countries to such an extent that in some parts, such as California and Arizona, they are regarded as native trees. They have become invasive in South Africa.

The wide range of species available means that a tree may be selected to suit any soil or cli­mate. However, it should be remembered before deciding to plant a eucalypt that many grow into trees too big for a suburban garden. They may not be deciduous but they drop leaves and twigs all year round. Some even drop occasional large branches. Many have spread­ing root systems, making them unsuitable for planting near houses. Seek professional advice if you are unsure which species to choose.

Species

Not all are available outside their native Australia.

E. caesia, zone 9, is a very decorative, mallee-type eucalypt from Western Australia. It is rare and endangered in the wild but is widely cultivated. The red-brown bark on the trunk peels in summer, exposing new green bark beneath. The branches are often pendulous and the leaves, buds and stems are covered with a gray-white bloom. The fairly large flowers are pink or crimson, with prominent gold anthers. Silver Princess or subsp. magna is the variety most often grown. This graceful, small tree grows 6-10 m (20-33 ft) high.

E. camald­ulensis, river red gum, zone 9, has the widest distribution of all eucalypts, occurring in patches through all states of Australia except Tasmania. It is adaptable to a wide range of cli­mates and conditions and may grow between 15 and 50 m (50-160 ft) high, depending on those conditions. The timber is resistant to termite attack and highly sought after for con­struction and fine decorative work. The flowers provide copious amounts of nectar, making it a good honey and pollen tree. Too large for the home garden, it is ideal in many rural areas.

E. cinerea, Argyle apple, grows 8-15 m (26-50 ft) tall and is noted for its silvery gray foliage, especially in its juvenile stage. This tree is often coppiced or hard pruned to maintain a supply of juvenile foliage. It is a good eucalypt for home gardens as it provides foliage contrast, with the foliage cover extending almost to the ground. This adaptable tree will tolerate quite a degree of frost.

E. cladocalyx, sugar gum, zone 9, has been widely planted as a windbreak and shelter belt tree in many areas. The smaller form 'Nana' is ideal for this purpose. The species is variable ranging from 8 m (26 ft) to more than 30 m (100 ft) tall. This frost-tolerant tree is also drought-tolerant once established.

E. curtisii, Plunkett make, grows 3-10 m (10-33 ft) tall and has multiple trunks. A fast-growing tree suitable for home gardens and street planting, it flowers profusely from an early age through late winter and spring. It is especially attractive to birds and insects.

E. deglupta, kamerere, is One of the few eucalypts not native to Australia. It occurs naturally in some parts of New Guinea, Indonesia and the Philippines. Native to tropi­cal forests with very high rainfall, it is fast grow­ing and may reach 60-70 m (200-230 ft). It is a colonizing species in its habitats and is culti­vated both for timber and wood pulp. This tree has a very colourful trunk as the bark peels in rib-bons revealing green, blue, purple and red to orange shades.

E. erythrocorys, illyarie or red cap gum, is one of the most striking eucalypts in cultivation. The buds have a bright red, waxy, four-sided cap, opening to the brightest yellow flower, providing a most eye-catching sight. It grows 4-10 m (13-33 ft) high and may even be grown in a very large tub. Although more adaptable than some Western Australian eucalypts, it does best in warm to hot areas where summer rain and humidity are low. It must have per­fectly drained soil.

E. globulus, Tasmanian blue gum or southern blue gum, may grow to over 50 m (160 ft) high in ideal conditions, but can be as low as 15 m (50 ft). There are a number of subspecies, though all are large trees. The young leaves are quite rounded and covered in blue-gray bloom, while adult leaves are sickle-shaped, dark green and up to 30 cm (12 in) long. This is an ideal shade and shelter tree for large gardens, parks and rural properties.

E. haemastoma, scribbly gum, is fairly small at 10-15 m (33-50 ft), often having a slightly bent or twisted trunk with characteristic scribbles formed by insects burrowing in the bark. It is an attractive small gum for home gardens and parks. The canopy is rather open, allowing understorey plants to flourish beneath.

E. macrocarpa, rnottlecah or rose-of-the-west, has the largest flower of any eucalypt. These are deep pink to crimson and appear on top of the untidy, sprawling growth of this malice-type eucalypt. The foliage is silvery gray. This species does best in fairly arid areas and appears unable to tolerate summer rain or humidity. Drainage must be perfect. All parts of the plant are used in floral work, including the woody, flattish cap­sules which are often 7 cm (3 in) in diameter.

E. melliodora, yellow box, is one of the best of all the honey-producing eucalypts. It is also grown for its durable timber. Although generally too large for suburban gardens, it is ideal in larger gardens, parks and rural areas. It may grow 10 to 30 m (33-100 ft) high and is adaptable to a wide range of soils.

E. microcorys, tallow wood, is large, 20-40 m (65-130 ft) high, but its wide-branching habit makes it an ideal shade and shelter tree. The trunk is covered in reddish brown, fibrous bark and white blossom appears over a long period through spring and early summer. The timber is one of the best pos­sible hardwoods available, with an extensive range of applications.

E. nicholii, willow pep­permint or narrow-leaved black peppermint, zone 8, is often planted by home gardeners in the mistaken belief that it is a small tree. Its gen­eral range is from 12 to 20 m (40-65 ft) high. The trunk is fibrous, gray-brown and the leaves are blue-green and very narrow. It is frost-toler­ant and also tolerates exposed, windy sites.

E. pauciflora subsp. niphophila, snow gum or white sally, zone 8, is now classified by some botanists as E. niphophila.

E. pauciflora occurs in a range of habitats, from coastal regions to areas well over 1500 m (5000 ft). The subspecies appears to occur only above 1500 m (5000 ft) in alpine areas of its range. Snow gums may have single or multiple trunks, often bent or twisted in shape. In higher altitudes, the smooth trunksshow decorative streaks and patches of pink, brown, white and green. It ranges in size from 8 to 20 m (26-65 ft) and has abundant blossom from mid-spring to midsummer.

E. pilularis, blackbutt, grows 25 to 40 m (65-130 ft). It is fast growing on good soils in mild areas with good rainfall. The bark is fibrous and dark on most of the trunk, becoming smooth only on the upper branches. It is an important timber tree and is ideal for park and landscape planting.

E. pyri­formis, pear-fruited malice, zone 9, is a small, decorative tree, 2-6 m (6-20 ft) high, with mul­tiple trunks. It is ideal for home gardens and responds well to hard pruning and coppicing. It is best grown in fairly arid areas with low summer rainfall and is drought-tolerant when established. The buds, flowers and ribbed fruit­ing capsules are very ornamental and are used in floral arts and crafts. The flowers may be coloured cream, yellow, pink or red.

E. robusta, swamp mahogany, grows 20-25 m (65-80 ft) high, occurring naturally on saline soils in coastal estuaries or other waterlogged areas. It is fast growing and makes a good street, park or large garden tree. Unfortunately, its appearance is often ruined by sap-sucking insects, known as lace lerps, which feed on its foliage.

E. scoparia, Wallangarra white gum, is a good choice for home gardens, normally growing from 8 to 12 m 126-40 ft). It has a smooth, light-coloured trunk and graceful, narrow, drooping foliage. The canopy is high, letting other plants grow successfully underneath. It is also quite fast growing.

E. sideroxylon, mugga or pink-flow­ered ironbark, zone 9, grows 10-30 m (33-100 ft) high. It is recognizable by its deeply furrowed, very dark bark and attractive, blue-green foliage. Flowering may be from midwinter to spring and flowers may be pink, red, white or yellow. The very hard wood is used in construction, and oil can be distilled from its foliage.

E. torquata, coral gum, is one of the most ornamental of all eucalypts. Flowering from a young age, the lovely blossom can be coral pink to red or occa­sionally white or cream. It grows 6-12 m (20-40 ft) and is useful in home gardens and parks and as a street tree. It is used widely in arid areas as it can tolerate either acid or alkaline soil. Ideally suited to arid inland conditions, it can also be grown in subtropical and cooler zones but is unlikely to do well in regions which have high humidity or high summer rainfall.

Cultivation

Many species of Eucalyptus have adapted to the warmer regions of the US, par­ticularly California and Arizona, and some can be grown outdoors in zone 8. In cooler areas they can be grown as young specimens in pots of soil-based potting compost in a cool, airy greenhouse or conservatory. Provide maximum light. In the summer they can be placed out-doors. Inside or out they need a neutral to slightly acid soil. In the garden, provide a sunny site with wind protection. To grow Eucalyptus as shrubs, cut back the stems annu­ally in early spring to within a few buds of the base (known as coppicing). Propagate from seed sown in spring. Germinate at 18°C (64°F).

Climate

Zone 10 unless otherwise specified above.