Common name: Clove tree

Cloves are the dried flower buds of a tree originating in the Moluccas Islands, a small group of islands in the Indonesian Archipelago. According to botanists, it is also the home of the Nutmeg tree (Myristica fragrans).

Nowadays, clove trees are widely distributed throughout the humid tropics but are the most intensively cultivated in Indonesia, India, Sri Lanka, Madagascar and Tanzania. An ancient spice crop, cloves were much valued by the Romans, Greeks and Chinese for their flavouring and medicinal properties.

It is a slow-growing, small to medium-sized tree, up to 20 m (65) in its native habitat, though it is typically 10 to 15 m (32 to 50 ft) tall in cultivation. The trunk is usually straight and slim, up to 30 cm (1 ft) in diameter, and supports a densely leafy crown. On open sites, young trees develop a pyramidal-shaped crown that becomes cylindrical with age as the branches assume a more ascending habit. The bark is grey or pale brown, smooth on young trees, becoming rough and cracked on mature trees.

Leaves are elongated-oval with a pointed tip, bright pink and soft when they emerge, becoming dark glossy green with crimson-coloured leaf stalks. They are arranged opposite along the ends of the branches and remain on the tree in all seasons. When crushed, they have a distinctive clove-like aroma.

The flowers are small and insignificant, consisting of pale yellow stamens amidst four tiny petals, all of which sit atop a tack-shaped flower bud. Buds arise at the ends of the branches, in clusters from a few to up to fifty or more. Green when young, they become pink when mature, just before flowering, and are the clove of the spice trade, their colour change signalling their readiness for harvest.

Flowering appears to be influenced by a change from wet to dry conditions, with bud initiation at the end of the rainy season, transitioning to the dry season, and then flowering occurring some six months later. In areas with two rainy-to-dry-season events per year, such as in Zanzibar, Tanzania, flowering and fruiting follows each event.

Fertilised flowers develop into fleshy oblong fruit about 3 cm (1.2 in) long. Green when young, they become reddish-purple when ripe and have one, occasionally two, small purplish seed inside.

Use

Clove the spice is the unopened flower bud after being thoroughly dried, traditionally under the sun. The pink, mature flower buds are laid out on mats or large Jute (Corchorus olitorius) bags and then turned and dried for about a week. After drying, they become dark brown and have lost about 65% of their initial weight. 

Dried cloves are used as a spice worldwide, either whole or powdered, to impart their unique flavours to sweet and savoury dishes. In North America and Europe, cloves are added whole to stews, soups, and pickling brines for pickling fish, vegetables and meats such as corned beef. In its powdered form, clove is commonly used to spice fruit pies, tarts, cakes, sweet biscuits, bread, puddings, sausage meats, sauces such as tomato ketchup and barbecue sauce and alcoholic beverages such as mulled wine and beer.

Other uses for cloves include blending it with Tobacco (Nicotiana tabacum), as in Indonesia, to make a popular local cigarette. Know as 'Kretek' cigarettes, they have other flavours added, such as extract of Vanilla (Vanilla planifolia) and Coffee (Coffea arabica). Kretek cigarettes are the preferred type for more than 80% of smokers in Indonesia. Cloves are also used to make alcoholic drinks such as bitters and vermouth and blended teas such as chai, which has become increasingly popular worldwide.

Dried cloves yield around 15% of a colourless to pale yellow liquid traded as 'Clove-Bud Oil' on hydro-distillation. It is a fluid oil with a sweet, spicy clove aroma and a warm, almost burning spicy flavour. The food, beverage, and pharmaceutical industries use it extensively as a flavouring agent.

Similar to other spice-derived essential oils, such as Cinnamon (Cinnamomum verum) leaf oil, clove bud oil is used to flavour confectionery, dairy products, baked goods and masticatory products such as chewing gum and chewing tobacco. Clove-bud oil is also utilised in perfumery to give perfumes a floral note and can be found in quality perfumes such as 'Fidji' by Guy Laroche and 'Ysatis' by Givenchy.

The main constituent of clove-bud oil is Eugenol, accounting for around 70 to 90% of the oil by volume. Eugenol is also present in commercial quantities in Clove basil (Ocimum gratissimum) and Allspice (Pimenta dioica) and is mostly synthesised into Vanillin, an organic compound that gives Vanilla its characteristic aroma and taste.

Eugenol is also present in the leaves and stems of the clove tree. On steam distillation, these materials yield 3% and 5% respectively of a colourless to pale yellow oil traded as 'Clove-Leaf Oil' and 'Clove-Stem Oil'. These oils are used mainly as a low-grade substitute for clove-bud oil.

Health use

Clove-bud oil has been demonstrated to have anti-microbial action. Its efficacy is due primarily to Eugenol, the oil's main constituent, possessing strong anti-inflammatory, antioxidant, antibacterial, and antiseptic activities. Clove-bud oil also has a long history of relieving toothache and is a component in many dental cement and filling products. And, because of its pleasing aroma and flavour, it has found its way into many toothpaste, mouth-wash and gargle products. 

Recent scientific studies have shown Eugenol to also exhibit good antibacterial activity against Salmonella enterica, a common bacterial contaminant of beef, poultry, raw milk and eggs. This holds out promise for Eugenol being perhaps used one day as a natural food preservative.

Climate

Grows naturally in humid tropical climates, generally frost-free areas with annual lows of 17 to 25°C, annual highs of 25 to 35°C, annual rainfall of 1400 to 4500 mm and a dry season of 3 months or less.

Irrigation can be used to extend Clove cultivation to areas having a dry season lasting up to 5 months. When grown under drier-than-normal conditions, the trees benefit from a micro-climate where irrigation can be misted over the top of their canopy.

Clove trees are known to flower and fruit at elevations of up to 1000 m (3280 ft) in southern India's Western Ghats region, where the average low of the warmest month is 18°C (64°F) or above.

Growing

New plants can be grown from seed or vegetative propagation using circumposing (air-layering) and grafting techniques. However, seedlings may vary in their characteristics, so vegetative propagation is preferred, particularly in commercial plantations. The seed take from two to six weeks to germinate and should be sown in a free-draining potting mix and tended in a nursery until they are around twelve months old or 50 cm (1.6 ft) tall. 

Clove trees perform best on rich, free-draining loam, sandy-loam and loamy-sand soils of a moderately acid to neutral nature, generally with a pH of 5.5 to 7.5, and on sites with partial to full sun exposure. However, the young plants need shading for the first few years of life to prevent sun-scorching. It is common to plant them under a fast-growing, short-lived perennial such as Banana (Musa acuminata).

Problem features

Clove is recorded as a weed in at least one reference publication. Still, there does not appear to be any record of it anywhere as a serious weed or invasive species.

Books

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• Attokaran, M. 2011, Natural food flavors and colorants, Institute of Food Technologists, Wiley-Blackwell Publishing, Oxfordshire

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• Crane, E., Walker, P. & Day, R. 1984, Directory of important world honey sources, International Bee Research Association, London

• Culbreth, D. M. R. 1927, A manual of materia medica and pharmacology : comprising the organic and inorganic drugs which are or have been recognized by the United States pharmacopeia, 7th ed., Febiger, Philadelphia

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

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• Fellows, P. 1997, Traditional foods : processing for profit, Intermediate Technology, Technical Centre for Agricultural and Rural Co-operation, London

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• Khan, I. A. & Abourashed, E. A. 2010, Leung's encyclopedia of common natural ingredients : used in food, drugs and cosmetics, 3rd edition, Wiley Publishing, Hoboken, New Jersey

• Lis-Balchin, Maria 2006, Aromatherapy science : a guide for healthcare professionals, Pharmaceutical Press, London

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

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• Stewart, A. 2013, The drunken botanist : the plants that create the world's great drinks, 1st ed., Algonquin Books, Chapel Hill, North Carolina

• Weiss, E. A 2002, Spice crops, CABI Publishing, Wallingford, United Kingdom

• Winter, R. 2009, A consumer's dictionary of cosmetic ingredients : complete information about the harmful and desirable ingredients found in cosmetics and cosmeceuticals, 7th ed, Three Rivers Press, New York