At Agro Vivero del Mediterráneo, we have been dedicated body and soul to the fascinating world of pistachios for years. Our experience has taught us that the success of a plantation does not lie solely in the choice of rootstock or good irrigation management, but in attention to the small details that make the difference. One of these details, which can catapult the profitability of a farm, is the smart and precise use of plant growth regulators. 👨‍🌾

The goal of every farmer is clear: to achieve the highest possible production with maximum quality. In the case of pistachios, caliber is one of the most determining factors in the selling price. A larger pistachio is not only more attractive to the consumer, but it also translates into a higher weight per hectare and, therefore, higher income. However, achieving that extra caliber consistently, year after year, can be a challenge. This is where agricultural science and technology provide us with invaluable tools: plant growth regulators.

Throughout this article, we will break down, based on our extensive experience and the trials we continuously carry out, how these substances can become our great allies to improve not only the size but also other quality parameters of the pistachio. We will talk about what they are, how they act, which are the most interesting active ingredients, when to apply them, and, very importantly, what precautions we must take to ensure effective and safe use. Because in modern agriculture, innovation and knowledge are the keys to maximizing return on investment. If you are thinking of taking a step further in your plantation, we invite you to continue reading and discover how to optimize your results.

Understanding Plant Growth Regulators: The Messengers of Plants

Before diving into specific applications and products, it is essential to understand what a plant growth regulator is. They are often known as plant hormones, and although the analogy is useful, we prefer to define them as “plant growth regulators.” They are organic compounds that, applied in very low concentrations, are capable of influencing the physiological processes of plants. They act as chemical messengers, sending signals to cells to initiate, stop, accelerate, or modify certain processes, such as cell division, cell elongation, flowering, pistachio fruit set, or ripening.

It is crucial to distinguish them from fertilizers. While a fertilizer provides the necessary nutrients for the plant to build its tissues (the “bricks”), a plant growth regulator gives it the “instructions” on how and when to use those bricks. For example, a plant growth regulator can tell the plant to allocate more resources to fattening the pistachio at a specific time in the cycle.

In our company, Agro Vivero del Mediterráneo, we not only sell the highest quality pistachio plants, but we also offer a complete range of technical advisory services. Understanding the role of plant growth regulators is an essential part of that advice, since correct use can mark a before and after in plantation profitability.

There are different families of plant growth regulators, each with specific functions. The most relevant for our goal of improving pistachio caliber and quality are cytokinins, gibberellins, and auxins. Below, we will analyze each of them in detail.

Cytokinins: The Drivers of Cell Division

Cytokinins are undoubtedly the stars when we talk about increasing pistachio caliber. Their main function in the plant is to promote cell division (cytokinesis). Let’s think about the initial development of the pistachio after fertilization. In this first phase, the final size it can reach will largely depend on the number of cells that are formed. The more cells, the greater the potential for subsequent growth. 📈

The exogenous application of cytokinins at the right time stimulates greater and faster cell division in the newly set pistachio. This is like building the foundation of a larger building; if we start from a base with more cells, the subsequent “filling” or fattening process (cell elongation) will result in a larger pistachio.

Key Active Ingredient: 6-Benzyladenine (6-BA)

Within the group of synthetic cytokinins, 6-Benzyladenine (6-BA) is one of the most studied and used in pistachio trees. Its effectiveness is more than proven, provided it is applied correctly.

• Application Timing: This is the most critical factor. The application of 6-BA must be carried out in a very specific time period, which generally ranges from 15 to 30-35 days after full fruit set. Why in this window? Because it is the peak of cell division in the pistachio. If we apply earlier, we may not have enough set pistachios for the treatment to be effective. If we apply later, the cell division phase will have ended and we will be in the elongation phase, where cytokinins no longer have such a powerful effect on size. The pistachio will no longer be receptive to the signal to “divide more.” From our experience, we recommend closely monitoring cluster development to determine the optimal day. A good indicator is when the small pistachios are between 3 and 5 millimeters in size.

• Dosage and Concentration: The dose varies depending on the commercial product, but concentrations usually range from 100 to 200 ppm (parts per million). It is essential not to exceed the doses recommended by the manufacturer and, if possible, to carry out tests in a small area of the plantation before widespread application. An excessive dose can cause unwanted effects, such as phytotoxicity on the leaves or even excessive competition between pistachios in the same cluster.

• Application Method: The application must be foliar, ensuring complete and uniform wetting of the clusters. It is important that the product directly reaches the developing pistachios. Therefore, a sufficient spray volume must be used (between 1,000 and 1,500 liters per hectare, depending on the planting layout and tree size) and nozzles that generate a fine drop for optimal coverage. Applications should be made during hours of low insolation and without wind to maximize absorption and avoid drift.

Expected Results with Cytokinins:

A well-executed treatment with 6-BA can mean a very significant increase in caliber. In our trials and in the plantations we advise, we have observed pistachio weight increases of between 10% and 20%. This translates directly into a higher proportion of pistachios in the highest caliber categories (18/20, 20/22), which are the ones that obtain the best prices on the market. In addition, a larger pistachio usually has a better appearance and is easier to open, which also improves its commercial quality.

Gibberellins: The Architects of Cell Elongation

If cytokinins are in charge of laying the “bricks” (cells), gibberellins are in charge of making those bricks bigger. Their main function is to promote cell elongation or lengthening. Once the cell division phase has concluded, the pistachio enters a rapid growth phase where the cells that have already formed increase in size by accumulating water and photosynthates.

In the pistachio tree, the use of gibberellins is more delicate and their effect on caliber is not as direct or consistent as that of cytokinins. However, in combination with other plant growth regulators or in specific situations, they can play an interesting role.

Key Active Ingredient: Gibberellic Acid (GA3)

Gibberellic Acid, specifically GA3, is the most commonly used gibberellin in agriculture.

• Applications for Caliber: Some strategies combine cytokinin applications (for division) followed, a few weeks later, by gibberellin applications (for elongation). This strategy seeks a synergistic effect. The application of GA3 would be carried out when the pistachio has already passed the cell division phase and is in full growth. However, one must be very cautious. High doses of gibberellins can have unwanted side effects on the pistachio tree, such as the inhibition of floral induction for the following season. That is, we can achieve a good caliber one year at the cost of drastically reducing the harvest the following year. 😟 For this reason, its use for fattening must be very measured and always under qualified technical advice.

• Other Applications of Interest: Gibberellins have another very interesting use in pistachios, although it is not directly related to the caliber of the commercial pistachio, it is related to the quality of the production process. In varieties like Kerman, which sometimes present female flowering ahead of male flowering, an application of GA3 in winter on male trees can delay their flowering by a few days, helping to synchronize it with that of female trees and thus improving pollination and fruit set. Better pollination is the first step to a good harvest.

If you are interested in exploring these advanced techniques and need personalized advice for your plantation, do not hesitate to get in contact with our team of experts. We will analyze your particular case and propose the most appropriate solutions.

Auxins: The Orchestra Conductors

Auxins are another fundamental group of plant growth regulators. They act as true orchestra conductors in the plant, regulating an enormous number of processes, from root growth to apical dominance and, what interests us here, nutrient flow and pistachio development.

One of the key functions of auxins is to create nutrient “sinks.” Areas of the plant with high auxin concentrations act as a magnet for sugars and other compounds produced in the leaves (photosynthates). The developing pistachio is, by nature, a strong sink. The exogenous application of auxins can enhance this effect, directing a greater flow of “food” towards the pistachios and thus favoring their filling and fattening.

Active Ingredients and Application:

The most commonly used synthetic auxins are Naphthaleneacetic Acid (NAA) and 4-Chlorophenoxyacetic Acid (4-CPA).

• Application Timing: As with cytokinins, timing is crucial. Auxins are generally applied during the active growth phase of the pistachio, after fruit set. Their goal is to maintain high metabolic activity in the pistachio and ensure it receives all the resources it needs to grow. A typical application could be between 30 and 60 days after full fruit set.

• Combined Effect and Precautions: Often, the most effective commercial products for pistachio fattening do not contain a single active ingredient, but a balanced combination. Formulations that mix a cytokinin (like 6-BA) with an auxin precursor or a low-activity auxin are very common. This synergy is very powerful: the cytokinin stimulates initial cell division, and the auxin ensures that those new cells fill correctly. However, with auxins, one must also be extremely careful. An incorrect dose or application at an inappropriate time can cause phytotoxic effects, such as deformations in leaves and shoots, and even pistachio drop. It is imperative to follow technical recommendations and not experiment on your own.

• Reduction of Blanks: Another interesting effect attributed to auxins, often in combination with other compounds such as calcium and boron, is the possible reduction in the percentage of empty or blank pistachios (a problem known as “blanking”). By improving the flow of nutrients to the pistachio, it helps the embryo to develop correctly and not abort, which is the cause of a pistachio remaining empty. A lower percentage of empty pistachios not only increases the yield in weight but also reduces subsequent processing costs.

Fundamental Precautions and Good Practices: The Key to Success

The use of plant growth regulators is not a game. They are very powerful tools that, if misused, can cause more problems than benefits. At Agro Vivero del Mediterráneo, we always insist to the farmers we advise on the importance of following a series of precautions and good practices. ✅

1. Prior Diagnosis: Before deciding to apply a plant growth regulator, the plantation must be analyzed. Are the trees healthy and well-nourished? Do they have enough water? A plant growth regulator does not work miracles. If the plant has nutritional deficiencies or suffers water stress, the treatment will not be effective and may even be harmful. The goal is to help a healthy tree express its maximum potential, not try to revive a sick tree.

2. Read and Understand the Label: It seems obvious, but it is the most important and sometimes the most forgotten step. The product label contains vital information about the dose, application timing, incompatibilities with other products, safety period, and protective measures for the applicator.

3. Respect Doses and Timings: The “application window” for plant growth regulators is usually very narrow. Applying outside that window can make the treatment useless or counterproductive. Similarly, the temptation to “add a little more just in case” is the most common and dangerous mistake. Overdosing is the main cause of phytotoxicity problems.

4. Water Quality and Application Conditions: The pH of the water used for the mixture can influence the stability and effectiveness of some plant growth regulators. It is advisable to adjust it to a slightly acidic pH (between 5.5 and 6.5) if necessary. As we already mentioned, applying during the least hot hours and without wind is fundamental to maximize absorption and avoid rapid evaporation and product drift.

5. Equipment Calibration: Make sure the sprayer or spray tank is perfectly calibrated. Poor calibration can lead to applying very different doses in different areas of the plot, with consequent irregular results and phytotoxicity risks in overdosed areas.

6. Conduct Small-Scale Tests: If you are going to use a new product or apply a strategy for the first time, we highly recommend that you do a test in a small sector of the plantation. Treat a few rows of trees and leave others as a control. This way, you can evaluate the results and possible side effects before making a larger investment and treating the entire farm.

7. Professional Technical Advice: The physiology of the pistachio tree is complex and every plantation is a world of its own (tree age, variety, soil type, climate…). Having the backing of a specialized technician who knows the crop and the products available on the market is the best guarantee of success. A good advisor will help you design a custom strategy, choose the right products, and define the optimal application times. If you want a detailed plan, you can request it through our reservation and quote form.

Beyond Caliber: Other Quality Benefits

Although the main objective of using these plant growth regulators is to increase caliber, their positive effects can extend to other quality parameters that influence final profitability:

• Higher Split Percentage: A larger, well-formed pistachio tends to have higher internal pressure during the final ripening phase, which facilitates the natural opening of the shell. A higher percentage of open pistachios (“split”) is highly valued commercially, as it reduces the need for mechanical processing and improves consumer perception.

• Harvest Homogeneity: A uniform application of plant growth regulators can contribute to the pistachios in the same cluster and throughout the tree having a more homogeneous development. This facilitates harvesting and simplifies subsequent grading processes at the processing plant.

• Reduction of Blanks: As mentioned earlier, by improving embryo nutrition and viability, some of these applications can help reduce the percentage of empty or blank pistachios, which means a direct increase in the net yield of the harvest.

• Better Appearance and Filling: A pistachio that has received the right stimuli at the right times is not only larger, but usually has better kernel filling, without wrinkles or malformations, and a cleaner, more attractive shell.

In summary, the use of plant growth regulators is a high-precision technique that, when implemented correctly and under expert advice, can provide a qualitative and quantitative leap in pistachio production. It is not a “magic recipe,” but a scientific tool that allows us to dialogue with the plant and guide its physiological processes to achieve our production goals. 🌿

At Agro Vivero del Mediterráneo, our philosophy is based on combining the best pistachio plant with the most advanced knowledge. We firmly believe in training and accompanying the farmer as pillars for success. The use of plant growth regulators is a clear example of how innovation can help us be more efficient, more productive, and, ultimately, improve the profitability of a pistachio plantation, an exciting crop with an extraordinary future. If you have any questions or want us to study your case, do not hesitate to contact us; our team will be happy to help you.