Macadamia nuts.
Macadamia nuts.
Inset: close-up of macadamia flower raceme.

Macadamia, Queensland Nut

Macadamia integrifolia, M. tetraphylla and Hybrids

Origin:

Macadamia is native to rain forest areas of South Eastern Queensland and North eastern NSW, but is now grown in many other countries including Hawaii, South Africa, Guatemala and Brazil.  Australia is now the major (>40%) worldwide producer, but it was in Hawaii where it was successfully developed into an important commercial crop.

Climate:

It grows well within 34 degrees of the Equator, with optimal temperatures of 13-30°C and well-distributed precipitation of 1200-2300mm pa.  Drier periods are preferred during flowering and early fruit set.  Trees are frost sensitive when young, slightly less so when mature. In Perth there is often some leaf scorch on west-facing leaves (caused by hot dry weather, and fierce afternoon sun).

Plant Description:

Trees are evergreen, long-lived, 14-18m tall but grafted plants less, with a canopy almost as wide if unpruned.  Dense leaves are light green or bronze when young, changing to shiny dark green with maturity.  They can be 10-25cm long, obovate and with an entire or spiny margin, the latter being more pronounced in tetraphylla (MT).  Leaves appear in whorls of 3 or 4 for integrifolia (MI) and MT respectively.  Growth occurs in a series of flushes in warmer months.

Relatives:

Proteaceae Family. This is largely a southern hemisphere Family of trees, shrubs and herbs; some other edible ones are the gevuina and red bopple nuts and the Atherton oak.  There are more than 10 species in the genus Macadamia. Care should be taken with the lesser-known species because they may need special processing to be made non-toxic.

Soils:

Preference for well-drained soils, not overly fertile, pH 5.5-6.5.  They will cope up to pH 7-8 but trace element nutrition may then need attention.  Mulching is beneficial to retain moisture and nutrients, and also to inhibit soil pathogens.

Propagation:

Superior plants are almost universally propagated by grafting; success rates improve if the scion is girdled a few weeks before severing.  Seedlings of MI are usually used as rootstocks; although MT seedlings are more vigorous and resistant to Phytophthora, they often have long term incompatibility problems.  Most orchards in Australia are primarily based on MI and MI x MT hybrids.  Newer DNA fingerprinting techniques have shown that even the so-called pure species of MI and MT have a proportion of their genome from the other type.

Cultivars:

Many of these are identified by number, plus those developed later have their history in the pioneering Hawaiian breeding programs.  In Australia, a large proportion of commercial cvs are hybrids.  Some varieties that you may be able to source are:  MI – 246, 741, 816, A38, H2, Own Choice  and Daddow;  hybrids – A4, A268, A16, Beaumont, Renown, and Guros.  Performance of particular cvs varies greatly with location, environment, climate etc.

Flowering and Pollination:

Inflorescences are long pendant racemes with several flowers in a whorl at each node.  The rachis is 6-250cm long, bearing up to 300 perfect apetalous individual flowers, 10mm long, with 4 petaloid sepals forming a perianth tube.  There are 4 stamens surrounding the pistil which has a single-carpelled ovary, 2 ovules and a 10mm long style.   Usually MI flowers are white and MT pink.  Night temperatures of 12-18C are best for flower induction.  Most inflorescences, terminal or axillary, are formed on 2-3 year old wood in August to October, with mature trees producing up to 8000 racemes.  Flowers are protandrous with maximum pollen germination 3 days after anthesis.  Cultivars have variable self-sterility; of those fruit that are set, only 10% may be self-pollinated.  This is improved with cross pollination.  Bees and other insects are the main pollinators; wind is likely a minor contributor.   

Cultivation:

They should be planted in full sun but will tolerate some shade as their origin is as an understory tree. Although in the Proteaceae, they still need their P (NPK ratios that the plant requires/absorbs not too different to other tree crops, 10:1:8). Once a dry period induces fruit set, they need water for proper maturation. Keep weeds from competing.

Wind Tolerance:

Branches can be brittle in strong winds.  This can be alleviated by selective pruning of weak narrow crotch angle branches and appropriate sheltering.

Pruning:

Leader forms of growth with whorls of  wide crotch angle scaffold branches are preferred and best established while the tree is young.  Inevitably some pruning will be required to control size and height.  The key feature to keep in mind regarding yield is light interception – if this is markedly reduced then yield will decline.

The Fruit:

This is a dehiscent follicle that splits along a single suture in the pericarp (husk).  The extremely hard kernel shell is actually derived from the seed testa.  MI shells are ‘smooth’ and those of MT ‘rough’.  Kernels consist of 75% oils, most of which are the healthy monounsaturated types, good levels of Mg and 13% carbohydrates.

Fruit Production and Harvesting:

Seedlings could take 8-12 years to begin fruiting with a variable quality outcome whereas grafted trees may take less than half this time with more predictable results. Yield will increase steadily over several years.  Fruit set with cross pollination may be as high as 35% but less than 1% develop into mature nuts, even less in higher temperatures.  Nuts take 30-35 weeks to mature from anthesis, with harvest in Mar-Jun.  Mature nuts usually abscise and drop to the ground when they should be regularly collected.  The staggered harvest period is of advantage to the home grower.  They should be de-husked within a few days  and then air-dried for a few weeks before attempting to crack them.  With good management and conditions, a mature tree can produce 20-40kg of nut-in-shell.

Fruit Uses:

Although home growers often eat the kernels as is, flavour is much enhanced when they are roasted, and it is this form that most commerce is based on, either as nuts or in confectionary or bakery items.  Extracted oil is also produced. MI nuts are easier to process than MT nuts which contain more oil and sugar and hence are more prone to go rancid and to scorch during roasting.

Pests and Diseases:

Major pests can be macadamia nut borer and flower caterpillar, fruit spotting bug, rats and the larger cockatoos.  Diseases may include root rots (Phytophthora and other pathogens), blossom blight and husk spot.

Comments:

Macadamia grows well in the south west of WA if moisture levels are managed appropriately.  Only named cvs should be planted if the goal is yield; the more self-fertile the better.  Dried nuts-in-shell keep very well, and  when combined with staggered harvesting the delicious reward is long-lived.  Yield will be improved if there is space to accommodate different cv trees or if single tree grafting is undertaken.  Roasting for the home grower is straightforward (see Recipes section) and well worth the effort.