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Nisão Walnut Production Blog: Ep. 4 - Issues, considerations: Plantation density

Plantation density in Walnuts
Issues, consideration

As previously mentioned, establishing a walnut business requires making several decisions. Though a lot of effort should be given to make the best of each of these decisions, it is also important to differentiate the real important ones from hoaxes. I have seen over and over people spending hours discussing some issues that I found to be not worth the effort. In this article, I would

like to focus on some of these issues, trying to give you my perspective on them. I hope this article will provide you some insights and help you in optimizing your decision process.

Plantation Density:

Sometimes it is easier to understand a concept using extreme examples. In a very broad sense, production of walnut is an outcome of a chemical reaction, whereby solar energy

Solar panels
Leaves work as Solar panels

absorbed by plant leaves are used to produce sugar, through the photosynthesis process we all know well. It is for this very reason that I often consider the plant leaves as solar panels. The orchard canopy is the area covered by layer of vegetation in the orchard and is essential for fruit production. In essence a majority portion of this area is covered by leaves. Obviously this begins to be a very small part of the overall area in the early years of plantation but growers exponentially as your trees continue growing. A good way of describing this is to look at the orchard surface. Your orchard canopy is basically the area of the orchard surface that is not receiving sun due to shades caused by the leaves. The larger your leaf area, the larger your orchard canopy is, and thus the more of the solar energy is being absorbed by your plants. Think about a solar panel installation. You would prefer to cover the entire area of your orchard with solar panels, in order to increase your utilization of the sun. While the analogy between the leaves and the solar panels is helpful, we need to keep one major distinction between them thing in mind. An orchard, made of plants is a living organism that changes its size, whereas the solar panels do not. Therefore it is important to assess the future when designing your orchard.

Raw and tree spacing demonstrated
Spacing demonstrated

Trees are planted in rows, making many agricutural operations (like spray, irrigation, pruning) easier. Tractors usually operate within this row. Therefore, the Raw Spacing is generally larger than the Tree Spacing, which is the distance of the trunks of the trees planted in the same raw. The combination of these two distances (7 x 5) for example, indicates what is often referred to as tree spacing or plantation density. This also indaces the space allocated for each tree (7 x 5) = 35 square meters. From this you can also derive the number of plants per ha. 10,000 / (3 x 5) = 286. Despite the allocated space per tree, during the initial years of a plantation, due to the small are that each tree covers, the orchard looks almost empty. But make no mistake about it, as it will chage very fast. The ideal canopy coverage for a mature orchard is about 80 %, although this may change according to your tree variety and pruning. And a mature walnut tree should be in the shape of a vertical triangle, increasing its leaf area as you move towards the soil. The lower leaves that will not receive sunlight due to overlap with other trees will stop producing nuts. Therefore, in an orchard with 100 % canopy the only nut producing area of the plant will be the upper section exposed to sun. By leaving some area for light getting through, you can create a more efficient plant that produces nuts in all of its sections.

Those who advocate the dense plantation or super dense plantation, argue that a denser plantation will increase the overall (maximum) cruise production. While I do not entirely agree with them, I need to accept that there is a logic behind it. For a moderate dense orchards, of say, (7 x 5) density, you would need about 7-9 years to reach the maximum desired canopy coverage, which is also you maximum productivity time. I am aware that I am making it oversiplified by not taking certain irrelevant parameters such as rootstock, climate and the soil conditions.

Canopy evolution
Canopy evolution

On the other hand, a much denser orchard of (5 x 2.5) plantation density, will have approximately 2.8 time more trees compared to the 7 x 5 model (800 plants/ha vs 286 plants/ha). With the same growth pace, it will take them significantly less time to reach the desired canopy level. Instead of 7th-9th year, this orchard may reach its cruise productivity on the 5th-6th year. However, in this case the genetic age of the plants (especially the maturity of the pollinators) may have an effect of the productivity and the pollen availability must be strategized even more carefully.

Canopy evolution in higher density plantation
Canopy evolution in higher density plantation

In both cases, given the same care, management and fertilization program per ha, both orchards will have similar Cruise Productivity (when they reach full maturity - Full Canopy).

The main difference in these two scenarios is the anticipated cruise production time, (lets say from the 9th year to the 5th). Obviously reaching the maximum productivity earlier is something valuable. However doing to by having a denser plantation comes with a price tag and certain challenges that no grower should ignore:

1- A denser plantation will increase your initial investment, due to higher tree cost as well as additional labor expenses during plantation. Depending on your irrigation system, it will also increase your infrastructure cost, although less than a proportionately.

2- The amount of water usage (at full maturity) will not change with your plantation density as the ETc. (Evapotranspiration Model is based on area, crop and canopy coverage) However, during the first years the denser plantation will need significantly more water than the other, reaching the full water need level earlier, as it will reach the full canopy coverage earlier.

3- The fertigation will increase especially in the early years as you will be feeding more plants. Same thing is expected for all spray applications including bio-stimulants, spray fertilizers, insecticides as well as fungicide applications.

4- Herbicide application will increase according to the increase of the number of plant rows (dense planting will have more rows to be applied with herbicide).

5- But perhaps the most important difference will come when the densely planted orchard reaches to the higher canopy level where branches will be covered by other plants preventing the sun to reach them. In a dense plantation you will get to this point not only very fast but at the same time in a very intense way. This is when management of the orchard will become significantly more difficult as frequent pruning will be required in order to provide sun for the overlapping branches that will stop producing walnut otherwise. Some orchards prefer to remove some plants to dilute the plantation at this point. Although this operation is possible, it is difficult and costly.

I am for having plantations with moderate density, of 7 x 5 meters for regular soils, where spacing can be reduced to 7 x 4 in less fertile, sandy soils, or increased to 7 x 5.5 or 7 x 6 meters in rich, deep and well drained soils.

Some Warnings:

1- DO NOT FALL INTO THE USUAL MISINFORMATION that advocates "per tree productivity" and then multiplying this with the number of plants per ha. I have seen so many surprisingly brilliant businessmen that have fallen to the clever misguidance provided to them by some. I have seen many growers with surreal productivity expectations such as 15 tons/ha, because of falling into this trap.

Per tree production Z kg

Trees per ha. Y trees

Production expectancy: Z x Y. As density increases (so will Y), and the formula will generate very unrealistic figures of productivity.

2- Productivity should also be defined and analyzed per unit area (da, ha) and not per tree.

Unless you are a very experienced, seen all, done all, type of walnut grower, start with a achievable yet challenging productivity goal of 6 tons/ha (at full maturity as an average figure of the entire orchard). You may end up having parcels that produce twice this amount, but there will also be ones producing less in certain years. Focus on the overall average.

One important consideration in determining plantation density is the fertility of the soil and the vigor of your plants and rootstocks. Plants usually prioritize growing larger in more fertile, deep soils, during which time production may be delayed. While larger trees eventually end up producing more than smaller ones, growers can take steps in slowing their trees´ growth for anticipating the production. Soils with less fertility and depth on the other hand, will limit your plants growth and help produce earlier. Therefore, as a rule of thumb, more spacing is needed when working in rich soils, compared to less fertile ones. More vigorous varieties and clonal rootstocks usually create larger trees. A 7 x 5 sizing is a safe place to start. Depending on these two variables, you may want to make minor adjustments to your plantation density.


Oct. 2023


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