Pollination in pistachio and its influence on the fruit
In this article we will discuss a topic of great interest to precision olive and fruit growing, but at the same time quite unknown: Pollination in pistachio, the structural characteristics of their flowers and their direct influence on the fruit, as well as the impact of climate on this entire process.
Plantation design and establishment
When starting a pistachio plantation, the first critical step is establishing the rootstock. Its selection must be based firmly on soil analysis and the specific agronomic practices of each farm. It is essential that the varieties used are hybrids with high resistance to Verticillium wilt, a soil-borne fungal pathogen that severely affects pistachio trees in areas with poor drainage or prone to waterlogging.
Once the rootstocks are ready for grafting, the scions are placed, distinguishing between female and male plants. Strategic distribution is fundamental: the optimal technical ratio should be approximately one male tree for every nine female trees. You can find more details about the different varieties in our article on pistachios in Spain.
Flowers and Pollination in Pistachio Trees
Bud Types and Cell Differentiation
Flowers arise from a cell meristem, which divides continuously in a spiral pattern at the golden angle of 137.5º, generating young embryonic tissue. These structures are located in the apical buds of two-year-old stems in male trees and one-year-old stems in female trees. The result of this development is inflorescences; that is, dense clusters composed of multiple flowers.
When pruning, it is vital to consider this arrangement to avoid removing productive buds, since only mature and properly fertilized female flowers will produce pistachio fruit.
Unlike most fruit trees, whose flowers are hermaphroditic (possessing both male and female parts in the same structure, thus facilitating fertilization), the pistachio tree is dioecious. The male and female flowers are very different.
Parts of Hermaphroditic Flowers
Unlike the pistachio, hermaphroditic plants combine both male and female parts within the same flower, a morphological factor that facilitates self-pollination.
Male inflorescences: Dioecious male flowers
Flowering of male pistachio trees begins after the winter dormancy period, usually occurring slightly earlier than that of the female trees. The completion of the accumulated chilling hours triggers flower opening. If these hours are shortened due to climatic anomalies, male flowering may occur excessively early, causing a lack of synchronization with the appearance of the female flowers and negatively impacting the viability of the harvest
Male shoot development: 1. Clusters of male pistachio flowers. 2. Different shades of the clusters depending on the age of the shoot. 3. Arrangement of the clusters in a pineapple shape. 4. Clusters without pollen at the end of pollen release and leaf buds.
Morphological characteristics of the male flower:
It completely lacks decorative petals and an ovary. It has two external protective bracts and a calyx divided into five lobes.
It has between two, five, and eight stamens with short filaments ending in massive, tetragonal anthers saturated with pollen. *(Note that the sequence 1, 1, 2, 3, 5, and 8 corresponds precisely to the numbers in the Fibonacci sequence.)*
The pollen grain is spherical-ovoid, bright yellow, and contains the germinative progenitor cells. It is protected by an inner layer (intine) and an outer layer (exine) composed of sporopollenin, one of the most resistant organic substances in nature. This biological shield allows it to travel exposed to the elements and reach the stigma intact, without being altered by physical or chemical factors. Furthermore, it has three germinal pores that facilitate the emission of the pollen tube during fertilization.
These flowers grow in compact clusters, about 65 mm long, containing approximately 500 individual flowers. They exhibit a greenish-brown color and are distributed according to the perfect geometry of the golden angle.
Female Inflorescences: Dioecious Female Flowers
Female flowers differ radically from male flowers, both in their external morphology and internal structure. They emerge from the meristems later, always on shoots developed during the previous growing season (one-year-old wood).
Morphological Characteristics of the Female Flower
They lack stamens and a corolla. They have two protective bracts and a calyx composed of two to five lobes.
Their nucleus consists of a superior ovary, ovoid in shape (located above the floral receptacle), which contains a single ovule, crowned by three styles and three flattened stigmas of a striking purple color covered with a fine receptive down.
They are structured in clusters of about 200 flowers, with an average length of 75 mm. Direct observation under a magnifying glass clearly reveals this floral architecture oriented towards pollen capture.
Development of the female shoot: 1. Initial female flower bud. 2. Beginning of evident pistil development. 3. Appearance of the first purple stigmas and leaf buds. 4. Maturation of several clusters of female flowers alongside the current year’s leaves.
Pollination Dynamics in Pistachio Trees
The effective transfer of pollen from the male anthers to the female stigmatic apparatus is defined as pollination. In the case of pistachio trees, this process is strictly anemophilous; that is, it depends solely and exclusively on the action of the wind. The intervention of pollinating insects is ineffective and counterproductive, since bees and other vectors are attracted only to the male clusters (due to the pollen) and ignore the female flowers, thus failing to complete the pollen transport cycle.
The optimal pollen cycle begins when the mature anthers open by dehiscence, releasing clouds of pollen into the air in a sustained manner for a critical interval of approximately 4 days per flower.
Critical Factors for Successful Pollination in pistachio
To maximize fruit set and ensure the profitability of the orchard, the following agronomic and climatic factors must be managed and combined:
- Wind Dynamics and Planting Layout: When designing the orchard, rows of male trees must be placed along the northern and southwestern edges, since the prevailing winds on the Iberian Peninsula during spring blow mainly from north to south or southwest to southeast. The absence of light breezes prevents the physical dispersal of pollen.
- Rainfall and Humidity: It is crucial that there is no rainfall during the peak pollination period, as rainwater washes away the suspended pollen and destroys the pollen grains through osmotic shock. Conversely, in the days immediately following pollen collection, an environment with optimal relative humidity significantly accelerates the germination rate of the pollen tube.
- Stratification of male varieties: To cover the full range of female pollination, it is essential to have early, mid, and late pollinating males (or to use multi-grafted trees that combine all three options on the same rootstock). Furthermore, through selective pruning, the males should be trained to grow taller than the females, allowing the wind to distribute the pollen by gravity throughout the plantation.
Flowering usually begins between the end of March and the beginning of April, appearing before the first leaves of the season sprout.
The ultimate success of this process will be consolidated weeks later, transforming these inflorescences into dense bunches laden with fruit, which will mature progressively until the moment of their harvesting and industrial processing.
Technical conclusion: Meticulously planned Pollination in pistachio, combined with strict control of the plant’s water and nutritional status using precision technology, is the only real guarantee to ensure stable, high-quality harvests with a minimal percentage of empty fruits.
