Macadamia tetraphylla Johnson

Macadamia tetraphylla is a small to medium sized, densely branched tree, up to 18 m with a diameter of 45 cm (dbh). The outer bark greyish-brown, smooth or finely wrinkled, with numerous cream horizontal lenticels. Branchlets brown to greyish-brown, young shoots hairy. Leaves in whorls of 4, simple, the margins always pricky-toothed with 33-40 teeth on each side, oblong or oblong lanceolate, 7-25 cm long, 2-5 cm wide, abruptly rounded to a short sharp point at the tip, rigid in texture. Inflorescence axillary or on the branches, 10-25 cm long; flowers creamy pink to mauve, 10 mm long. Fruit 20-35 mm in diameter, greyish-green, turning brown. Seed 15-30 mm long, pointed at the apex and warty. The name tetraphyla is derived from tetra-four phylon-leaf, in allusion to the grouping of leaves in whorls of four.

Flowers appear August –October and fruits ripen in January.

Soils are well-drained, fertile red loams or alluvia derived largely from basic igneous rocks such as basalt.

M. tetraphylla is found on sub-coastal low hills and plateau, basaltic shelfs, rocky slopes, ravines, rocky off-shore islands and headlands and in warm, protected fertile valleys of coastal river systems. It is restricted to eastern Australia and extends from extreme north eastern New South Wales to overlap with M. integrifolia in southern Queensland. It occurs in warm sub-humid and humid zones. The average number of frosts vary from 1-4 annually. It withstands mild frosts but only for a short period. Rainfall is distributed through out the year, with a summer maximum. Associated vegetation include: Argyrodendron trifoliatum, Dysoxylum muelleri and Castanospermun australe. In the drier notophyll vineforest dominant genera are Ficus, Streblus and Casine.

Detailed horticultural investigations began in Hawaii in the 20th century with material from Australia in 1892. Satisfactory methods of silviculture (propagation and management) have so far been developed and it is now grown in commercial scale in many countries. For instance in Kenya, it was introduced in 1946 in the central highlands where its systematic cultivation began in 1953.

Budding and grafting are the preferred techniques used. The rootstocks are usually seedlings 9-12 months old, produced by sowing freshly harvested, de-husked nuts of selected cultivars. The rootstocks are soaked in tap water for 24 hours in seed boxes, containing coarse river sand. Grafted trees come into production in 3 years.

Pruning is necessary to remove excessive side branching as the trees are prone to breakage and wind-throw on exposed sites. Weed control and fertilizer application may be necessary before the canopy closes. A light crop can be harvested in 4-5 years after planting. Where initial spacing is 14 x 4 m, yields of 6 tons/ha/year can be expected. Macadamia trees are expected to give 40-50 years of productive life.

There are a number of economically important pests attacking flowers, fruits, foliage and twigs. Fungal diseases include Phytophthora cinamomi, husk spot, blossom blight and husk rot.

Mature seed may be stored at 1-2 deg C. for several months in airtight containers.

Shade or shelter: M. tetraphylla is beautifully proportioned with handsome foliage, making it a fine shade tree. Intercropping: In Kenya, it has been inter-cropped with coffee and food crops without affecting the yield of these crops (ICRAF, 1992).

Food: Raw kernels (macadamia nuts) are used either alone or in a wide range of confectionery and processed foods. Macadamia oil is also produced. Hawaii dominates the macadamia industry followed by Australia and Costa Rica. The nuts have an excellent flavour, containing up to 76% colorless oil, suitable for human food; the residual cake, after crushing for oil, contains 35-36 % protein. The nut has no starch. Fancy pastries, candies and ice cream, have been made from it. It has the advantage of retaining texture and flavour without becoming stale when used this way. Fuel: Macadamia shell may be used as fuel, generating sufficient energy to dry wet, in shell nuts. Timber: The wood is reddish, hard and tough, attractively marked, used in small turnery jobs. The timber is not generally exploited. Other products: The decomposed husk is used in potting soils and the ground shell supplied to the plastic industry.