Persea americana

Common name: Avocado

Other common names: Avocado pear, Alligator pear, Butter pear, Pear, West Indian avocado

Names in non-English languages: Spanish Thailand Portuguese German

Description

Originating from Central America, the Avocado tree has a natural range extending from Mexico, through neighbouring Guatemala to Nicaragua, but is today widely cultivated around the world for its fruit, which is eaten as a salad vegetable.

There are many varieties of Avocado, derived from three main types commonly known as the Mexican, Guatemalan and West Indian types. The Guatemalan and West Indian types bear large round, oval or pear-shaped fruit, distinct from the Mexican type which bears smaller, typically only pear-shaped fruit.

The three types also differ in their climates requirements, the West Indian type growing best in tropical lowlands areas and the  Mexican and Guatemalan types in tropical highland, subtropical and warm-temperate areas.

Trees in the wild may attain heights up to 20 m (65 ft), though in cultivation they are more typically 5 to 15 m (16 to 50 ft) tall with a short, stout trunk supporting a wide-spreading umbrella-shaped crown. The bark is grey or brown, slightly rough and fissured.

Leaves elongated oval with tapered ends, 10 to 20 cm (4 to 8 in) long, dark glossy green and prominently veined on top, underneath dull grey-green. They are alternately arranged along the ends of the branches and remain on the tree in all seasons.

Flowers small, insignificant, greenish-yellow and held in much-branched clusters arising at the ends of the branches. They come into bloom from winter through to autumn, their timing varying considerably amongst the different types and varieties thereof, some coming into flower very early in the season and others quite late.

The fertilised flowers are followed by, depending on the type and variety, fruit that are pear-shaped, oval or round, with skin that is smooth or warty, green or dark purple, and weight from a few grams up to 3 kg (7 lbs). They ripen six to fourteen months after fruit-set, depending on the variety and climate conditions, becoming soft to the touch, with thin leathery skin surrounding buttery yellow pulp and with a large roundish seed at the centre.

Use

The soft, ripe, buttery pulp is eaten in different ways around the world. In Mexico, it is crushed and mixed with spices and lime juice to make Guacamole, a dish now also widely popular in the United States. In North America, Europe, Australia and New Zealand it is eaten in salads, spread on toast and incorporated in sandwiches. In the Caribbean, it is commonly eaten as a cooling accompaniment to soups. And in Brazil, it is blended with milk and ice cream to make smoothies.

A pale yellow, edible oil is expressed from the pulp of fruit rejected by the fresh fruit trade, usually for being over-ripe, blemished, malformed, misshapen or of the wrong size.

The oil is cold-pressed, meaning it is expressed from the pulp using an unheated process. This yields from 10 to 30% of oil by weight, depending on the type (Mexican, Guatemalan or West Indian), growing conditions and the ripeness of the pulp. Its taste is described as subtle, with a delicate, buttery feel in the mouth and without the pungent notes of most cold-pressed salad oils.

Avocado oil has one of the highest smoke points of any cooking oil, at about 270 °C (518 °F), which makes it also suitable for deep-frying. By comparison, refined Safflower oil (from Carthamus tinctorius) has a smoke point of 266 °C (510 °F), Coconut oil (from Cocos nucifera) around 232 °C (450 °F), and Olive oil (from Olea europaea) only 210 °C (410 °F). Avocado oil is also sourced for use in cosmetics, as it is reputed to have beneficial effects on the skin when applied.

The flowers secrete abundant nectar under favourable conditions and it is reported as a major honey plant in the Dominican Republic, El Salvador, Haiti, Martinique, Mexico and Mozambique. Avocado honey is dark amber, somewhat resembling cane syrup, thick, strong-flavoured with a hint of molasses and caramelised sugar and is slow to granulate.

The wood is light- to medium-weight, averaging around 640 kgs per cubic meter (40 lbs per cubic ft), and has low natural resistance to rot and decay. It has no special uses or any commercial value and the tree is not usually felled in areas where the fruit is eaten and has economic importance. After pruning, the branch-wood can be dried and used for firewood.

Health and nutrition

The oil-rich pulp is a good source of energy and contains good levels of essential vitamins, particularly the water-soluble vitamins B (Folate) and C, and fat-soluble vitamins A, E and K. Because fat-soluble vitamins are absorbed through the skin, the oil is widely used in skin creams and cosmetics. Avocado oil also contains good levels of Potassium and Phosphorus and is high in Oleic acid, an Omega-9 fatty acid that helps maintain healthy blood cholesterol levels.

The ancient Mayans used the oil for treating burns and as a pomade (an oil or ointment used for dressing the hair).

The seed contain 1 to 2% of a brownish oil extracted by cold-pressing. This is used topically for treating dry skin and skin blemishes.

General interest

In Jamaica, river mullet are caught on small hooks baited with pieces of semi-ripe avocado.

Climate

Avocado trees grow naturally and produce good quality fruit in humid warm-temperate, subtropical and tropical climates, generally frost-free areas with annual lows of 8 to 25 °C, annual highs of 18 to 35 °C, annual rainfall of 900 to 4000 mm and a dry season of 5 months or less, extending to 12 months with irrigation or groundwater.

In its native Central America, trees of the West Indian type occur at elevations near sea level up to 750 m (2460 ft), where the average low of the warmest month is 19 °C (66 °F) or higher. This suggests that varieties of this type are unlikely to thrive at elevations above, for example, 350 m (1150 ft) in Hawaii, 730 m (2395 ft) in Jamaica or 960 m (3150 ft) in Colombia, because of too-cool temperatures.

By contrast, Guatemalan and Mexican types occur in Central America at elevations from 900 m to 2500 m (2950 to 8200 ft), where the average low of the warmest month ranges from 10 to 18 °C (50 to 64 °F), making them much more cold tolerant than the West Indian type. As such, they are the best choice for cultivating in cool-climate areas like California, Spain, Israel, South Africa, southern Australia and high-elevation areas in the tropics.

How to grow

New plants can be started from seed, but seedlings do not come true-to-type, so vegetative propagation is preferred when seeking predictable results, such as high yields and fruit quality.

Grafting cuttings of select varieties onto ordinary seedling rootstock has given the best results. This propagation technique also enables the harvest season to be extended, through grafting different varieties onto the same tree or onto different rootstock in the same orchard. For example, cultivating grafted 'Pollock', 'Waldin', 'Booth 7' and 'Choquette' varieties will extend the harvest season by months over cultivating a single variety, 'Pollock' being the earliest to bear fruit and 'Choquette' being the last to do so.

Avocado trees perform best on free-draining loam, sandy-loam and loamy-sand soils of a slightly acid to moderately alkaline nature, generally with a pH of 6.2 to 8.0, and on sites with full to partial sun exposure. The trees have poor tolerance to slow-draining, clayey or permanently wet soil, or to pronounced and prolonged drought conditions.

Five year old Guatemalan type Avocado trees in well-managed orchards in Australia yield around 80 kg (176 lbs) of fruit per tree per season, while fifteen year old trees yield around 250 kg (550 lbs). Yields of seedling West Indian Avocados vary greatly, but up to 22,000 kg per hectare (19,630 lbs per acre) is possible in mature, well-managed orchards.

Problem features

The leaves are reportedly toxic to ruminant animals such as cows, sheep and goats.

Where it will grow

With irrigation or groundwater

References

Books

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Articles, Journals, Reports and Working Papers

  • Buddenhagen C.E., Chimera C. & Clifford P. 2009, Assessing Biofuel Crop Invasiveness: A Case Study, PLoS ONE 4

  • Hamilton, R.A. 1987, Ten tropical fruits of potential value for crop diversification in Hawaii, Research Extension Series : RES-085, University of Hawaii, Honolulu

  • Morton, J.F. 1964, Honeybee Plants of South Florida, Proceedings of the Florida State Horticultural Society, Vol 77:415-436.

  • Percival, S. & Findley, B. 2007 (Reviewed April 2014), What's in Your Tropical Fruit?, Fact Sheet HN 0708, University of Florida IFAS Extension Service, Gainesville, Florida

  • Watson, B.J., & Moncur, M. 1985, Guideline criteria for determining survival, commercial and best mean minimum July temperatures for various tropical fruit in Australia (Southern Hemisphere), Department of Primary Industries Queensland (DPI QLD), Wet Tropics Regional Publication, Queensland

  • Wenkam N.S. & Miller C.D. 1965, Composition of Hawaii fruits (Bulletin 135), University of Hawaii, Honolulu

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