Plant Profile- Macadamia

When I first read Mark Shepard’s remarkable book “Restoration agriculture” and watched every talk of his I could find I reached out and asked his advice as I was very excited about the possibility of growing chestnuts or hazelnuts. His excellent advice was to focus first on the potential species that are already adapted to your local conditions and start from there. Managing landscapes is difficult enough without trying to re-invent the wheel. Shepard’s main thrust is to grow species adapted to local conditions, and facilitate their reproduction and selection to balance vigour and productivity. Imitate nature and plant many individuals then select out the best and repeat the process.

The macadamia is a rather surprising agricultural oddity. This remarkable nut tree did not exist as a commercial crop only one human lifetime ago. The genus itself has grown in Australia for millions of years, and more recently was eaten occasionally by aboriginals. Of the original four wild species two produce edible nuts two were hybridised to produce most of the commercial plantings around the world. Many domesticated plants are the result of hybridisation between related wild species followed by human selection including potato, wheat and apples. Mixing two or more distinct species creates a hybrid population with very high levels of genetic diversity from which desirable combinations of traits can be selected. We now know that many species of plants, insects and even vertebrates are created by hybridisation events, even modern humans. The ongoing interaction between the wild species and domesticated crop acts as a source of important genetic diversity, allowing further evolution of the domesticated forms.

In our area macadamia represent the ideal tree for zero input agriculture. Mature trees are common in the district and productive even when neglected. Their hard shells make cracking them slow going, but also means that the crop is not readily lost to parrots and rats like many others. When I wish to establish trees I first collect seed from the most productive trees I can access. A quick dunk in a bucket of water allows the blank floaters to be discarded. If I am confident of the quality of the seed I will sow one seed each in a 4 x 4 cm tree tube. They take a few months to germinate, during which time the tray of pots needs to be kept in a box under chicken mesh to stop rats digging them out. As the seeds swell and split they become more vulnerable to attack. Even after germination has commenced the rats can disturb the pots to get at the remaining nut, making it difficult to time removing the seedlings from the protective box before they grow through the mesh. For seed with more uncertain quality I will sow them thickly in group 25 cm pots with chicken wire on top to reduce time wasted making up and protecting individual tubes that end up empty. Shortly after germination I repot these into tubes. This year’s batch had about 60 % germination so I chose the latter path. Repotting at this stage damages the roots and sets the seedlings back a bit. I needed to pinch out the roots on the larger ones to fit into the small pots. Once transplanted they will never have a fully functional taproot, making them less vigorous, less drought tolerant and more likely to be toppled by a storm as they age. As an alternative to establishing in pots I trialled direct sowing seed in my exposed weedy paddocks, getting about a 5 % germination rate. This might seem very low, but consider that the natural germination rate for a tree would be much lower than this. Direct sowing into better conditions with more leaf litter and shade might yield higher seedling numbers. If I had larger amounts of seed plus no access to irrigation water, greenhouses, rat protection, potting soil and plastic pots then direct seeding could be a viable option with practice.

Seedlings are transplanted out once they get to about 10-15 cm tall. These hardy trees tolerate the abuse of being crammed into tiny pots and left to dry out periodically with minimal deaths. I treat them harshly to reduce transplant shock (I think my plants often experience transplant relief to be out of their cramped pots with a source of ground water more reliable than my hose). I avoid fertilising my seedlings as I have found herbivores seem to home in on anything with higher mineral levels than the surrounding vegetation. I prefer to transplant out at this small size since it makes carrying the trees into the field much easier (I can handle 30-40 at a time in buckets) and planting only requires a small hole that can be dug very quickly. By comparison with a typical size grafted tree found in a nursery I could carry maybe four at a time and digging the much larger holes takes a much longer time. I measured, staked out, cleared grass, applied a little manure, dug holes and distributed core flute tree guards and stakes in advance on planting out, focusing on one job per day. The trees establish even better without tree guards if they are under a Tithonia diversifolia as a nurse tree. The planting itself took myself and a friend a morning to complete. The plants got a top dressing of camphor laurel mulch, applied loosely in the tree guard to partially cover the trees and shade them further, gradually exposing the trees to the sun as it settled over a few weeks. The trees also got a single dose of 1 L of water each, applied via an inverted PET drink bottle with two 4 mm holes drilled in the lid to give the water time to percolate. This system of establishment irrigation for transplants allows me to carry about 20 L of water at a time preloaded into the bottles without spilling it on the way. I have two sets of such bottles so I can carry out a full set then retrieve the empties on the same trip. Establishment irrigation means I can continue transplanting trees during dry and cool periods when the added water goes a lot further. During hot and dry times I don’t transplant, and during wet times I don’t irrigate new transplants (but I am often too busy establishing annual crops to plant trees). The small seedlings establish readily. I planted out an orchard of 130 seedlings last spring during a dry spell. At the end of summer I revisited this planting and found only 12 of them had died despite my utter neglect through a dry spring and early summer. I had spare seedlings left over from spring still in the greenhouse so replanted the gaps in about an hour.

I can hear some of you saying that I won’t get any nuts from seed grown trees and should have paid the extra money for guaranteed grafted clones. In my zero input situation the only thing guaranteed with a grafted tree is the price tag. The next most dependable thing is all the extra resources I need to put into managing that grafted tree to stop it dying and taking my investment with it. If I decided to establish the orchard before a severe drought then I could lose the lot no matter how much I pour into them. With virtually free seedlings I can simply let them die and try again next year. The varieties selected for commercial orchards tend to be those that produce abundantly when supplied with drip irrigation and ample fertiliser, things I will not be providing my trees. Grafted trees typically produce sooner but also burn out and die sooner as well. Commercial orchards also prefer to plant large blocks of single varieties so they all drop their crop at the same time, reducing the amount of labour to repeatedly sweep the orchard. For a home grower with no machinery this would be a disaster as the single wave of nuts would overwhelm their ability to harvest. A mixed population that crops over a longer time period is easier to manage instead. And finally on the topic of quality and productivity. Commercial growers prefer nuts to be uniform to make sorting and processing with machines easier. A home grower doesn’t care as much about these things. With productivity I know that half my trees will be above average. So I planted my orchard densely enough that on average more than half of the trees will need to be removed as they develop. Trees showing early maturity will be retained. Trees showing heavy yields will be favoured. Trees showing poor vigour or production will be culled, especially if they have a promising immediate neighbour. Gaps in the orchard will be replanted with seed from my best trees. This means there will be a continuity of different aged trees so the orchard can keep produce indefinitely. By contrast commercial orchards are all planted on the same day and when the trees reach a certain age are all ripped out together as well.

The final point is what to do with macadamias beyond plain production. The genetics that has been developed to date is a remarkable step forward on the wild species, however it does lack genetic diversity and in time the crop will probably suffer increasing levels of pests and diseases. As such I am planning to source some local diversity of the original wild species to cross into the more selected lines and see where we end up. I still have a lot of potential space to plant with this remarkable plant. With correct spacing it may make a useful understory tree combined with bunya nuts, a Gondwanan equivalent of starchy chestnuts and oily hazelnuts. Both of these local trees appear to be tolerant of goat grazing, opening opportunities for a diverse system combining starch, oil and dairy. Unlike fruit crops it is quite easy to store substantial quantities of macadamia nuts through the winter and spring, providing a valuable source of fat and protein through the chilly time of year. The biggest take away lesson from the story of the macadamia is to wonder what other amazing crops are right under our noses, waiting for the chance to team up with humans to conquer the world together.

Group planted seedlings with protective mesh

Macadamia seedlings ready for potting on. These could be planted out but would require a little TLC or better conditions than our current dry spell to do well.

Freshly potted macadamia seedlings in small tubes, due for planting out in late summer once rains arrive.

A three year old seedling around 1 m tall, planted and forgotten yet thriving

A surprise self sown seedling in my flower beds, a sure sign of a species well adapted to local conditions.