Macadamia: A Basic Understanding

by Thompson H. "Tom," Cooper

As a consultant and as a nurseryman and grower of Macadamia trees, I frequently have noticed the shortage of correct information about these trees.

The Macadamia nut tree is indigenous to the eastern shore of Australia,between 25 degrees south latitude near Maryborough, Queensland territory (where M. integrifolia is predominant) and 31 degrees south latitude near Coffs Harbour, in the territory of New South Wales (where M. tetraphylla makes up the forest stand).

Macadamia was introduced to Hawaii by W.H. Purvis and by some well meaning person or persons to California about 1888. There are at least four or five of those trees still in existence here, and after 107 years, they still are producing.

Real development of Macadamia in Hawaii commenced about 1930, and in California after World War II, about 1950. I estimate that in California there has been planted between 1,500 to 2,000 acres, while in Hawaii over 24,000 acres.

Definition: the CLASS ANGIOSPERMAE

(flowering or fruit bearing plants)

Subclass: Dicotyledoneae

Order: Proteales

Family: Proteaceae--Macadamia For purposes of Horticulturists Macadamia have been assigned the following although there is but one specie and that is Macadamia.

Edible Species: integrifolia, tetraphylla

Inedible Species: ternifolia

Varieties:

The varieties are given names or research numbers assigned by different groups or individuals. For example HAES-344 (Hawaiian Agricultural Experimental Station variety #344 with an assigned name of 'Kau').

In M integrifolia, M. Tetraphylla, and their hybrids, there are many sexually propagated varieties or cultivars. "Sexually propagated" is the result of planting a seed which attains varietal status through selection. Accordingly, asexually propagated varieties are ones which are grafts, clones, or rooted cuttings of the sexually propagated varieties.

An F1 hybrid is the result of cross-pollination of the above two species, and has a mixture of their characteristics. It is not considered another species. The progeny of a cross of an F1 and an F1 is called an F2. All Macadamia trees are extremely heterozygous (carrying both dominant and recessive characteristics). Nurserymen and other propagators of large numbersof trees commonly do not use Fl seeds as a source of rootstock becauseof lack of uniformity in the seedlings and possibly unwanted characteristics with some of the gene combinations. In practice, the Hawaiian industry prescribes the use of integrifolia rootstock and to graft onto it integrifolia. In California, we have prescribed the use of tetraphylla rootstock because if its precocity, and since generally both species and hybrids can be grafted to it with good results. Unfortunately in time this practice was proved false economy in the preparation of these trees as the tetraphylla rootstock was very sensitive to winter cold and many trees of tetraphylla selection were terminated or severely damaged in the California winters.

Although trials have been limited, integrifolias that have been tried in California do not make good rootstock for grafting tetraphylla scions onto. The phrase "graft incompatibility" is used. The graft may not take at all or what you see are trees that prosper for awhile then (1) commence a dying process usually at the terminal buds and the tender leaves which are directly below, or (2) exhibit much chlorosis or yellowing, and, in general, little growth expansion. Sometimes it is the cause of overgrowth above the graft at the union. Negative genes can cause similar characteristics so propagators need to exercise care and be critical of all plants they produce in order to deliver high quality final product to the public.

The differences of the two species are distinct. Integrifolias have petioles (leaf stalks), tetraphylla leaves are sessile (no leaf stalks). Hybrids generally have short petioles. The integrifolias have three leaves at a node. Tetraphyllas have four leaves, and occasionally two to as many as eight at a node. Hybrids vary generally from three to five. Tetraphyllas have leaves with many spines along the edges. Integrifolia leaves are entire to as many as fourteen spines. Leaves in hybrids are generally more spiny than in integrifolia but less than in tetraphylla.

A raceme is a flower cluster in which flowers, each with its own pedicel,are spaced along an axis. The proximal flowers bloom first.

The color of the flowers has some consistency with species but is still variable. Integrifolia flowers usually are white to cream colored.Tetraphylla flower-color ranges the spectrum from red to cream and yellow beige to almost brown. Hybrid flowers are white, cream, pink or red.

The fruit produced by a Macadamia is a one-seeded follicle. The husk is the wall of the ovary and inside is the seed. The shell is the outer seed coat. Inside of the shell is the embryo or nutmeat. The shells of integrifolia are spherical and generally almost smooth. The color is brown and often with a lighter mottling. Tetraphylla shells are ellipsoidal, almost football-shaped. The surface varies from slightly rough to very coarse or pebbled. The shell colors are similar to integrifolia. Shell thickness in both species vary from paper thin to more than 5 millimeters. Opinion exists that thickness is affected by altitude and exposure to ultraviolet light, but I feel, from my observations, that it is more closely related to genetic variation. This is not to say that shell thickening cannot be induced. In Hawaii in the early 1980s, a drought caused shell thickening in orchards without irrigation, and smaller nutmeats. With the installation of irrigation two years later the shells were normal in size. Thick and thin shells each have good and bad points. For example, thick shells usually have less nutmeat, but more protective integrity to the meat quality until shelled; thin-shelled nuts are assumed to be easier to enter, for insects as well as humans. A thin shell often causes splitting open and oxidation of the nutmeat. Additionally, thin-shelled nuts even though dried carefully tend to squash in automated cracking machines while thicker-shelled (particularly integrifolia nuts) crack the easiest in mechanized crackers.

Nuts that do not drop naturally are referred to as stick-tights, and these sometimes open and pregerminate while still attached to the tree, this makes the nutmeat inedible (cyanogenic, and bitter). Nutmeat color in both species is white or cream or slightly cream-yellow.

The flavor is affected by carbohydrates and essential oils. Integrifolias are bland, the hybrids generally have the best Macadamia flavor, and Tetraphyllas are the sweetest.

The tree shape is variable. It may have an umbrella shape at maturity as in the 'Cate' cultivar; columnar like the 'Pahala,' and 'Smooth Queen'; pyramidal like the 'Kau', 'James,' and 'Beaumont;' and broad like the 'Kakea'and 'Keahou.'

Most Macadamia trees start bearing in three to five years. The length of time it takes is not related to production at a later age. Unfortunately many people think that an early bearing tree will always produce more nuts.

In all cases the first year's production will probably not be more than a pound of nuts in-shell. This will increase through age to numbers that depend on the variety, the care and the location of the tree. Some integrifolias have been reported to bear 200 pounds of in-shell nuts, but common producing varieties in Hawaii in large acreage average about 65 LB. per tree.

Many of the tetraphyllas in California are alternate-bearers,with one year a good crop, and the next almost half the previous. In general tetraphylla trees produce less nuts than integrifolias and hybrids.

Since alternate-bearing is common in Macadamia one must use the measurements of many trees of a species and variety in order to gain production averages that are meaningful.

Maturity of Macadamia trees occurs at different times. Maturity meaning when the tree reaches maximum size and its average annual production levels off, it varies with variety and location. Certain Hawaiian developed trees mature in about 11 years while the same varieties in Costa Rica mature in about 8 years. In California, many of the varieties do not mature in less than15 years.

Mature nuts generally drop to the ground. There are a few varieties which require pick-harvesting. Although two to three months is common,the length of time of nut-drop is extremely variable. Shaking is done in some trees toward the end of the drop cycle and chemical release agents such as ETHREL are also used.

The structural wood (the harder inner wood cells that support the tree) is generally hard and brittle. One should choose trees which are more upright and have a central leader. Trees which are not developed or pruned correctly in early years tend to be multi-trunked and, because of their brittleness, often split, break and may have to be replaced.

There was an attempt by Dr. Zentmeyer at the University of California,years ago, to induce the fungus Phytophthora cinnamomi, commonly called avocado root rot, into the two species of Macadamia. Of 200 trees inoculated, two integrifolias and one tetraphylla were infected causing a canker on the trunks, but no effect to the roots, and, therefore,were put on the list of plants susceptible to P. cinnamomi by the University of California. Unfortunately, this has caused conflicting opinions and interferes with decisions in what to plant, when for all practical purposes Macadamia are totally resistant to the fungus.

The 2 species of edible Macadamia were found in Australia in distinctly different climates although close in proximity. An integrifolia tree is considered a tropical species. It grows best in an area near the equator but no further away than the TROPIC OF CAPRICORN, 20 degrees latitude.Tetraphylla trees supposedly grow best in subtropical zones, those areas away from the equator and past the two tropic lines but no further than the 35th latitude. This is an oversimplification, but is basically true and obviously there are exception to this conclusion. To be more specific, integrifolia trees prefer a climate which has the following parameters: temperature in the range 1OC-30C (50F-86F), is considered suitable with ideals of 20C-30C (68F-86F). Included in this is humidity in the range of 40 to 80+ percent. Tetraphylla trees could be called temperate zone trees. They need temperatures that are never hot nor cold. These ranges would be 7C-25C (45F-77F) , with optimums of 12C-26C (54F-79F). Californians mistakenly grow these tetraphyllas as ones that would best tolerate the cool to cold winters we have, when in fact integrifolias which do better in a warmer zone will tolerate cold weather better than tetraphyllas. Problems occur in production, though, because integrifolias bloom in our mid-winter season and in years when it is extremely cold flower production is low and a poor crop is the result. Again there are exceptions, but this is common. Many hybrid varieties are quite similar to integrifolia and prosper better as their flowering period is in April-May and well past the cold periods. This may be why hybrids are most prolific producers in our subtropical zone.

It is important to remember, that in many of the areas where Macadamia are grown in California, temperatures may range from as high as 113F to as low as 15F. Humidity commonly ranges from 10% to 100%.

Tetraphylla was not the choice of species by the Hawaiians, and though it has been tried here in California, the varieties grown here are showing the same characteristics that caused the Hawaiian industry to reject them. Primary reasons: General inconsistency, alternate bearing, small and large nuts and nutmeats, thin and thick shells, excesses of carbohydrates making processing difficult. It is important to note that roasting is almost mandatory in tetraphyllas in order to convert the chemistry of the nutmeat into one that has a longer shelf life once it has been removed from the shell. Oxidation happens rapidly in nutmeats that have been removed from the shell. Also characteristics such as ragged trees with poor shapes, much susceptibility to heavy wind damage, and cold sensitivity have added to the reasons for tetraphyllas being rejected.

In choosing a variety to grow, popularity should not be your first consideration. Site, climate and soil type are the important factors. Talk to someone who knows Macadamia trees before doing anything; they can save you much time and money.

I am amazed at the versatility of this Genus Macadamia and I continue to grow in the understanding of it and its management. As I have remarked to many persons in the past, we who work with this plant are hesitant to make too many "final" comments or conclusions about it because just about that time, the trees will make a liar of you and do something else.

Addendum:

Hawaiian researchers and growers formulated the position that the correct growing areas for Macadamia were a minimum of 600 feet above sea level, and a maximum of 1800 feet. Many other persons in the world took this as a formula and planted their orchards within the same parameters.

In my travels particularly in Central America I have met growers that have violated these parameters, usually because their property was at some other elevation. It is interesting to note that Macadamia trees are growing and prospering in the mountainous parts of Mexico where the pine trees start, and higher, up to7,000 feet. I see production in many parts of Mexico at 6,000 feet with a good annual yield. Most of these trees are hybrids, or as the Mexicans call them criollas.

I am working with growers there, and introducing many new varieties for them to use as a comparison of best varieties to plant.

New growers or interested persons should remember what the Hawaiian growers did was correct for Hawaii but not necessarily correct for other places in the world. There are too many microclimes in this world and each area should be studied before introducing major plantings.