At Agro Vivero del Mediterráneo, we have been dedicated body and soul for years to the fascinating world of pistachios. Our passion and experience have made us a benchmark in the sector, and today we want to share with you one of the most advanced and at the same time delicate tools in the management of pistachio plantations: the application of growth regulators. 🧑‍🔬

We understand that the goal of every farmer is to maximize the profitability of their farm, obtaining high-quality harvests consistently. Growth regulators, also known as phytoregulators or PGRs (from English Plant Growth Regulators), are synthetic or naturally occurring compounds that, applied at very specific times and doses, allow us to influence the physiological processes of the tree. They are, in essence, a way of dialoguing with the plant in its own hormonal language to guide its development towards our productive goals. 🎯

However, it is crucial to emphasize that they are not a magic solution. Their success depends on a deep knowledge of the state of the plantation, environmental conditions, and meticulous application. Incorrect use can have null or even counterproductive effects. Therefore, at Agro Vivero del Mediterráneo, we advocate for responsible, informed use, always integrated within a global plantation management plan. Join us on this detailed tour on how and why to use these powerful tools to take your pistachio plantations to the next level.

Understanding the hormonal language of the pistachio tree

Before diving into practical application, it is fundamental to understand what growth regulators are and how they act. Plants, like animals, produce their own hormones (phytohormones) that regulate absolutely all their vital processes: from seed germination to leaf fall in autumn. The main groups of phytohormones are:

• Auxins: Promote cell growth, rooting, and fruit development.

• Gibberellins (GAs): Stimulate stem elongation, germination, and fruit development.

• Cytokinins (CKs): Promote cell division, delay tissue aging (senescence), and are key in fruit set.

• Abscisic Acid (ABA): Known as the stress hormone, regulates stomatal closure to prevent water loss and promotes bud dormancy.

• Ethylene: Is a gas that acts as a hormone, regulating fruit ripening, leaf and flower drop.

Growth regulators that we apply externally mimic or interfere with the action of these natural hormones. By doing so, we can, for example, improve fruit set, increase nut size, induce thinning to avoid alternate bearing, or facilitate harvesting. It is high-precision management that requires expert knowledge. 🤓

Main objectives of regulator application in pistachio

In our plantations and those of the clients we advise, we use growth regulators to achieve very specific objectives. It is not about applying for the sake of applying, but responding to a specific need of the plantation at a given time.

1. Improve fruit set and reduce fruit drop: One of the most critical moments of the cycle. Good fruit set is the basis of a good harvest.

2. Increase nut size and weight: Larger size not only increases the total weight of the harvest but also improves the selling price.

3. Manage alternate bearing (productive alternation): The pistachio tree has a marked tendency towards alternate bearing, alternating years of high production with others of very low production. Regulators can help us mitigate this phenomenon.

4. Facilitate harvesting: Synchronizing and facilitating nut detachment can greatly optimize harvesting tasks, reducing costs and damage to the tree.

5. Induce endocarp dehiscence (opening): For certain varieties and markets, a high percentage of open fruits is desirable, and we can influence this process.

6. Delay flowering in areas with risk of late frosts: Although it is a more complex and less widespread application, there are strategies to try to “escape” spring frosts that can devastate production.

Let’s now see, in detail, how to address some of these objectives using the most common growth regulators in pistachio management.

Application strategies to improve pistachio fruit set

The transition from flower to fruit (fruit set) is a delicate process involving climatic, nutritional, and hormonal factors. After pollination, fierce competition for resources begins among thousands of newly formed small fruits. Many of them will fall naturally. Our goal is to ensure that an optimal number of them continue. 💪

Cytokinins are the key hormones in this process, as they promote cell division in newly formed embryos. Exogenous application of synthetic cytokinins, such as forchlorfenuron (CPPU), can act as a signal to the tree, indicating it to “invest” more resources in those fruits and prevent their drop.

• Application timing: This is undoubtedly the most critical factor. The application window is very narrow. We must perform the treatment when young fruits have a diameter between 3 and 5 millimeters. This usually occurs between 15 and 30 days after the peak of female flowering. Too early application can be ineffective, and too late may not have the desired effect or even cause unwanted effects.

• Dosage: Doses are very low, in the order of 5-10 parts per million (ppm). It is essential to calculate precisely the concentration and volume of spray solution to apply per hectare. An excess can cause fruit malformations or unnecessary stress on the plant.

• Application conditions: We always recommend applying early in the morning or late in the afternoon, avoiding hours of maximum insolation and high temperatures (above 25-28 °C). Relative humidity should be high to favor product absorption through the fruit epidermis. Using a good non-ionic wetting agent is essential to ensure uniform coverage.

• Considerations: It is important to note that excessive fruit set increase can lead to greater competition among fruits, resulting in smaller final size and worsening of alternate bearing the following year. Therefore, this technique must be accompanied by impeccable nutritional and irrigation management so the tree can support that greater load. At Agro Vivero del Mediterráneo, we always perform a prior analysis of floral load and tree vigor before recommending a treatment to improve fruit set.

Increasing nut size: the role of gibberellins

Once we have ensured a good number of fruits on the tree, the next goal is for them to reach the largest possible size and weight. This is where gibberellins come into play. These hormones promote cell elongation and expansion, key processes during the fruit growth phase.

The application of Gibberellic Acid (GA₃) during the second phase of fruit development, which is when exponential pericarp growth occurs, can translate into a significant increase in final size.

• Application timing: Application should coincide with the beginning of endocarp (woody shell) hardening. This moment can be identified by sampling, cutting fruits transversely. When the shell begins to offer resistance to cutting with a knife, but is not yet fully lignified, it is the ideal time. This usually occurs between 6 and 8 weeks after full bloom.

• Dosage: Concentrations used usually vary between 25 and 50 ppm. The exact dose will depend on the variety, tree vigor, and fruit load. It is a practice that requires fine-tuning.

• Application conditions: As with cytokinins, environmental conditions are key. Avoid extreme heat and wind, and ensure good coverage of all clusters. Absorption is foliar and through the fruit itself.

• Synergies: In some strategies, gibberellin applications are combined with cytokinins or sequential applications are made to maximize both fruit number and final size. However, these combinations increase complexity and risk, and should only be carried out by very experienced personnel. From Agro Vivero del Mediterráneo, we always recommend starting with simple treatments and, once the technique is mastered, exploring more complex synergies.

The great challenge: managing alternate bearing with regulators

Alternate bearing or productive alternation is, perhaps, the biggest challenge in pistachio cultivation. The tree invests a huge amount of energy in developing one year’s harvest (“ON” year), which depletes its reserves and prevents it from forming enough flower buds for the following year (“OFF” year). The result is a cycle of one year of high production followed by one of very low or zero production. This greatly complicates the economic planning of the farm. 📉📈

One of the most powerful tools to mitigate alternate bearing is fruit thinning in “ON” years. By reducing fruit load, we free up resources that the tree can allocate to floral induction for the following season. This thinning can be done manually (economically unfeasible in large plantations) or chemically.

For chemical thinning, synthetic auxins such as Naphthaleneacetic Acid (NAA) or Ethephon, a compound that releases ethylene upon decomposition in plant tissues, are used. Ethylene promotes the formation of an abscission layer on the peduncle of weaker fruits, causing them to fall.

• Application timing: It is extremely critical. Treatment must be carried out when the natural abscission decision is already underway, to reinforce it. This usually occurs between 3 and 5 weeks after full bloom. If applied too early, flowering and initial fruit set can be damaged. If applied too late, it will have no effect or may cause leaf drop.

• Dosage and product: The choice of product (NAA or Ethephon) and dose is very delicate. NAA concentrations can range from 10 to 20 ppm, while Ethephon concentrations can range between 100 and 300 ppm. The exact dose will depend on the fruit load to be removed. The goal is not to remove all fruits, but to reduce the load by 20-40%, depending on the intensity of the “ON” year.

• Factors to consider: The effectiveness of chemical thinning is highly influenced by temperature. At higher temperatures, the product effect is enhanced, so the dose must be adjusted downwards. It is a high-risk technique. An error in dose or application timing can result in excessive thinning, losing much of the current year’s harvest. For this reason, at Agro Vivero del Mediterráneo, we insist on the need to conduct tests on small plots before applying the treatment to the entire plantation. In addition, chemical thinning must be part of a comprehensive strategy that includes balanced pruning and adequate nutrition to promote floral induction.

Optimizing harvesting: the use of Ethephon

Pistachio harvesting must be fast and efficient to preserve nut quality and avoid aflatoxin problems. Ideally, we want all fruits to ripen at once and detach easily from the tree when the shaker passes. 🚜

Ethephon, the same product we can use for thinning, is also a fantastic tool to facilitate and uniformize harvesting. By releasing ethylene, it accelerates ripening processes and promotes the formation of the abscission layer between fruit and peduncle. This has several advantages:

1. Uniformity in ripening: Allows advancing and concentrating the harvest in a shorter period of time.

2. Facilitates detachment: Reduces the force needed to detach the fruit, which translates into less shaking time per tree, less damage to branches, and a higher percentage of knocked down fruits.

3. Harvest advancement: Can allow advancing harvesting by a few days, which can be strategic to avoid autumn rains or to reach the market earlier.

• Application timing: It should be applied when the fruit is already physiologically mature but has not yet massively started its natural abscission process. A good indicator is when the episperm (the thin skin covering the seed) changes from a pale green color to a pink or purple tone in a significant percentage of fruits (around 10-15%). This usually occurs between 1 and 3 weeks before the expected harvest date.

• Dosage: Concentrations for this use are higher than for thinning, ranging from 500 to 1,000 ppm. The dose must be adjusted based on variety and weather conditions. High temperatures enhance its effect.

• Precautions: Be very careful! Too early application or excessive dose can cause premature leaf drop (defoliation), which would severely affect reserve accumulation in the tree for the next season. Always assess the state of the plantation. Trees with water or nutritional stress are much more sensitive to defoliation. Harvesting should be planned to take place between 7 and 14 days after treatment. The product cannot be applied and harvest delayed, as fruits would fall to the ground.

Practical aspects for successful application

Success in applying growth regulators lies not only in choosing the right product and timing, but in the application technique. Poor treatment can throw away all effort and investment. From our experience at Agro Vivero del Mediterráneo, these are the keys to success:

1. Prior diagnosis: Before deciding to apply a regulator, walk the plantation. Assess vigor, nutritional and water status, floral or fruit load, and weather forecast. A stressed plantation cannot be treated. Limiting factors (water, nutrients) must be corrected first.

2. Equipment calibration: It is absolutely essential. We must know exactly how many liters of spray solution per hectare we are applying. Poor calibration can lead us to apply double the desired dose or half, with disastrous consequences in both cases. We recommend using equipment with anti-drift nozzles that generate a fine and uniform droplet.

3. Water quality: The pH of the water used for mixing is crucial. Many growth regulators are unstable at basic pH (above 7). They degrade rapidly (alkaline hydrolysis) and lose their effectiveness. It is essential to measure water pH and, if necessary, correct it with an acidifying product to a range of 5.5-6.5 before adding the phytoregulator.

4. Use of adjuvants: Using a good surfactant or non-ionic wetting agent is almost mandatory. These products break the surface tension of water, allowing droplets to spread over the surface of leaves and fruits, instead of forming beads that roll and fall. This multiplies the contact surface and, therefore, absorption and treatment efficacy.

5. Spray volume: Complete and homogeneous coverage of the entire tree must be ensured, especially fruit clusters, which are our main target in most applications. It is preferable to use higher spray volumes (1,000-1,500 L/ha) to ensure the product reaches everywhere.

6. Environmental conditions: As already mentioned, avoid central hours of the day with high temperatures, low humidity, and wind. Optimal conditions are mild temperatures (18-25 °C), high relative humidity (>60%), and absence of wind.

7. Record and traceability: Note everything: date, time, plot, product, dose, spray volume, weather conditions, crop phenological stage, etc. This record will allow us to analyze results and adjust strategies in future campaigns. 📝

Final considerations: a tool for experts

At Agro Vivero del Mediterráneo, we consider growth regulators to be a tool of “precision agriculture”. They are not general-use inputs, but surgical solutions for specific problems. Their implementation requires a learning curve and, above all, deep respect for tree physiology.

Our recommendation is always to start with caution. If you lack experience, ideally start testing in a small area of the plantation, with different doses, to observe the tree’s response in our specific conditions. Having the advice of a specialized technician, who knows the crop and products, is the best investment to avoid costly mistakes.

The application of phytoregulators does not replace, but complements, good cultivation practices. A well-pruned, nourished, and irrigated tree will respond much better to these treatments. They are the icing on the cake of excellent management, the tool that allows us to make a leap in quality and profitability when the basis of our crop is solid.

We hope this detailed tour has been useful to you. The world of growth regulators in pistachio is complex but exciting, and mastering them can make a substantial difference in the success of your farm. At Agro Vivero del Mediterráneo, we are at your disposal to accompany you on this path, sharing our knowledge and experience so that your plantations reach their maximum potential. Together, we will continue to grow the best pistachio! 🌱💚