Planting an orchard is, in itself, interfering with nature. You’re doing it to produce food … you’re a farmer … manipulating nature to feed more and more animals (and I lovingly include humans in this class). So, where can we manipulate and how far do we go? There are innumerable factors that we can ‘tweak’ to maximise food production:

Yes, even the ones that seem unlikely. As humans, we can do anything, including wind management (wind-breaks), sunlight management (shade cloth) and animal control (fences). It seems there are no limits. But we often manage each element in isolation, focusing too closely on the challenge right in front of us and thereby ending up in an out-of-control situation as our actions impact other factors.

Bottom line – there is SO much still to learn, in SO many areas. It becomes overwhelming. This year, I am going to delve into a few of these areas and bring you insight from top farmers and the best experts. This article is hopefully going to help us understand Water Management in Macadamia Farming.

I went straight to the go-to guys – the Smit brothers; Theunis and Armand. They are young, energetic, enthusiastic and way too intelligent for me (seriously – I had to listen to the recording of our interview over and over again before I felt confident enough to try and put the information down in what I hope is a meaningful way for you to use). I met them in Nelspruit on my country trip this year. That was Feb and they were hoping to have emigrated to Australia by around about this time (end March/early April) but I imagine Corona has put a spanner in those plans …

Anyway, most of what I pass on is from their wisdom, and as two students (although Theunis is a almost a doctor and Armand, probably also on his way) of the topic, it is definitely a worth-while read …

When considering water management on your farm, these are the primary aspects:

  1. ALL the environmental factors. Water is only ONE factor in the tree’s environment – it is inextricably linked to ALL the other factors and has to be managed as such.
  2. Water availability and options in getting it to the tree – water supply, quality and consistency, irrigation system.
  3. The tree itself – age, variety, condition, yield.

Theunis and Armand kicked our discussion off by emphasising the reality of water scarcity across the globe. The truth is that we need to learn to farm with less. Some farmers are accepting of this but too many are continuing as though water will always be freely available. Weather patterns are changing with a distinctive trend towards drought and/or erratic rainfall. Soft, gentle summer rains seem to have been replaced by angry, lashing floods, making consistent water availability more concerning than the volume. There’s no argument that, if we learn to farm profitably with less, we will be better off.

But how much less? How much water does a mac tree need to yield well? There is no standard answer to this question. Location and environment impact a tree’s water usage. Big trees need more than small trees. Different varieties consume at different rates etc etc.

Our experts can help us understand exactly how a mac tree works. As farmers you need to take that understanding and apply it to your context. So, let’s get into the detail:

In the world of domesticated agriculture, it is important to realise that, with a mac tree, we are dealing with a wild beast. In the last edition of TropicalBytes we looked at the History of the industry. Through that, we realise that our South African orchards are only about 2 generations down from the wild macs that grow in the forests of Australia. Unlike citrus, for example, that has been “domesticated” and grown as a commercial crop for about 30 generations (of trees).

What does that mean? Mac trees are essentially still wild, and as such, they are skilful survivalists. They have specialised systems that flourish in the wild. They neither need, nor want, our pampering. They need balanced ecosystems where life is wholistic and natural. Understanding their specific skills will help us get more out of them.

The trunk: Simplest one first. Have you ever compared a citrus graft point to a mac graft point? Notice how the citrus root-stock trunk is far thicker than the trunk to the grafted tree? When it comes to moving water, the citrus tree encounters a bottle-neck at this point.

Now look at your mac graft points – there is no reduction in the circumference of the trunk as the root stock meets the grafted wood. This enables free-flow of water from the roots to the canopy – a major advantage when moving life-giving fluids to where they are needed.

The Roots: The experts stress that it is vital that you get up close and personal with your trees at this point; grab a spade (or your TLB) and go out there … dig a trench at the tree’s dripline, get in and take a look.

You should find a matt of fibrous roots dominating the first 30-40cm of your excavation. Depending on the health of your tree (and your eye sight) you may also find root clusters, more scientifically referred to as proteiod roots. They are typical of the Proteaceae family, to which the macadamia belongs. They are an extreme adaption to phosphorous-limited environments. These are specialized feeding roots – the ones that take water and nutrients into the tree. (NB: these roots do not develop in phosphorous-rich environments. Macadamias are susceptible to phosphorous toxicity and these roots will disappear in the presence of excess phosphorous or anaerobic conditions (i.e. overwatering or water-logged soils). The lack of these cluster roots limits the tree’s uptake of P and it also inhibits the uptake of water and other essential nutrients – so the presence or absence of these roots is a good indication of your soil health in general.

Here are the characteristics of proteiod root clusters:

  • They are shallow – from surface to about 40cm deep (but they can be found deeper).
  • They are extremely thin and delicate
  • They are sensitive and inclined to fungal infections, phosphorous and waterlogging

Although there is also a tap root that extends to deeper levels, and this root also takes up water, its prime purpose is to anchor the tree. In an agricultural context, trying to supply water to this tap root would be a waste of resources. So, we focus instead on these feeding the fibrous roots.

Characteristic Explanation Implication
Shallow In the wild, forest setting, these shallow roots are foraging in the dense matt of rich, decomposing organic matter created by the forest. The importance of mulching our orchards is immediately apparent.
Thin and delicate The root clusters constantly renew and extend abundantly in healthy, damp conditions but, when conditions are too wet or compact or exposed, they will diminish (often fungi develop and kill these fine roots in wet conditions) and reduce the trees feeding capacity, thereby limiting its productivity. Maintain suitable soil conditions – limit compaction, do not clear mulch away from roots, keep soil damp but not wet. Wet, oxygen-starved soils are perfect environments for fungi, such as phytophthora, which can kill your tree.
Excellent foragers The surface area of these roots is HUGE, increasing their foraging returns even when nutrients are limited. Do not over-fertilise, especially phosphorous. If a natural, well-balanced, organically-rich environment is maintained, the trees will find what they need.
Require oxygen These roots thrive in air. Being efficient feeders they can take up water effectively, but only when required. You will do less damage to a mac by under-irrigation (within limits) than you will by over-irrigation

Our experts make the following recommendations when it comes to serving the needs of our mac trees:

  • Less water more often. Shorten your irrigation cycles ACCORDING TO YOUR SOIL TYPE. Focus on keeping the fibrous root zone damp. Do not over-irrigate. Water-logged soil will kill the root clusters and limit the availability of certain nutrients.
  • Protect the feeding cluster roots. By replicating the forest conditions of good (undisturbed) mulch wherein micro-organisms can create healthy food, we will encourage the trees natural ability to feed itself. Healthy cluster roots will ensure the survival of mac trees through adverse conditions such as drought. Any interference in this root zone (mulch removal, sweeping etc.) will impact the work of these roots do.

The leaves: Mac leaves are truly remarkable. They are sclerophyllous leaves which are thick, leathery and built to resist environmental demands that compromise so many other, less-hardy plants. Ever wondered why mature leaves on a mac tree never wilt? Understanding their functionality will help you, to help them, produce better. First, we look at the biology of all leaves:

They have stomata which are little valves. These valves open to take in carbon dioxide and release water and oxygen. This happens through the same stomata. Carbon dioxide is essential for photosynthesis. If this element is restricted, photosynthesis (productivity) is restricted. The functioning or regulation of these stomata is what sets the mac tree apart from most other plants.

Some of you in the sugar cane industry will be familiar with the term ‘Potential evapotranspiration’ which is an indicator of how much plants will transpire (release/lose) water on a particular day. Very dry days, when the atmosphere has low humidity = there will be a high ET (evapotranspiration) indicator. Humid days, when the atmosphere is full of moisture = there will be a low ET indicator. Now most plants, including sugar cane, will release whatever moisture the environment demands so dry days result in dehydrated, wilted plants, unless you irrigate more. But the plants never shut down, they continue to take up water and give it off to the atmosphere as demanded (see diagram below, left).

Mac trees are smarter (or just more adapt to surviving in the wild). They reach a point of evapotranspiration where they say, “no more”. They close their stomata, denying the atmosphere any further water. This has implications that are important for the farmer:

  • Do not irrigate more when evapotranspiration indices are high (when humidity is low).

I have spent time with farmers who have explained how they use their micro-irrigation systems to ‘cool’ or humidify their orchards. And while I thought this was a dam fine idea at the time, our experts say it is not advisable. An orchard is an open environment and, as such, is impossible to cool or humidify enough using micropsrinklers to change the trees’ behaviour. The water will be subject to evaporation (directly from the air, which is wasteful) AND the tree will not be taking up the water in the soil when ET is high. The stomata have closed. The tree has shutdown. Any water supplied will lie around the roots, creating further stress as they drown or rot in the excessive moisture. Rather work WITH your trees and irrigate again when they can use the water (see graph below).

  • Do not expect mac trees to thrive in a very dry area. This is important when evaluating new orchard sites. In a predominantly dry environment, the stomata will spend a lot of time closed. When the stomata close, the tree cannot take in carbon dioxide and therefore cannot photosynthesise. The engine is off. It is in survival mode. Too many days off = poor productivity. So, even if you have all the water in the Orange River, you cannot farm macs in the Karoo.
  • We are specifically talking about humidity here, not temperature. Macs will manage in high temperatures as long as it is in a highly humid environment but they will not thrive in low humidity, even if the temperatures are moderate.

NB: Wind also plays a part in evapotranspiration in that it moves moisture away from the trees i.e.: dry, windy days have an even higher evapotranspiration rate than dry, still days.

By understanding the stomatic activity of mac leaves, we can schedule irrigation better:

Irrigate when the tree is operating i.e.: mid- to high-humid mornings, or nights if necessary. Run short, regular cycles rather than long, infrequent ones. It is very important that you calculate the ideal  length of your irrigation cycle by assessing your unique soil characteristics – see below – it will almost certainly be different for every orchard.

Although it seems counter-intuitive, I hope you now understand why a mac tree, in a dry environment, would actually fare better with less water whereas one in a humid environment would require more water. Another realisation you may have reached is why no one can tell you how much water your trees need without a comprehensive study of your environment (not only humidity but soil and a number of other factors as well) and trees.

By listening to your tree you have now learnt where to water (wherever you found those cluster roots), when to water (when evaporation is low, when the stomata are open and the tree is transpiring* – early mornings & late afternoons when ET is low), and how much to water (just enough to keep that root zone damp, not wet**) and how to conserve water (by mulching the root zone and not irrigating during peak heat periods when evaporation is high).

*The citrus trees show you when transpiration has stopped – they curl their leaves. You may have observed this on a really hot, dry day. And this is an obvious signal to stop irrigation.

**While you have that trench open, along the tree’s dripline, irrigate and observe how long it takes to moisten the root-zone. That’s your irrigation time. Schedule based on this observation.

And now I happily track back to the opening lines in this discussion – we need to learn to farm with less water. And I am confident that you now understand why this is not only possible, but will ultimately benefit your production and success as a mac farmer.

Of course our experts have tested their understanding of how mac trees use water. Much of their research has been funded by the Water Research Commission Project through University of Pretoria and is therefore owned by this organisation but they did share that the results of their many tests endorse their claim that mac trees thrive with less water. I was privileged to visit a farm on which one of their studies was conducted – when I interviewed a Nelspruit “Jaff” – and this farmer was mystified by the results he saw right in front of him; the tree that was denied all water (had a cover placed over it so that it did not even get rain water) produced the best yield. The one that received only rain was second and the fully irrigated trees – the worst. Now, Theunis and Armand are not suggesting that we all go dry-land farming! Far from it. They are advising that farmers carefully examine their soils and other environmental factors and marry that with a suitable irrigation schedule. If they do this, it will almost always involve far less water, given in shorter cycles, than they were using before.

This stands to reason if you consider our own body’s water requirements. Even though, as humans, we need an average of 15 litres a week, if we were to get all that on a Monday morning and nothing else for the rest of the week, we wouldn’t last long. Not only would we dehydrate on day 2, we would also have washed essential nutrients from our bodies on day 1. But, a litre in the morning and a litre in the afternoon, every day, would result in a healthy, well-hydrated specimen.

I found another parallel they drew to animals fascinating and I hope it helps you too – consider a mother cow and her calf. The longer you leave her calf suckling, the longer she’ll take to pick up condition and ready herself for another calf. Similarly, the longer you leave nuts on a tree, the longer it will take to pick up condition in preparation for the next harvest – so don’t leave nuts on trees for longer than what is necessary. As soon as they’re ready, harvest. Unless of course, you are in the middle of a Corona pandemic and have limited options. 🤔

I also learnt something about shade-cloth covered orchards – remember I mentioned earlier that it is almost impossible to cool or humidify an orchard because the ‘environment’ is open? Well, by covering it with shade-cloth, you close that ‘environment’ and are then able to better manipulate it. i.e.: it is possible to increase the humidity under the shade-cloth. This will extend the period that the trees transpire – their factories are on and working. This will increase productivity. These trees will also use more water because their factories run longer hours – seems counter-intuitive but I hope you now understand why it happens. When it comes to macs, much more research and cultivar development is required before we use this shade-cloth strategy – macs are still rather wild and may respond unexpectedly to a shade-cloth ceiling.

So now that we have carefully examined the tree and how it works, it is time to discuss the next topic: which irrigation system is best.

I hate to disappoint but I couldn’t get the experts to commit. Like true professionals they remind me of the basics and logic: water is water – it does not matter whether you deliver it in a bucket, a hosepipe, a micro-jet system or a multi-million rand drip irrigation system. The important questions are around where (in fibrous root zone), when and how much water to give – and we have answered those questions. Theunis did say that his PERSONAL preference was for micros but he says that low-flow drip irrigation (or add continuous fertiliser into that system for an open hydroponics strategy) is an excellent method for optimal results with the lowest risk because the chances of over-irrigating are minimised. Once installed, a low flow drip irrigation system also takes very little management to regulate and administer. The choice of which irrigation system to choose is more about the farmer than the tree – each farmer is unique and operating in a unique set of circumstances. You cannot decide for him what will serve him best. As long as he respects the trees requirements it doesn’t matter how he delivers the water.

Although I understand that each farmer will work best within his own unique parameters, I was still interested in what Theunis and Armand would do if they had a mac farm of their own … firstly, they said that they would choose to keep it small and size is a factor in what system works so they emphasised that this be kept in mind when discussing their choices. They would install the piping for a micro system but use button drip irrigation on the young trees for the first 2 to 3 years, giving each tree around 10l every day. The trees would all be heavily mulched so, sometimes, 10l may only be required every second day. Once the tree canopy had developed, they would put the micros in and make sure that the shadow area of the tree was where the water fell, delivering about 20l per day for a producing tree. Mac roots operate best at 15°C°- maintaining a mulch layer over those roots will help you achieve this sweet-spot temperature. All irrigation would happen in the mornings. To sustain through drought conditions, they would reduce the radius of the microjet by half, effectively watering the circle immediately around the tree trunk. I was interested to learn that they would place the micros up against the tree trunk and they would spray 360°, one micro per tree.

Deficit Irrigation: Armand is currently doing his Masters on this topic. He is interested in how you can give the tree less than what we think they need without impacting productivity. This is a well-worked practice in citrus, especially in Spain. He is trying to establish what the best-practice is for macs and this involves in-depth study of soil types, cultivars, stage of phenology etc etc. Given our threatened and inconsistent water reserves, this study is going to yield essential information. So far, he can assure us that healthy mac trees are such efficient feeders that they can easily get by with half of what they “should” get. BUT – this water then needs to be delivered in the right place (feeder root zone) at the right time (when the tree is active) in the right amounts (small and frequent) and under the right conditions (well-mulched, forest-floor-like substrates) i.e.: use the water much more effectively.

Surviving drought: The Australians, who are lucky enough to be gaining the Smit brothers, face drought as much, if not more, than we do, so much of their expertise will be invested in surviving drought. In a simplified version of what this would look like, Theunis says, “cut the tree in half, mulch the cuttings, and place that over the root zone.” Sounds extreme and very simple but the logic is all there – reducing the canopy by half will reduce the water requirement by half. Mulching the roots will heighten their ability to forage more effectively. Remember – this is a drought survival strategy not an exercise in productivity. He also says that, in drought, water should be diverted to small trees where a greater return on investment will be realised; the bigger trees have root structures that allow them to access deeper water reserves, increasing their chances of survival but the undeveloped, small trees lack the structure to survive drought unsupported.

I was very interested to learn that there is more than one type of drought:

  • Lack of rain drought: dams are empty, rivers are dry.
  • Atmospheric drought: dams are full but the air is so dry. Very low humidity levels. As already explained – this results in sub-tropical crops like macadamias shutting down and subsequent productivity drops.
  • Physiological drought: like when you are on a life-raft in the middle of the sea, surrounded by all the water in the world but you can’t drink it. Theunis advises that we can prepare for this NOW by adding gypsum to the mulch we place over the root zone. During a ‘lack of water’ drought salt content is often high in any water that we do source – creating the additional problem of physiological drought – and gypsum will help to balance this, in the soil.

There is very little risk in putting down too much gypsum but the experts recommend standard mac feeding practices i.e.: rather give 8 applications of 500kgs/hectare over the next year, than one dump of 4 tonnes per hectare.

And how’s this for a mind-bender: placing a mac tree in too much water can also create a physiological drought. The symptoms are the same – if there is an atmospheric drought happening (low humidity) the tree cannot take up the water it is lying in and, simultaneously, its roots will be suffocating by drowning! As consultants, when these guys see “dry trees” they cannot assume it is because there is no water. It may be over-irrigation, poor quality (salty) water, extended levels of low humidity or any other factor that is starving the tree of water uptake.

The knock-on effects of these various types of droughts also need to be considered. You may have had sufficient water to irrigate throughout a season as normal but the following season has a poor yield – why? If there was an atmospheric drought, in the preceding season, that has shut the plants down for extended periods, they would have had limited photosynthesising time and not have been able to build up enough reserves for the following season’s fruit. OR – they could have had a salinity issue in the soil, so although there was sufficient water, it was still a drought (this time physiological in nature) with similar low productivity results in the following season.

The good news is that macs (and citrus) trees are actually far better off with less water (but not less humidity) than most other plants.

The bad news is that, with climate change progressing at the pace it is, we are not guaranteed of stable conditions anywhere. 10 years ago, Theunis would never have expected macs to produce a crop in the Eastern Cape but now, they are managing to be commercially viable in this area.  Ocean currents, minuscule temperature changes, atmospheric pressure deviations – all these things and many more, collude to produce climatic changes that will either result in an area becoming suitable or unsuitable for subtropical crops. Macs along the coastline generally do better, and will continue to do so, because of the moderating effects the ocean has on climate changes.

So, does temperature have NOTHING to do with the success or failure of a mac tree? Not at all – macs cannot survive at low temperatures because frost kills them. Excessively high temperatures are also dangerous, as they are dangerous to every living organism. This is because chemicals and enzymes start to break down or change structure in extreme temps. Nothing does well under these conditions, including mac trees.

I have absolutely LOVED writing this article. I learnt so much and feel so lucky to have had Theunis and Armand all to myself while I drained their brains. I wrapped up with a final discussion on whether it is possible to manipulate a mac tree’s yield, using water. The experts say it is not that easy because this beast is so wild. Sugar levels in citrus, table grapes and even sugar cane can be manipulated via deficit irrigation to increase the quality of the crop but macadamias are not working with sugar, they are working with oil which takes 10 times as much energy to produce than sugar. The sheer amount of energy involved in producing oil and the wild (and very efficient) nature of these trees makes manipulation nearby impossible. The possibility of successful manipulation with nutrients is more likely than manipulating with water. So, deficit irrigation in macs is more about saving water (or managing with less water) and still getting a good crop than it is about increasing the quality of the crop. Water stress could manipulate vegetative growth – but the experts do not recommend it as it could easily go wrong. Mac trees are built for survival (long-term, subtropical, evergreen crop) so, if stressed incorrectly, they will sacrifice the crop rather than themselves. Manipulation can also give a false positive result in that it can trigger an alternate bearing pattern i.e.: the crop this year may be great but it halves in the following season because the tree went into a survival mode. A smaller, consistent crop may be more beneficial than an irregular harvest.

Before the brothers dash off they summarise: less is more when it comes to getting the most out of your macadamia trees. To substantiate that, they explain that the evergreen nature of a mac tree is yet another indication of its highly adapted survival skills – valuable energy is stored in those thick, leathery leaves and they do not drop them easily. They are conservative with their nutrients, neither wasting nor taking up any excess. This has led our experts to speculate that we may also be over-fertilising mac trees, especially when it comes to nitrogen. Same principle applies: smaller, more regular meals rather than fewer, larger meals. (We will most certainly clarify and expand on this topic in our NUTRITION article due to be published in August)

And then they’re off. SO many farmers want a piece of these men before they leave our shores. Although I am tempted to bemoan the loss of such amazing minds, instead I will be grateful for the interaction and time we have had with them and be proud that they will carry our flag high, wherever they go.

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