Macadamia Nut Tree

Commercially, only M. integrifolia and M. tetraphylla and their hybrids are important. They are very similar to each other and botanically very closely related to a third species, M. ternifolia F. Muell., which produces a small, bitter kernel unsuitable as a table nut. M. integrifolia is commonly referred to as the "smooth shell" species. The fruit consists of a white kernel, high in oil content (72% oil and 4% sugar when dry), very uniform, and of excellent quality. It is enclosed in a round, hard shell about 1 inch (2.5 cm) in diameter surrounded by a smooth, bright green pericarp (husk). The leaves are stiff, oblong to lanceolate, 4-10 inches (10-25 cm) long in nodal whorls of three, rarely four, and either light green or bronze when young. The small, perfect white flowers are borne in racemes 4-8 inches (10-20 cm) long. Only a few flowers in a raceme will set fruit. M. tetraphylla is called the "rough shelled" species because of the pebbliness of the shell's surface. The husk is somewhat spindle-shaped, grayish green, and covered with a dense, white pubescence. Kernels have a grayish base, are darker in color and more variable in quality than those of M. integrifolia. The oil content averages 67% in the dry nut, with 6% to 8% sugar. The leaves are characteristically sessile and serrated along the margins, in whorls of four at the nodes. The flowers are pink and in racemes 6-18 inches (15-45 cm) long. Cultivars which are hybrids of the two species possess characteristics of both, and the quality of their nuts compares favorably with that of M. integrifolia. The trees of both species are tall and spreading, reaching 60 feet (20 m) or more in height. The wood is hard and brittle. Exposed bark sunscalds very easily. Macadamias are well adapted to warm, subtropical conditions. Mature trees can withstand winter temperatures of as low as 25-26°F (3-5°C) for short periods with minor damage to the foliage. However, young trees and foliage are very tender and are killed by temperatures very near freezing. Temperatures below 28°F (-2°C) cause damage to flowers and young fruit and reduce production. In the tropics, macadamias are better adapted to medium elevations of 2100 to 3600 feet (700 to 1200 m), but in Hawaii, macadamias are not planted commercially above 2500 feet (800 m). Although the plant is quite resistant to drought, supplemental irrigation is very important, particularly during the flowering and fruit setting season. Severe moisture stress results in considerable drop of young fruit.