Common name: Clove basil

Clove basil is an aromatic shrub and close relative of common Basil (Ocimum basilicum), yielding a commercially important essential oil. 

Originating in Africa and the Indian subcontinent, its natural range extends from East Africa, through Madagascar and the Seychelles, to India, Sri Lanka and Bangladesh. Nowadays, it is widely introduced and cultivated for its oil, mainly in Brazil, Tahiti and Indonesia. 

There are two oil-yielding types or varieties which are almost indistinguishable in appearance and have similar growing requirements. However, their essential oils are quite different, one having an oil content high in thymol, the other high in eugenol. Thymol is commonly extracted from Thyme (Thymus vulgaris) or Ajwan (Trachyspermum ammi), whereas eugenol is extracted from Cloves (Syzygium aromaticum).

It is a much-branched perennial shrub 1 to 2 m (3 to 6 ft) tall with an untidy, bushy or weedy appearance and strongly aromatic herbaceous foliage on top of a woody base. The leaves are dull green, heart-shaped, toothed on the margins, finely haired on both sides and get their strong aroma from oil glands dotted on the upper surface.

The flowers are small or insignificant, white or green-yellow borne on erect flowering spikes at the tips of the branches. They bloom in winter, which coincides with the dry season in its native range, and are soon followed by very small, roundish fruit, made up of four dry, one-seeded nutlets.

Use

The herbage, which includes the leaves, soft stems and flowers, yield an essential oil on steam distillation that, depending on the variety, is high in thymol or eugenol. The primary use of these oils is as a substitute for thyme and clove oils.

Thymol is a dark yellow to orange-yellow liquid with a spicy, medicinal, somewhat herbaceous aroma and a warm, slightly bitter, burning flavour with a medicinal aftertaste. It is a common ingredient in cold and cough syrups, mouthwashes and soaps, and a disinfectant in ointments.

Eugenol is a pale yellow to brownish-yellow liquid with a warm, spicy aroma reminiscent of cloves and with a sweetly woody, floral top note. It is used as a flavouring in food, confectionery and pharmaceuticals, as a fragrance in perfumes and as an antiseptic in toothpaste. Vanillin, which gives vanilla its flavour, is synthesised from eugenol. 

The oils also contain small amounts of geraniol, a substance found in many plants with insect repellent activity and in its refined form, is used in quite a few commercial insect repellents.

Climate

Grows naturally and produces herbage with a high oil content in sub-humid to moderately humid subtropical and tropical climates, generally areas with annual lows of 17 to 25°C, annual highs of 27 to 36°C, annual rainfall of 700 to 2000 mm and a dry season of 2 to 7 months, extending to 12 months with irrigation. Although it also grows well in wetter climates with no dry season, the oil content of the herbage is relatively low and of poor quality.

Growing

New plants are usually started from seed that germinate readily. It grows on a wide range of soils. Still, it performs best on well-manured, free-draining loam, sandy-loam and loamy-sand soils of a moderately acid to moderately alkaline nature, generally with a pH of 6.0 to 8.0, and on sites with full sun exposure. It is intolerant of slow-draining or saturated soils and herbage grown under shade has a low oil content.

Crops in intensive systems are irrigated once to twice a week, depending on soil moisture levels. The herbage is harvested three times a year in the early stages of flowering. All herbaceous above-ground parts are taken, including the soft stems, leaves, and flowers. Repeat harvesting continues for up to five years before the plant loses vigour and replanting becomes necessary.

Around 50,000 kilograms of fresh herbage is harvested per hectare, per year, in commercial operations. And with an oil content of 0.3 to 0.6%, it yields 150 to 300 kilograms of oil on distillation, the equivalent of 134 to 268 pounds of oil, per acre, per year.

Problem features

The seed are small and easily dispersed by wind and water. It is widely distributed and naturalised in the tropics, including parts of Africa, Central and South America. It is a declared weed in some Caribbean and Pacific island nations.

Books

• Adams, C. D. 1972, Flowering plants of Jamaica, University of the West Indies, Mona, Greater Kingston

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

• 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

• Debboun, M. & Frances, S. P. & Strickman, D. 2006, Insect repellents : principles, methods, and uses, CRC Press, Boca Raton, Florida

• Farooqi, A. A. & Sreeramu, B. S. 2004, Cultivation of medicinal and aromatic crops, Hyderabad University Press, Hyderabad

• Guenther, E. & Althausen, D. 1948 to 1952, The essential oils (6 volumes), Van Nostrand Publishing, New York

• Oyen, L. P. A. & Nguyen X. D. 1999, Plant Resources of South-East Asia (PROSEA) 19 : Essential-oil plants, Backhuys Publishers, Leiden

• Parrotta, J. A. 2001, Healing plants of peninsular India, CABI Publishing, Wallingford, Oxfordshire

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

• Seidemann, J. 2005, World spice plants: economic usage botany taxonomy, Springer-Verlag, Berlin

• Space, J. C., et al. 2000, Invasive plant species on Kosrae : Federated states of Micronesia, USDA Forest Service, Institute of Pacific Islands Forestry, Hilo, Hawaii