At Agro Vivero del Mediterráneo, we have been dedicated body and soul to the fascinating world of pistachios for years. As experts in the sector, we perfectly understand the challenges farmers face. One of the most significant and impactful on plantation profitability is undoubtedly alternate bearing, also known as production alternation. This phenomenon, characterized by the alternation of an abundant harvest year (“ON year”) with a scarce or zero harvest year (“OFF year”), is one of the greatest concerns for any producer.

From our experience at Agro Vivero del Mediterráneo, we have observed that inadequate management of alternate bearing can compromise the economic viability of a project. Therefore, we have decided to prepare this comprehensive guide, where we will share our knowledge and the most effective strategies we have developed and perfected to control and mitigate this natural behavior of the pistachio tree. The goal is clear: to achieve more stable and predictable productions year after year, thus maximizing profits.

Understanding the Phenomenon of Alternate Bearing in Pistachios

Alternate bearing is not a whim of the tree, but a complex physiological response. The pistachio tree, like many other woody species, has a survival mechanism that leads it to regulate its resources. In a high-production year, the tree invests a huge amount of energy and nutrients in the development and filling of the pistachios. This enormous demand for resources depletes its reserves, mainly carbohydrates, and limits its ability to form flower buds for the next season. 💐

As a result, the following year, the tree enters a productive “rest” cycle or “OFF year,” during which it prioritizes the recovery of reserves and vegetative growth over harvest production. This biennial cycle has a direct and very negative impact on the farmer’s economy. Irregular income makes financial planning, labor management for harvesting, and long-term commercial agreements difficult. For all these reasons, understanding the factors that cause it and learning to manage it is fundamental to the success of any pistachio plantation.

At Agro Vivero del Mediterráneo we not only provide the best pistachio plant, but we also offer comprehensive advice so that our clients can face this and other agronomic management challenges.

Key Factors Influencing Production Alternation

To be able to control alternate bearing, we must first identify the factors that trigger and accentuate it. They are multiple and interrelated, but we can group them into three main categories: physiological factors, management factors, and environmental factors.

Physiological and Genetic Factors

The genetic component of the rootstock and the variety is determining. Some pistachio varieties are intrinsically more prone to alternation than others. For example, the Kerman variety, one of the most widespread worldwide, is notoriously prone to alternate bearing. On the other hand, other more modern varieties like Larnaka or Sirora, although they also present some alternation, tend to be more regular in their production if managed properly.

The physiological key lies in the competition for carbohydrates. During the “ON year,” developing fruits act as very powerful energy sinks. They consume most of the photoassimilates (sugars produced in photosynthesis) that the plant generates. This competition is so fierce that there are barely any resources left for the differentiation of buds from vegetative to floral, which is the process that will define the following year’s harvest. This floral differentiation process occurs in summer, coinciding with the period of maximum pistachio fattening, which aggravates the situation.

In addition, developing seeds produce hormones, such as gibberellins, which inhibit floral induction. A high crop load implies a high production of these inhibitory hormones, which drastically reduces the productive potential of the next cycle. Choosing a good rootstock and variety combination is the first step to mitigating this problem, and in our advisory services, we always emphasize this initial strategic decision.

Agronomic Management Factors

This is where we as farmers have the greatest power to act. Poor plantation management not only does not help control alternate bearing, but it can dramatically accentuate it.

• Pruning ✂️: Inadequate pruning is one of the main mistakes we see in the field. The absence of pruning or too light pruning in “OFF years” causes an excess of flower buds for the next season, ensuring an “ON year” of excessive load that will exhaust the tree and perpetuate the alternate bearing cycle. Conversely, excessively severe pruning in an “ON year” can reduce production so much that it completely unbalances planning. Pruning must be a regulation tool, seeking a constant balance between vegetative growth and production.

• Irrigation 💧: Water management is critical. Water stress, especially during key phases of pistachio development (fruit set, kernel filling, and bud differentiation), aggravates competition for resources and enhances alternate bearing. A tree with a water deficit will not be able to photosynthesize at full capacity, limiting carbohydrate production and its ability to support the load and, at the same time, prepare the future harvest. Well-planned irrigation adjusted to the tree’s needs at all times is essential to keep it strong and balanced.

• Nutrition 🌿: Fertilization plays a crucial role. An unbalanced fertilization plan, with deficiencies or excesses of certain nutrients, weakens the tree. Nitrogen, phosphorus, and potassium are fundamental, but so are micronutrients like zinc and boron, which are directly involved in processes such as flowering and fruit set. In an “ON year,” nutrient extractions are massive. If they are not adequately replenished, the tree will arrive exhausted at the end of the cycle, unable to accumulate the necessary reserves for the following year. At Agro Vivero del Mediterráneo, we always recommend conducting soil and foliar analyses to adjust the fertilization plan to the real needs of the plantation.

Environmental and Climatic Factors

Although we have less control over them, climatic factors are decisive.

• Late frosts ❄️: A late spring frost can destroy flowers or newly set fruits. If this happens in what was going to be an “ON year,” a forced unloading occurs. The tree, not having to invest energy in that lost harvest, will allocate all its resources to exuberant vegetative growth and the formation of a massive amount of flower buds, resulting in an “ON year” of extreme load the following year, restarting or intensifying the alternate bearing cycle.

• Conditions during flowering: Inadequate temperatures, persistent rains, or strong winds during the flowering period can negatively affect pollination and, therefore, fruit set. Poor fruit set in an “ON year” can have an effect similar to that of a frost, although usually less drastic.

• Extreme summers ☀️: Very high temperatures and high solar radiation can cause stress to the plant, even with adequate irrigation. This stress can lead to stomatal closure to prevent water loss, which in turn reduces the photosynthetic rate. This decrease in energy production at a critical time exacerbates internal competition and favors alternation.

Understanding the interaction of all these factors is the first step to designing an effective mitigation strategy. There is no single solution; success lies in integrated and constant management over time. If you want a personalized analysis for your farm, do not hesitate to contact us.

Alternate Bearing Mitigation Strategies: Our Approach

At Agro Vivero del Mediterráneo we advocate a proactive and multifactorial approach to break the alternate bearing cycle. The goal is not to eliminate it completely, as it is inherent to the tree’s physiology, but to soften it to the point that it does not compromise the profitability of the plantation. Below, we detail the strategies that, in our experience, offer the best results.

1. Regulation Pruning: The Art of Balance

Pruning is undoubtedly the most powerful and at the same time most complex tool to control alternate bearing. It is not about cutting for the sake of cutting; it is about “dialoguing” with the tree and guiding it towards a sustainable balance. The pruning strategy must adapt to the state of the tree, that is, whether it is in a high-load (“ON”) or low-load (“OFF”) year.

• Pruning in “OFF Year” (Winter prior to “ON Year”): During the winter preceding a foreseeable high-production year, pruning must be more intense. The goal is to reduce the number of flower buds to avoid an excessive load. Weak, poorly located branches or those that produced the previous year (since the pistachio produces on the previous year’s wood) must be removed. We recommend removing between 25% and 40% of the flower buds, depending on the age of the tree and the intensity of the alternate bearing it has been showing. This may seem contradictory, as we are eliminating potential harvest, but it is a fundamental investment. By reducing the future load, we ensure that the tree has sufficient resources to fill the remaining pistachios well, obtain a good caliber, and, most importantly, have energy to differentiate flower buds for the next season. It is an anticipation pruning.

• Pruning in “ON Year” (Winter prior to “OFF Year”): In the winter following a large harvest and preceding a low-production year, pruning must be much lighter and more selective. The tree is exhausted and has few flower buds. The goal here is to conserve the maximum possible productive potential and stimulate the vegetative growth that will give rise to the productive branches of the future. Mainly cleaning cuts are made, removing dry or damaged wood and sanitizing the tree’s structure. Severe pruning at this time would be disastrous, as we would eliminate the little expected harvest and could even delay the tree’s recovery.

This differentiated pruning, year after year, is the cornerstone for regulating production. It requires observation, knowledge, and experience.

2. Precision Irrigation and Nutrition: The Tree’s Life Support

As we have mentioned, a stressed tree is an alternate bearing tree. Therefore, an optimal and constant supply of water and nutrients is non-negotiable.

• Irrigation Management: The implementation of a drip irrigation system is almost essential in new pistachio plantations. This system allows us to provide water in a localized and efficient manner. It is crucial to monitor soil moisture using sensors (capacitance probes, tensiometers) to adjust irrigation doses and frequencies to the real demand of the tree and climatic conditions. The most critical moments where water cannot be lacking are:

  • Post-fruit set: To ensure the initial development of the pistachio.

  • Kernel filling (July-August): It is the phase of greatest water and energy demand. Stress in this period not only affects the caliber and the percentage of split nuts, but directly impacts bud differentiation for the following year.

  • Post-harvest: An often forgotten moment! After harvesting, the tree needs to replenish its carbohydrate reserves. A couple of supplemental irrigations after harvest help the tree “recharge its batteries” and face the winter in better conditions, which will affect budding and flowering the following spring.

• Strategic Fertilization Plan: Fertilization must be dynamic and adjusted to the phase of the productive cycle. We cannot apply the same fertilization plan in an “ON year” as in an “OFF year.”

  • In “ON Year”: The needs, especially for nitrogen (N) and potassium (K), are very high. Potassium is fundamental for kernel filling and sugar transport. It is vital to apply these nutrients in fractions throughout the cycle (fertigation) so that the tree uses them efficiently. The application of micronutrients such as zinc (Zn) and boron (B) via foliar spray before flowering has also been shown to improve fruit set and pollen quality.

  • In “OFF Year”: The needs are lower, but not non-existent. The goal is to promote good vegetative development and reserve accumulation. N and K applications are reduced, but special attention must be paid to phosphorus (P), which plays a key role in energy storage and root development. Foliar analyses carried out in July will give us the most precise information about the nutritional status of the tree and will allow us to correct any deficiency in time.

3. Fruit Thinning: A Direct Intervention

Fruit thinning is a direct intervention technique that consists of removing a part of the harvest in an “ON year” when the fruits are still small. Although it can be costly in terms of labor (manual thinning) or require specific machinery (mechanical thinning), its benefits are enormous.

• Why thin? By reducing the number of fruits, we decrease competition for carbohydrates and water. This allows the remaining pistachios to reach a larger caliber and a higher percentage of split nuts, which translates into a better selling price. But the most important benefit is that, by reducing the load and the production of inhibitory hormones by the seeds, the tree has enough energy to induce the formation of flower buds for the next season. It is one of the most effective ways to break the alternate bearing cycle.

• When and how to thin? The optimal time for thinning is about 4-6 weeks after full fruit set, when the fruits are small and easy to detach. It can be done manually in young trees or small plantations, or mechanically with trunk or branch shakers in larger plantations. The goal is to remove between 30% and 50% of the initial load. It is a decision that requires courage, but the medium and long-term results justify the investment.

4. Varietal Choice: Starting on the Right Foot

The most important decision that will mark the future of the plantation is made even before planting the first tree. The choice of the rootstock/variety combination is fundamental.

• Rootstocks: The rootstock is the base of the tree. Its vigor and root system directly influence the tree’s ability to capture water and nutrients. Vigorous rootstocks like UCB-1, which at Agro Vivero del Mediterráneo we produce and widely recommend for their great adaptation to our conditions, provide the tree with a greater capacity to support the load and recover from productive stress. A more vigorous tree with a powerful root system will always be less prone to severe alternate bearing.

• Varieties: As we mentioned, there are varieties with a more marked genetic tendency to alternation than others. While Kerman is the classic example of an alternate bearing variety, other options like Larnaka or Sirora, which we also offer in our nursery, present a more regular behavior. Researching and choosing a variety that, in addition to being productive and adapting to the local climate, has a lower tendency to alternate bearing, is a smart long-term strategy.

If you are thinking of starting a plantation, we invite you to fill out our reservation and quote form. Our technical team will advise you to choose the perfect combination for your project.

Integrated and Long-Term Management: The Key to Success

Controlling alternate bearing in pistachios is not a task achieved with a single specific action. It is the result of integrated, constant, and meticulous agronomic management year after year. It is a work philosophy that seeks the balance and sustainability of the plantation.

At Agro Vivero del Mediterráneo, we are convinced that a well-trained and informed farmer is a successful farmer. Production alternation is a complex challenge, but with the right tools and knowledge, it can be managed effectively. The combination of precise regulation pruning, an irrigation and fertilization plan adjusted to the needs of each cycle, the brave decision to thin in load years, and a smart varietal choice from the beginning, are the pillars on which a profitable and stable pistachio plantation is built.

The path requires effort, observation, and dedication, but the results are worth it: more regular harvests, higher quality pistachios, and an economically sustainable and predictable farm. We are here to accompany you every step of this exciting journey. 🌳💚