Macadamia is a nut-bearing tree originating in eastern Australia, in subtropical rainforests extending from coastal northern New South Wales to southeast Queensland.
Through cultivation, its range has expanded to other parts of the subtropics. Within Australia to just south of the town of Mackay, and outside of Australia to countries such as Hawaii, where it is now a commercially important nut crop.
In natural forests, it may be a small tree up to 15 m (50 ft) tall with a straight trunk up to 30 cm (1 ft) in diameter. On open sites and in cultivation, it is more typically 5 to 10 m (15 to 30 ft) tall with a short, low-branching trunk and a densely leafy pyramidal or rounded crown. The bark is brown, smooth on young trees, becoming slightly rough on older trees.
Leaves are club-shaped, leathery, 10 to 30 cm ( 4 to 12 in) long, bronze-red when they emerge, turning glossy green and have wavy margins that are sometimes toothed or prickly. They are spirally arranged towards the ends of the branches and remain on the tree throughout the year.
Flowers are small, creamy-white, petalless tubes borne in numbers of one to three hundred or more, in flower spikes 10 to 30 cm (4 to 12 in) long, resembling bottle brushes. They bloom in the dry season, from winter to spring in subtropical Australia, the spikes numbering possibly in the thousands hanging from the ends of the branches, almost completely hiding the green foliage from view.
Although the flowers are perfect, having both female and male parts, only a few flowers on each spike develop into fruit. The fruit, when mature, are roundish with a prominent nipple and with smooth, green leathery peel, somewhat resembling a young lime.
The peel surrounds a round nut with a glossy brown shell that is very hard and difficult to crack open. It protects an edible, round kernel with creamy, nutty, oily white flesh. As the fruit ripens, the peel splits, releasing the nut, which falls to the ground.
The fallen nuts are collected from the ground, stored and left to dry for a week before they are eaten or processed. Macadamias are a popular nut, cultivated commercially and sold for eating as they are or in various snack foods and confectionery, either raw, roasted, salted, unsalted, coated in chocolate (Theobroma cacao), baked in cookies or added to ice cream. They also yield an edible oil marketed as a high-end salad oil. More recently, they are being used to make a milk substitute sold under the 'Milkadamia' brand.
The flowers produce enough nectar for honey production to be economical, with yields of 25 to 50 kgs (55 to 110 lbs) per colony per season reported for Australia. The pure honey is light to medium amber, of medium density, slow to granulate and with a rich, moderately sweet flavour reminiscent of butterscotch or malt and caramel.
Macadamia grows naturally and is productive in humid subtropical and tropical mid-elevation climates, generally frost-free areas with annual lows of 11 to 19°C, annual highs of 21 to 32°C, annual rainfall of 800 to 3700 mm and a dry season of 5 months or less, extending to 8 months with irrigation or groundwater.
A more cold-hardy species of Macadamia known as the Rough-Shelled Macadamia (Macadamia tetraphylla) has been cultivated successfully in cooler climates, such as the San Francisco Bay area in California. Most Macadamia cultivated in California is a hybrid cross (Macadamia integrifolia x Macadamia tetraphylla)
Macadamia trees may fail to flower and fruit or do so poorly in areas where the average low of the coldest month is above 16°C (61°F).
New plants are usually produced vegetatively, using cuttings and air-layering (circumposing) techniques. Although new plants are also started from seed, there is great variation in yield and nut characteristics in seedling trees. Vegetatively produced trees also come into bearing sooner, around two years earlier than seedling trees, when three to four years old.
Performs best on deep, rich, free-draining clay and loam soils of a moderately acid to neutral nature, generally with a pH of 5.0 to 7.0, and on sites with full to partial sun exposure. It has poor tolerance to slow-draining or saturated soils and strong winds.
Yields reported for Australia range from 12 to 15 kgs (26 to 33 lbs) of in-the-shell nuts per tree per year. However, yields in Hawaii are significantly higher.
It is listed 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, despite it being widely introduced and cultivated in the tropics. It is assessed as a low weed risk species for Hawaii by the Hawaii Pacific Weed Risk Assessment (HPWRA) project.
The milky sap exuded by part of the plant is high in Hydrogen cyanide (HCN), also known as prussic acid. This poisonous compound should be treated with caution.
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