Eucalyptus exserta

Common name: Queensland peppermint

Description

Queensland peppermint is a timber and essential oil yielding eucalypt originating in Australia's Queensland state. Its natural range extends from just west of Cairns, in the state's tropical north, south to northern New South Wales, and from the coast to 800 km (500 mi) inland.

On favourable sites with plenty of moisture, it is a handsome tree up to 25 m (80 ft) tall with a straight trunk 1 m (3 ft) in diameter and without branches for more than half the tree height. The crown is moderately branched open and rounded. On dry sites, it is usually a small multi-trunked tree. The bark is grey, smooth on young trees, on mature trees, rough with vertical fissures.

As with many eucalyptus species, the leaves change shape as the tree matures. In this species, from long, narrow, dull-green linear-shaped leaves on seedling and juvenile trees to long, dull green and slightly curved lance-shaped leaves on mature trees. They remain on the tree in all seasons and give off a characteristic eucalyptus-oil aroma when crushed.

The flowers are typical of eucalypts, being perfect (bisexual) and small, with long, creamy-white filaments set in a cup-shaped base. They arise at the leaf axis, usually in groups of seven and bloom from late spring through summer, coinciding with the rainy season in its native range.

Fertilised flowers develop into small, goblet-shaped seed capsules that become dark brown and woody when mature with tiny, black, strongly toothed, pyramidal-shaped seed inside.

Use

Queensland peppermint produces a hard, durable and heavy wood in the 900 to 1000 kg per cubic meter (56 to 62 lbs per cubic ft) range, with attractive pinkish-brown heartwood. It is suitable for general construction, especially house framing, and in split or round form for fencing and firewood.

It is widely planted in southern China, particularly Guangdong and Hainan provinces, to protect river embankments, fix sand, and conserve water and soil that would otherwise be washed away by destructive typhoons. 

On steam distillation, the leaves yield around 0.7 to 1.2% eucalyptus oil of the cineole (aka. eucalyptol) type, which is the most valuable of the eucalyptus oils because of its medicinal and antiseptic qualities. When extracted, the oil is colourless, but ages to pale yellow, especially under long or poor storage conditions and is strongly camphor-smelling and mint-like in taste. 

Cineole-type eucalyptus oil is a proven expectorant, antiseptic and antibacterial, particularly against Streptococcus. It is used in cough drops and syrups, vaporiser fluids, ointments, liniments, gargles and oral hygiene products such as toothpaste and mouthwash. It is also commonly used in dentistry, particularly in solvents and sealers for tooth fillings after root canal surgery. 

Cineole type oils are also a good stain remover, having proven effective in removing grease, glue and dried stains from both skin and clothing. Eucalyptus oils are more generally used in cleaning, mould removal, disinfecting, perfumery, pharmaceutical, personal care and pest insect control products. China is by far the largest producer of eucalyptus oil extracted from Queensland Peppermint.

Other Eucalyptus species producing commercial quantities of cineole-type eucalyptus oils include Tasmanian Blue gum (Eucalyptus globulus), Gully gum (Eucalyptus smithii), Blue mallee (Eucalyptus polybractea) and Narrow-leaved peppermint (Eucalyptus radiata).

Although not a reliable nectar producer, the flowers produce at times sufficient nectar for honey production. The honey is light amber with a good flavour and moderately dense.

The leaves are a favourite food for koalas in Queensland state.

Climate

Grows naturally in sub-humid to humid subtropical and tropical climates, generally areas with annual lows of 12 to 22°C, annual highs of 25 to 30°C, annual rainfall of 500 to 2000 mm and a winter dry season of 3 to 7 months. However, it reaches its best development as a timber tree in areas with annual rainfall of 900 mm or more and a dry season of up to 5 months.

Growing

New plants are usually started from seed. Perform best on free-draining loam, sandy-loam and loamy-sand soils of a moderately acid to alkaline nature, generally with a pH of 6.0 to 8.0 and on sites with full sun exposure. It has good tolerance to drought conditions.

Problem features

None known

Where it grows

With irrigation or groundwater

References

Books

  • Blake, S. T. & Roff, C. 1987, The honey flora of Queensland, 3rd ed., Queensland Department of Primary Industries (QLD DPI), Brisbane

  • Boland D.J., Brophy J.J. & House A. P. N. 1991, Eucalyptus leaf oils, use, chemistry, distillation and marketing, Australian Centre for International Agricultural Research (ACIAR) & Commonwealth Scientific Industrial Research(CSIRO), Inkata Press, Melbourne

  • Boland, D. & Brooker, I. & McDonald, M. W. 2006, Forest trees of Australia, 5th ed., CSIRO Publishing (Ensis), Melbourne

  • Hall, N. 1972, The use of trees and shrubs in the dry country of Australia, Australian Government Publishing Service, Canberra, Australian Capital Territory

  • Jacobs M. R. 1979, Eucalypts for planting, Food and Agriculture Organization of the United Nations (FAO), Rome

  • National Research Council (Board on Science and Technology for International Development) 1983, Firewood crops : shrub and tree species for energy production (Volume 2), The National Academies Press, Washington D. C.

  • Randall, R. P. 2002, A global compendium of weeds, R.G. and F.J. Richardson Press, Melbourne

  • Streets, R. J. & Troup, R. S. 1962, Exotic forest trees in the British Commonwealth, Oxford University Press, Oxford, England

Articles, Journals, Reports and Working Papers

  • Ciesla W.M. 2002, Non-wood forest products from temperate broad-leaved trees, FAO Technical Papers 15, Food and Agriculture Organization of the United Nations (FAO), Rome

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