How winter cold affects pistachio pollination?

Oct 5, 2023 | Pollination

By Gal Sapir

Vernalization – How it affects flower synchronization in pistachio?

Deciduous fruit trees naturally go through a dormant phase when the days become shorter and the temperature drops in the fall. This helps them survive the winter months and protect themselves from harsh cold temperatures and other environmental factors that could cause damage to the tree. The process known as vernalization involves the use of a prolonged cold period, typically winter, to promote flowering in plants (1).

As winter approaches, trees enter dormancy and cease producing leaves and flowers. At this time, the roots also stop growing. This slowdown in metabolism makes the tree less active but more resilient to stress, allowing it to better survive the winter months. In addition to affecting the tree’s metabolism, chill hours also play a role in the tree’s flowering and fruiting. To ensure proper bud break and flowering in the spring, fruit trees require a specific amount of chill hours (below 45°F/7°C) to break dormancy. This phenomenon varies based on the genetics and origin of each species and cultivars (2). California growers have access to a prediction tool ( that combines winter chilling hours and spring heat hours (known as growing degree hours) to determine the start of the bloom. However, it’s important to note that this tool is not specific to each cultivar and variations can occur between different cultivars and between male and female varieties.

When a tree doesn’t get enough chill hours, it may produce fruit inconsistently and at a lower rate. Additionally, insufficient chill hours can affect the synchronization of flowering across different varieties of plants. Flower synchronization is when plants coordinate the timing of their flower development and blooming to increase the likelihood of successful pollination by other members of their species.

For pistachio trees to successfully pollinate, their flowers must be synchronized between male and female trees. This synchronization allows for cross-pollination between the male flowers that produce pollen and the female flowers that receive the pollen to facilitate fertilization and the consequence – fruits. The transfer of pollen from male to female trees is a critical step in fertilization and is essential for achieving high yields in pistachio breeding programs.

Male pistachio tree already flowering while female trees are still dormant. California 2021

Combinations of varieties such as the Californian-bred female cultivars ‘Golden Hills’ and ‘Lost Hills’ and their male pollinator cultivar ‘Randy’ were done over several years that had sufficient chilling hours (3). However, due to global warming and reduced chilling hours in recent years, their synchronization is now at risk. This is causing a gap between male and female inflorescence, resulting in reduced cross-pollination and lower yields.

A method called mechanical pollination, such as the one developed in EDETE, can solve issues like this. This involves producing pollen from male trees during their blooming period and spreading it onto the female tree during the peak of its blossom. This ensures maximum yield regardless of synchronization of flowering.

(1) Kim, D. H., Doyle, M. R., Sung, S., & Amasino, R. M. (2009). Vernalization: winter and the timing of flowering in plants. Annual Review of Cell and Developmental, 25, 277-299.‏

(2) Pope, K. S., Brown, P. H., Dose, V., Da Silva, D., & DeJong, T. M. (2013, May). Yield potential analysis to model dormancy requirements in pistachio. In VI International Symposium on Almonds and Pistachios 1028 (pp. 103-106).‏

(3) Chao, C. T., Parfitt, D. E., Ferguson, L., Kallsen, C., & Maranto, J. (1997, August). Breeding and genetics of pistachio: The California program. In II International Symposium on Pistachios and Almonds 470 (pp. 152-161).

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