Macadamia is a genus of four species of trees indigenous to Australia, and constituting part of the plant family Proteaceae. They are native to north eastern New South Wales and central and south eastern Queensland. Three species of the genus are commercially important for their fruit, the macadamia nut/ˌmækəˈdeɪmiə/ (or simply macadamia), with a total global production of 160,000 tonnes (180,000 short tons) in 2015. Other names include Queensland nut, bush nut, maroochi nut, bauple nut, and Hawaii nut. In Australian Aboriginal languages, the fruit is known by names such as bauple, gyndl, jindilli, and boombera.
Fresh macadamia nut with husk or pericarp cut in half
Macadamia nut in its shell and a roasted nut
Macadamia nut with sawn nutshell, and special key used to pry open the nut
Macadamia is an evergreen genus that grows 2–12 m (7–40 ft) tall.
The leaves are arranged in whorls of three to six, lanceolate to obovate or elliptic in shape, 60–300 mm (2–10 in) long and 30–130 mm (1–5 in) broad, with an entire or spiny-serrated margin. The flowers are produced in a long, slender, simple raceme 50–300 mm (2–10 in) long, the individual flowers 10–15 mm (0.4–0.6 in) long, white to pink or purple, with four tepals. The fruit is a hard, woody, globose follicle with a pointed apex, containing one or two seeds. The nut shell ("coat") is particularly tough, and requires around 2000 N to crack. The shell material is 5 times harder than hazelnut shells, and has mechanical properties similar to aluminium. It has a Vickers hardness of 35.
German-Australian botanist Ferdinand von Mueller gave the genus the scientific name Macadamia – named after von Mueller’s friend Dr. John Macadam, a noted scientist and secretary of the Philosophical Institute of Australia.
Walter Hill, superintendent of the Brisbane Botanic Gardens (Australia), observed a boy eating the kernel without ill effect, becoming the first nonindigenous person recorded to eat macadamia nuts.
King Jacky, aboriginal elder of the Logan River clan, south of Brisbane, Queensland, was the first known macadamia entrepreneur, as his tribe and he regularly collected and traded the macadamias with settlers.
Tom Petrie planted macadamias at Yebri Creek (near Petrie) from nuts obtained from Aboriginals at Buderim;
Nuts from M. jansenii contain toxic amounts of cyanogenic glycosides, The other three species are cultivated in the commercial production of macadamia nuts for human consumption.
Previously, more species with disjunct distributions were named as members of this genus Macadamia. Genetics and morphological studies published in 2008 show they have separated from the genus Macadamia, correlating less closely than thought from earlier morphological studies. The species previously named in the genus Macadamia may still be referred to overall by the descriptive, non-scientific name of macadamia.
The macadamia tree is usually propagated by grafting, and does not begin to produce commercial quantities of seeds until it is 7–10 years old, but once established, may continue bearing for over 100 years. Macadamias prefer fertile, well-drained soils, a rainfall of 1,000–2,000 mm (40–80 in), and temperatures not falling below 10 °C (50 °F) (although once established, they can withstand light frosts), with an optimum temperature of 25 °C (80 °F). The roots are shallow and trees can be blown down in storms; they are also susceptible to Phytophthora root disease.
Macadamia 'Beaumont' in new growth
A Macadamia integrifolia / M. tetraphylla hybrid commercial variety is widely planted in Australia and New Zealand; it was discovered by Dr. J. H. Beaumont. It is high in oil, but is not sweet. New leaves are reddish, and flowers are bright pink, borne on long racemes. It is one of the quickest varieties to come into bearing once planted in the garden, usually carrying a useful crop by the fourth year, and improving from then on. It crops prodigiously when well pollinated. The impressive, grape-like clusters are sometimes so heavy, they break the branchlets to which they are attached. In commercial orchards, it has reached 18 kg (40 lb) per tree by eight years old. On the downside, the macadamias do not drop from the tree when ripe, and the leaves are a bit prickly when one reaches into the interior of the tree during harvest. Its shell is easier to open than that of most commercial varieties.
Macadamia 'Maroochy' new growth
A pure M. tetraphylla variety from Australia, this strain is cultivated for its productive crop yield, flavor, and suitability for pollinating 'Beaumont'.
A South African M. integrifolia / M. tetraphylla hybrid cultivar, it has a sweet seed, which means it has to be cooked carefully so that the sugars do not caramelise. The sweet seed is usually not fully processed, as it generally does not taste as good, but many people enjoy eating it uncooked. It has an open micropyle (hole in the shell) which may let in mould. The crack-out percentage (ratio of nut meat to whole nut, by weight) is high. Ten-year-old trees average 22 kg (50 lb) per tree. It is a popular variety because of its pollination of 'Beaumont', and the yields are almost comparable.
A M. integrifolia / M. tetraphylla hybrid, this is a rather spreading tree. On the plus side, it is high yielding commercially, 17 kg (37 lb) from a 9-year-old tree has been recorded, and the nuts drop to the ground. However, they are thick-shelled, with not much flavor.
The first commercial orchard of macadamia trees was planted in the early 1880s by Rous Mill, 12 km (7.5 mi) southeast of Lismore, New South Wales, consisting of M. tetraphylla. Besides the development of a small boutique industry in Australia during the late 19th and early 20th centuries, macadamia was extensively planted as a commercial crop in Hawaii from the 1920s. Macadamia seeds were first imported into Hawaii in 1882 by William H. Purvis, who planted seeds that year at Kapulena. The Hawaiian-produced macadamia established the well-known seed internationally, and in 2017 Hawaii produced over 22,000 tonnes.
In 2019, researchers collected samples from hundreds of trees in Queensland, and compared the genetic profiles to samples from Hawaiian orchards. They determined that essentially all the Hawaiian trees must've descended from a small population of Australian trees from Gympie, possibly just a single tree. This lack of genetic diversity in the commercial crop puts it at risk of succumbing to pathogens (as has happened in the past to banana cultivars). Growers may seek to diversify the cultivated population, by hybridizing with wild specimens.
Macadamias are toxic to dogs. Ingestion may result in macadamia toxicity marked by weakness and hind limb paralysis with the inability to stand, occurring within 12 hours of ingestion. Depending on the quantity ingested and size of the dog, symptoms may also include muscle tremors, joint pain, and severe abdominal pain. In high doses of toxin, opiate medication may be required for symptom relief until the toxic effects diminish, with full recovery usually within 24 to 48 hours.
^"Macadamia%". Australian Plant Name Index (APNI), Integrated Botanical Information System (IBIS) database (listing by % wildcard matching of all taxa relevant to Australia). Centre for Plant Biodiversity Research, Australian Government. Retrieved 26 April 2013.