Common name: Broad-leaved peppermint

Broad-leaved peppermint is an essential-oil yielding eucalyptus originating in Australia, where it occurs in the country's southeast. Its natural range extends from eastern and southern Victoria into the high country of New South Wales, as far north as the town of Armidale.

On favourable forests sites, it may reach heights of up to 25 m (82 ft) with a trunk diameter of 0.7 m (2.3 ft). Though it is typically 12 to 20 m (40 to 65 ft) tall with a straight trunk around half of the tree's height, supporting a moderately branched, columnar crown. On open sites, it grows somewhat shorter and is low-branching, with a short trunk supporting a large, rounded crown. The bark is dark grey or brown, on young trees smooth, with age becoming fibrous, rough and flaking off in patches.

As with many Eucalyptus species, the leaves change shape as the tree matures. In this species, from stalk-less, ovate, thick, dull bluish-green leaves arranged in pairs on seedling and juvenile trees to long-stalked, broadly lance-shaped, glossy green leaves arranged alternately on mature trees. They remain on the tree throughout the year and release a strongly menthol-like aroma when crushed.

The flowers are typical of a eucalypt, small, with long, creamy-white filaments set in a cup-shaped base and perfect (bisexual). They arise at the sides of the branches, usually in groups of eleven to fifteen and bloom from spring through summer.

Fertilised flowers are followed by small, goblet-shaped seed capsules that become dark brown and woody when mature with tiny, glossy brown, pyramidal-shaped seed inside. 

Use

Depending on the variety, the fresh leaves on steam distillation yield around 3 to 4% eucalyptus oil of three types. The variety known as 'C' produces eucalyptus oil with a high cineole (aka. eucalyptol) content, which is the most valuable of the Eucalyptus oils because of its medicinal and antiseptic qualities. It is almost identical, in composition, to the oil from the Narrow-leaved peppermint (Eucalyptus radiata), a closely related eucalyptus species overlapping its native distribution.

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, more generally, are used in cleaning, mould removal, disinfecting, perfumery, pharmaceutical, personal care and pest insect control products.

Other varieties include so-named 'A' and 'B'. The latter produces an oil containing a large amount of 'laevo-Piperitone', which is used in the production of synthetic Menthol. Menthol is an organic compound used in a wide range of products, including lip balms, topical analgesics such as Tiger Balm, topical decongestants such as Vicks VapoRub and oral hygiene products such as mouthwash and toothpaste. It is also used as a flavouring agent in foods such as chewing gum. Variety 'A' contains less laevo-Piperitone and more 'Phellandrene', useful as a solvent and in perfumery. It also shows promise as a natural insecticide, particularly against herbivorous caterpillar species. And as a fungicide against Botrytis cinerea and Penicillium cyclopium.

When extracted, all oil types are colourless but age to pale yellow, especially under long or poor storage conditions.

The wood is moderately heavy, weighing around 670 kg per cubic meter (42 lbs per cubic ft), but has low natural resistance to trot, decay and wood-boring insects. It has limited commercial value and is not used much for timber.

Climate

It grows naturally in humid warm-temperate climates, generally areas with annual lows of 5 to 14°C, annual highs of 17 to 23°C, annual rainfall of 600 to 1400 mm and a dry season of 5 months or less.

Growing

New plants are usually started from seed, which benefit from being stratified and sown in a non-sterile potting mix to increase survival rates and enhance germination. 

Broad-leaved peppermint performs best on free-draining clay-loam, loam, sandy-loam and loamy-sand soils of a moderately acid to slightly alkaline nature, generally with a pH of 5.0 to 7.5, and on sites with full to partial sun exposure. It has poor tolerance to waterlogged or poorly drained soils.

Problem features

None known

Books

• Arctander, S. 1960, Perfume and flavor materials of natural origin, Elizabeth, New Jersey

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

• Brady, G. S. & Clauser, H. R & Vaccari, J. A. 2002, Materials handbook : an encyclopedia for managers, technical professionals, purchasing and production managers, technicians and supervisors, 15th ed., McGraw-Hill, New York

• Cribb, A. B. & Cribb, J. W. 1982, Useful wild plants in Australia, William Collins, Sydney

• F. R. Beuhne 1922, Honey flora of Victoria, Melbourne Albert J. Mullett, Govt. printer

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

• 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