Avocado (Persea americana, Mill. 1768)

Identity

The avocado (Persea americana Mill., 1768) is an evergreen tree in the laurel family, Lauraceae, related to the camphor tree and to bay laurel. One trait sets it sharply apart from these similar-leaved cousins: each avocado flower opens in two stages, female on one day and male the next, a form of dichogamy that prevents a single flower from pollinating itself. Its adult size, 8–20 m (26–65 ft) depending on how it is managed, and its fruit with one large seed make it easy to tell from other ornamental Lauraceae. The authority Mill. refers to the English botanist Philip Miller, who published the name in his Gardener's Dictionary in 1768. ^{1 2}

Behind the word avocado lies the Spanish aguacate, borrowed from the Nahuatl āhuacatl, which originally meant testicle, after the shape of the fruit. Guacamole shares that root: ahuacamolli literally means "avocado sauce." The old English name alligator pear describes the dark, pebbly skin of some fruits. ^{3 2}

Recognition

Orchard trees kept in shape commonly reach 8–12 m (26–40 ft), and lightly pruned or wild trees can grow to 20 m (65 ft). The grayish bark cracks with age, and the crown stays dense and leafy all year, even though some older leaves drop when the new flush appears. ^{3 4}

The alternate, elliptic leaves measure 20–30 cm (8–12 in) long, dark green and glossy above, duller beneath. Depending on the horticultural race, crushed leaves may or may not smell of anise. That makes a handy clue: the Mexican race has a distinct anise scent, while the Guatemalan and West Indian races do not. ^{5 3}

Tiny greenish yellow flowers gather in dense panicles in the leaf axils. The fruit, a single-seeded berry, is 7–20 cm (3–8 in) long and weighs 100 g to 1 kg (3.5 oz to 2.2 lb), pear-shaped to ovoid depending on the cultivar. ³

Useful taxonomy

POWO accepts a single name, Persea americana, with a native range given as central Mexico to Costa Rica. Yet three historical horticultural races shape all the diversity in cultivation: the Mexican race (the most cold-hardy, with thin skin and anise-scented leaves), the Guatemalan race (thick, pebbly skin) and the West Indian race (the least cold-tolerant, with pliable skin and low oil content). ^{1 5}

The three races cross readily, and most commercial cultivars, Hass and Fuerte among them, descend from such hybrids. The stable taxonomic reference remains the accepted name; race and cultivar describe horticultural behavior, not separate species. ¹

Habitat and range

Strictly native from southern Mexico to Guatemala and Costa Rica, the avocado grows in tropical and subtropical forest with a marked dry season. It favors well-drained hillsides and does not tolerate waterlogged soil. ^{1 4}

People have domesticated it for at least 10,000 years, judging by remains found at Huaca Prieta in Peru and in the Coxcatlán cave in Mexico, and it later spread far beyond its original range. Today the avocado is grown from Mexico to Peru, in California, Israel, South Africa and Spain, and even along parts of the French Mediterranean coast. The IUCN lists the species as Least Concern. ^{2 6}

Life cycle and reproduction

Avocado flowering is a rare case of two-type protogynous dichogamy: in type A cultivars, the flower opens as female on the morning of the first day, closes, then reopens as male on the afternoon of the second day; in type B cultivars, the order is reversed. On a lone tree, this offset prevents nearly all self-pollination, because female and male flowers on the same tree never open together. ^{2 5}

Growers pair a type A cultivar (Hass, Reed, Pinkerton) with a type B cultivar (Fuerte, Bacon, Zutano) to encourage the cross-pollination this dichogamous species needs. A seedling grown alone can bloom heavily without ever setting a single fruit. ⁵

Commercial propagation relies on grafting onto seedling rootstock, which fixes the cultivar and brings the first harvest forward to 2 to 3 years, compared with 6 to 8 years for a seedling. The fruit ripens mainly after picking, never truly on the tree, so it can be harvested well before it is eaten. ^{4 5}

Uses and risks

Rich in fat, the ripe fruit is eaten fresh, mashed into guacamole or pressed for cooking oil; the leaves are sometimes used for tea or as a seasoning. Nothing points to a particular risk for people once the fruit has ripened properly, apart from rare allergic reactions. ^{4 3}

Animals face most of the risk. Leaves, bark, pit and unripe fruit contain persin, a toxin that has caused mastitis, edema and respiratory distress in horses, goats, cattle and rabbits, sometimes fatal within just a few hours. The ASPCA lists avocado as toxic to horses in particular, and pet birds are also highly sensitive. ^{7 5 8}

Poorly drained or overwatered soil favors Phytophthora cinnamomi, the pathogen behind root rot and the main threat in orchards; anthracnose and fruit flies round out the common problems. Soil that stays soggy after rain is a more reliable warning sign than leaf symptoms, which show up only once root rot has already taken hold. ⁵

Varieties, look-alikes and related plants

Hass, a type A cultivar with pebbly skin that turns black when ripe, now dominates the world market. Fuerte, a type B cultivar with thinner skin, drove the California boom before Hass. Bacon, Zutano and Ettinger offer better cold tolerance and often serve as pollinators. ⁵

Avocado foliage can be confused with other Lauraceae of similar habit and texture, such as the camphor tree or bay laurel; only the pear-shaped fruit with its large seed settles the question beyond doubt. A pit-grown seedling, often kept as a houseplant curiosity, does not come true to the cultivar of the original fruit and usually takes 6 to 8 years to bear, if it ever does. ^{2 4}

Main sources

• Plants of the World Online / Kew, Persea americana Mill.
• Plants For A Future, Persea americana
• NC State Extension, Persea americana, Avocado
• ASPCA, Avocado toxicity entry
• Purdue University NewCROP / Julia Morton, Fruits of Warm Climates, Avocado
• Wikipedia EN, Avocado
• Wikipedia FR, Avocatier
• Wikidata, Persea americana (Q37153)
• IUCN Red List, Persea americana assessment