Understanding pistachio bloom
As California pistachio orchards wake from dormancy in the spring, the bloom phase offers one of the most critical and visually distinct windows into tree biology and orchard performance for the year ahead. Pistachios are dioecious, meaning male and female flowers grow on separate trees — a fact that defines both orchard layout and bloom dynamics.
Male pistachio trees, very typically planted every five trees, every five rows, produce clusters of small, pollen-bearing flowers in early spring. These staminate flowers are tightly packed and lack petals, essentially engineered by nature for efficiency, not beauty. Male trees begin to bloom slightly earlier than females (though some years of insufficient chill has produced the opposite effect), and their progression is carefully tracked through stages as defined by Bob Beede, Emeritus advisor of Kings County Cooperative Extension: tight bud, bud swell, pre-bloom (cabbage head), bloom, and post-bloom. A simple tap of a male branch containing buds in bloom produces a visual distribution of pollen.
Female pistachio trees usually bloom just after the males begin to shed pollen. Their inflorescences are composed of apetalous (no petal) pistillate flowers that are entirely wind-pollinated. Unlike almonds or other insect-pollinated crops, pistachios do not require pollinators; instead, the wind must carry pollen from male to female trees in sufficient quantities during a relatively narrow bloom window.
Synchrony between male pollen shed and female receptivity is therefore essential. Any mismatch — due to uneven budbreak, chilling variability, or weather anomalies — can negatively affect fruit set.
Female bloom also progresses through several distinct stages also defined by Beede: tight bud, bud swell, pre-bloom, bloom, and eventually post-bloom, or BB stage, when nutlets begin to develop. These stages are carefully rated by advisors and growers to determine how well bloom is progressing across blocks and to anticipate the success of pollination.
The progress of bloom may also indicate crop or cluster uniformity, or lack thereof, that lends to various decisions to be made before and at harvest.
Environmental factors like temperature, humidity, and wind can accelerate or disrupt these bloom stages. Cool, wet weather can slow pollen shed or reduce its viability, while high winds may cause uneven dispersal. In contrast, warm and dry conditions often lead to a tight, efficient bloom that maximizes pollination potential.
Newly growing vegetative and fruit bearing tissue is also sensitive to freezing temperatures, and short spells of freezing temperatures have shown in 2017 and 2024 in Golden Hills variety to cause injury best described as black speckling on the rachis and petioles that eventually leads to aborted nuts and reduced nut set.
This phenomenon is rare, and some advisors have associated this with botrytis or botryoshaeria and recommend fungicidal treatment, but plating of the affected tissue at Extension labs yielded inconsistent results, and was therefore considered to be due to environmental conditions, specifically a very short dip below 32° F and rupture of succulent plant cells.
Dust deposition during the bloom period has also been shown to reduce final nut set, and certain cultural practices are generally advised against during this time.
2025 bloom observations
Golden Hills variety across Fresno County appeared to be very uniform in emergence with Randy’s and Tejon males slightly ahead of females within the first two weeks of bloom. The bloom stages progressed from tight green tips to mostly pre-bloom and bloom, with an occasional very small BB, fairly rapidly at the beginning of April, and is largely at post bloom, BB, and nutlet stages with full leaf out.
A large amount of clusters in female trees have me optimistic that we should see a step up in industry-wide Golden Hills production. Some areas at higher elevation seem to be slightly behind this, but still appear to have a very nice count of clusters.
Kerman variety has shown a bit of a mixed bag of timing, and a bit of a wider range of bloom stages, ranging from tight to pre-bloom and a small amount of bloom across the first two weeks of bloom, to a range of pre-bloom and BB stage through late April.
Peter’s male pollinators show a similar wide range of bud development, so I do think the overlap will be sufficient in the farms I’m watching. Kerman, being more chill sensitive than Golden Hills, and having the tendency to alternate bear, had a very low production year in 2024, partly due to insufficient chill and perhaps due to “off-year” timing.”
Some growers did not harvest once-high yielding crops where affected the most.
Chill hours and portions of 2025 were largely considered adequate, so the production expectation for this year is generally a rebound to higher yield. I’m keeping my eyes on Kerman and have a feeling that harvest could be prolonged in a similar fashion (2-3 shakes) to 2024.
I’ve looked at far fewer Lost Hills ranches than other varieties, however those that I have spent time in appear to have strong fruit set with, however, a significant delay in full leaf out. The bloom progress was, like Kerman, a wider range of stages at any given time and could be multiple shake fields.
Other observations
Many cover crops are still intact, and depending on when the grower decides to terminate, he or she may have missed the opportunity to destroy the mummies that may have been blown into the cover crop during sanitation and berm cleaning.
Navel Orangeworm (NOW) typically emerges from overwintering mummies between 250-325 degree days from January 1, which, depending on heat accumulation, can be between March 20 through April 20, and that first flight of the year can persist for 3-6 weeks.
While it’s not advisable to create dust during bloom by mowing so as to not damage nut set, it’s something to consider completing at post bloom as a function of NOW sanitation.
Depending on the type of cover crop, some growers may choose to leave it intact after bloom, so speak with your pest control advisor about the potential effects on NOW pressure, as well as the humidity effects of the cover crop going into the summer on diseases like alternaria. Many who do not plant cover crops have already completed the sanitation task.
Gill’s mealybug crawlers were found in many orchards aggregating on the new green tips of trees and lots of applications were made during March and April. Centaur is out of stock in many retail outlets, which is a sign that there will be another year of pressure to deal with.
Some weed species are beginning to pop on berms, depending on your pre-emergent or overall weed control plan, and some growers are beginning to consider and make herbicide applications.
In short, pistachio bloom is more than a signal of spring—it’s a finely balanced biological dance that sets the stage for the entire crop, and a shotgun start to a slew of other farm tasks to manage. I wish everyone a happy springtime as summer approaches, and please don’t hesitate to reach out to [email protected] for any level of guidance on your pistachio farm.
Main image: Early formation of pistachio nuts.
Photo: Crystal Nay, Modern Ag Media
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