Common name: Avocado

Originating from Central America, the Avocado tree has a natural range extending from Mexico, through Guatemala, to Nicaragua. Nowadays, it is cultivated worldwide for its fruit, eaten mostly as a salad vegetable.

There are numerous varieties of avocado, all of which derive from three main types, referred to as the Mexican, Guatemalan and West Indian. The Guatemalan and West Indian types bear large round, oval or pear-shaped fruit, distinct from the Mexican type, which bears smaller, mostly pear-shaped fruit.

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

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

Leaves are 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 are small, greenish-yellow and in much-branched clusters at the branches' ends. They bloom from winter through to autumn, with the timing varying considerably amongst the different types and varieties. Some come into flower very early in the season, and others quite late.

Fertilised flowers develop into pear-shaped, oval or round fruit, with smooth or warty, green or dark purple skin and weigh from a few grams up to 3 kg (7 lbs), depending on the type and variety. They ripen six to fourteen months after fruit set, depending on the climate, and again the type and variety, becoming soft to the touch, with thin leathery skin enclosing buttery yellow pulp and a large round, brown seed.

Use

Avocados are used in many different dishes around the world. People enjoy their soft, buttery pulp in a variety of ways. In Mexico, avocados are mashed with spices and lime juice to make guacamole, a dish that is now popular in many countries, including the United States. In North America, Europe, Australia, and New Zealand, avocados are most often added to salads, spread on toast, or put in sandwiches. In the Caribbean, they are served as a cool side with soups. In Brazil, avocados are mixed with milk and ice cream to make smoothies.

Avocado oil has a very high smoke point, about 270°C (518°F), so it works well for deep-frying. For comparison, refined safflower oil (from Carthamus tinctorius) has a smoke point of 266°C (510°F), coconut oil (from Cocos nucifera) is around 232°C (450°F), and Olive oil (from Olea europaea) is 210°C (410°F). Avocado oil is also used in cosmetics and is believed to be good for the skin.

For many years, avocado seeds were simply thrown away as waste. In Mexico, a company called Biofase now uses these seeds to make a resin that can be heated and shaped into sheets similar to plastic. These sheets are then cut into disposable, biodegradable utensils like knives, forks, spoons, drinking straws, and coffee cup lids.

When conditions are right, avocado flowers produce abundant nectar. The tree is an important source of honey in places such as the Dominican Republic, El Salvador, Haiti, Martinique, Mexico, and Mozambique. Avocado honey is dark amber, thick, and has a strong flavour with hints of molasses and caramelised sugar. It granulates slowly.

Avocado wood is light to medium in weight, with an average density of about 640 kg per cubic meter (40 pounds per cubic foot). It does not resist rot or decay well. The wood has little commercial value and is not usually harvested where the fruit is grown for food. Pruned branches can be dried and used as firewood.

Health and nutrition

The oil-rich pulp is a good source of energy. It contains high 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 used widely in skin creams and cosmetics. Avocado oil also contains good levels of Potassium and Phosphorus. It is also high in Oleic acid, an Omega-9 fatty acid that helps maintain healthy blood cholesterol levels.

The ancient Mayans used the oil to treat burns and as a pomade (an oil or ointment used to dress the hair).

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

General interest

In Jamaica, river mullets 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 7 to 25°C, annual highs of 17 to 35°C, annual rainfall of 600 to 4000 mm and a summer or winter dry season of 4 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 from 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 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.

In 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 more cold tolerant than the West Indian type. As such, they are the best choice for cultivating in cool-climate regions such as California, Spain, Israel, South Africa, southern Australia and high-elevation areas in the tropics. However, the trees can take 5 or 6 years to flower and bear fruit in cool climate regions. 

Some of the best-suited varieties for locations with occasional minor frosts include 'Bacon', 'Fuerte', 'Hass', 'Holiday', 'Mexicola', 'Oro Negro', 'Rincon', 'Ryan', 'Sharwill', 'Stewart', 'Reed' and 'Zutano'. And the hardiest of these are 'Fuerte', 'Mexicola' and 'Zutano'.

Growing

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

The best results have been grafting cuttings of select varieties onto ordinary seedling rootstock. This propagation technique also facilitates extending the harvest season to be had through grafting different varieties onto the same tree or 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. In comparison, fifteen-year-old trees yield around 250 kgs (550 lbs). Yields of seedling West Indian Avocados vary greatly. Still, 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.

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

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• 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

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