Macadamia trees belong to the genus Macadamia
which contains four species of tree or shrub in the family Proteaceae which are grown for their edible seeds (nuts). Cultivated macadamia is a derivative of the subtropical species' Macadamia integrifoia
and Macadamia tetraphylla
and their hybrids. Macadamia plants naturally have multiple stems but are trained to a central leader system (single stem) in plantations. The leaves of the trees are oval or oblong and are arranged in whorls of 3 or 4 depending on variety. The tree produces creamy white or pink-red flowers on racemes of 100–300 flowers and a rounded fleshy fruit up to 27 mm (1 in) in diameter. The flesh covers a single spherical or elliptical seed (nut) with a white or gray kernel depending on the variety of tree. Macadamia can reach a height of 20 m (65.6 ft) and have a commercial lifespan of 40–60 years. Macadamia may also be referred to as Queensland nut or Australian nut and originates from Australia.
Macadamia tree and fruit
Macadamia leaf close-up
Macadamia with fruits
The kernels of the macadamia nut are mainly consumed as a snack food. The kernels from M. integrifolia
and M. tetraphylla
can be eaten raw from the shell while the remaining Macadamia species contain toxins which must be removed prior to consumption. Macadamia may also be used in baked goods and confectionery.
Macadamia trees originate from the sub-tropical rainforests of Australia and therefore tend to grow best in areas of high humidity and high rainfall. They will grow optimally at average annual temperatures between 20 and 25°C (68–77°F) in a deep, well draining loam and sandy loam with a pH between 5.0 and 5.5. Trees should not be planted in heavy clays. It is recommended that macadamia only be planted in frost free areas, young trees will be killed by temperatures of -6°C (21°F) although older, established trees can tolerate some light frost.
When planting numerous macadamia trees in a plantation, consideration must be given to drainage, irrigation, row alignment and wind protection. Planting distances tend to be site specific and are dependent on factors such as the macadamia variety, soil and rainfall. In general, macadamia trees are planted 4 to 8 m (13-26 ft) apart within a row, allowing 7 to 11 m (23-36 ft) between rows. Young trees are planted in Spring and Fall by digging holes which are 45 cm deep and 60 cm in diameter. Once the tree is in position, the hole is backfilled with soil. it is recommended that some compost and rock phosphate be added to the backfill at time of planting. The soil around the tree should then be gently tamped and the tree watered. Staking will help to protect the young trees from wind damage. Grafted trees usually begin to produce fruit 3 years after planting.
General care and maintenance
During the first four years after planting, animal manure and a thick layer of organic mulch can be applied around the trees year round. after this period, the amount of manure added should be decreased to prevent the trees being provided with too much nitrogen and being deficient in potassium. In the first 4 years following planting, vegetative growth should be stimulated through pruning. Once the leading shoot has reached a height of approximately 80 cm (31.5 in) it should be clipped to stimulate branching. Established trees are usually pruned in the Spring when old, unproductive and diseased branches are removed from the canopy. Any nuts remaining on the tree should also be removed. Mulches are best applied after nut harvest to allow the material to decompose before the next crop. Avoid piling mulch up against the trunks to prevent disease.
Grafted trees will begin to produce a crop 3 years after planting but it is not until the 7th year that the crop will be of any economic significance. Trees will give maximum yields 15 to 20 years after planting. Nuts should be collected when they fall from the tree and this should be done every 1-2 weeks during the nut producing periods. In commercial plantations, nuts can be collected mechanically or by hand.
Augstburger, F., Berger, J., Censkowsky, U., Heid, P., Milz, J. & Streit, C. (2000). Macadamia nuts. Naturland e.V. Available at: http://www.naturland.de/fileadmin/MDB/documents/Publication/English/macadamia.pdf. [Accessed 16 February 15]. Free to access
CABI Crop Protection Compendium. (2014). Macadamia integrifolia (macadamia nut) datasheet. Available at: http://www.cabi.org/cpc/datasheet/32791. [Accessed 16 February 15]. Paid subscription required
Cooke, T., Persley, D. & House, S. (2009). Diseases of Fruit Crops in Australia. CSIRO Publishing Ltd. Preview available at: http://books.google.com/books?id=GepY5GLJjHIC&pg=PA149&lpg=PA149&dq=macadamia+anthracnose&source=bl&ots=Mj7sLv9lYO&sig=uyjTIu6PbY6v_cnzN7RNAuBQ4oE&hl=en&sa=X&ei=n4KwUor8FYHLsQSK_oGACw&ved=0CHUQ6AEwDQ#v=onepage&q&f=true. [Accessed 16 February 15]. Free to access
Duke, J.A. (1987). Macadamia integrifolia Maiden & Betche, Macadamia tetraphylla L. Johnson. In Handbook of Energy Crops. unpublished. Available at: http://www.hort.purdue.edu/newcrop/duke_energy/Macadamia.html. [Accessed 16 February 15]. Free to access
Teviotdale, B. L., Michailides, T. J. & Pscheidt, J. W. (eds) (2002). Compendium of Nut Crop Diseases in Temperate Zones. American Phytopathological Society Press. Available at: http://www.apsnet.org/apsstore/shopapspress/Pages/42848.aspx. Available for purchase from APS Press
Quinlan, K. (2007). Organic macadamia growing. NSW Department of Primary Industries. Available at: http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0020/191144/Organic-macadamia-growing.pdf. [Accessed 16 February 15]. Free to access
Common Pests and Diseases
Category : Fungal
Black lesions on leaves and fruit; soft black lesions on husks of nuts followed by decay of nuts on the ground; senescent mature nuts may remain attached to the tree; shells of infected husks may turn brown-gray in color but the kernel inside remains unaffected
Avoid stressing trees by providing them with adequate irrigation and fertilization; prune out dead or dying limbs from tree canopy to reduce levels of disease inoculum; control insect pests to prevent stress to trees
Initial symptoms of the disease are chlorotic to yellow flecks on the husks which enlarge and develop tan brown centers; lesions coalesce and develop diffuse bright yellow halos; lesions may be covered in powdery gray spore masses during periods of wet weather; lesions mature into tough, woody spots; circular tan spots may develop inside the husk but shells and kernels remain unaffected
If variety of macadamia being grown is susceptible to the disease then an appropriate protective fungicide should be applied to the fruits when they are approximately the size of a match head; remove and old and diseased husks from the tree to reduce inoculum levels
Small brown spots on flower petals which spread to racemes (flower stalks); infection may affect small flower buds, florets and rachis; racemes turning black and dying; entire raceme may become blighted within a few days; necrotic flowers remain on tree and become covered in fuzzy gray fungal growth
Fungicide application is not usually warranted unless infection occurs during wet weather which can cause severe infections
Slow and quick tree decline
Macadamia tree infected with quick decline with healthy trees
Yellowing or browning of some leaves in the tree canopy; subtle change in color of tree canopy from dark to light green; entire tree turning brown; in final stages of disease, whole tree is brown and distinct from green trees around it; tree death may occur in a few months after initial symptoms have appeared
Remove any dead or diseased trees from the orchard; avoid wounding tree trunks
Category : Oomycete
Phytophthora trunk and stem canker
Nursery trees and young plantations trees are stunted and chlorotic and may be partially defoliated; if lesions girdle the stem then the tree will die; in mature trees, dark discoloration of wood is visible by removing the bark from the trunk of the tree; dark cankers may extend from trunk at soil line to the lower branches of the tree
Plant only disease-free nursery stock; plant trees in well-draining soils on a slight mound to promote drainage and prevent water-logging; avoid wounds to the trunk of the tree which allow fungi to enter; incorporate organic matter into the soil around the tree; apply appropriate protective fungicides to tree trunks prior to wet season;
Category : Other
Brown rat, Rattus norvegicus
Extensive loss and damage of developing nuts or nuts that have dropped from tree; nuts have holes approximately 1 cm in diameter gnawed through shell to gain access to kernels
All food sources for rats should be removed from the orchard, this includes old nuts, nut waste and wild tobacco; remove any rat nests from trees; keep grass around trees mown short and, if possible, maintain a clear mown area around the orchard to deter rats from entering and to aid predators such as owls in detecting and hunting rats