1. Cherry Petals Begin Falling After Full Bloom, Even in Still Air
Before a cherry tree reaches full bloom, it holds onto its petals even if the wind blows. However, once full bloom passes, petals begin to fall even in still air [1]. If wind were the only cause, petals would fall equally before and after full bloom. According to Weathernews, the key lies at the base of the petal. A layer of cells called an "abscission layer" forms there, cutting the petal off from the receptacle, which is the base that supports the flower [1]. This means the separation process begins before the petal actually drops. Because petals are hard to dislodge by wind before full bloom, wind alone cannot explain the timing of the fall. This process is not unique to cherry trees. In flowers where petals detach, an abscission layer forms between the petal and the base of the flower, after which the petal falls [2]. However, how this layer forms is not well studied in many ornamental plants [2].
2. Petals Fall Whether or Not the Flower Was Pollinated
It is tempting to think petals fall because pollination is complete. But a 2026 study by researchers including those from Seoul National University found that Somei Yoshino cherries drop petals quickly regardless of whether pollination occurred [4]. Flowers that were hand-pollinated and those that were not fell in the same way. The research team interprets this to mean that petal fall is built into the flower's developmental schedule [4]. This schedule varies by species. In the same study, the Oyamazakura cherry retained the petals of unpollinated flowers and later dropped the entire flower stalk [4]. So, the "early fall" of Somei Yoshino is not common to all cherry relatives. In short, flowers that did not receive pollen and those that did release their petals at the same speed. Falling is not the result of failed pollination, but part of a pre-set plan.
3. How Many Days Pass Between Opening and Petal Fall
The Japan Meteorological Agency defines the "opening date" as the first day when 5 to 6 or more flowers are open on a sample tree, and the "full bloom date" as the first day when about 80% or more are open [3]. There is no official observation category for the "falling date" [3]. Private weather companies estimate that it takes about 7 days from opening to full bloom in regions from Kyushu to Kanto, and about 5 days from Hokuriku to Hokkaido. A few days after full bloom, petals begin to fall all at once [1]. These are rough guidelines, not official statistics. The duration changes with temperature. Tree doctor Hiroyuki Wada states that if temperatures drop due to cold spells, the blossoms may last about 2 weeks, but if temperatures are high, they may fall in 1 week [1]. It is not a clock that runs on a fixed number of days, but one that changes speed based on temperature.
4. Petal Separation Is Prepared Days Before the Petal Falls
The separation does not seem to start suddenly right before the petal falls. In the same study, petal detachment was observed about 3 to 4 days after opening, which is considered the first stage of a 5-stage detachment process [4]. In experiments where flowers were given a substance called ACC, which is a precursor to the plant hormone ethylene, petal detachment happened sooner. The authors interpret this to show that the area with the abscission layer is prepared in advance and ready to respond to ethylene [4]. This 3 to 4 day figure is from the specific conditions of the study. It cannot be directly added to or subtracted from the weather company's guideline of "a few days after full bloom." The study tracks the movement of individual flowers, while "full bloom" refers to the appearance of the whole tree, so they are looking at different clocks.
5. Why Somei Yoshino Petals Fall All at Once in a Blizzard
For a "cherry blossom blizzard" to occur, many petals must fall at the same time. Somei Yoshino cherries are all clones with the same genes. If local conditions like temperature are similar, they tend to bloom, reach full bloom, and fall around the same time [1]. Even trees with the same genes will fall at different times if the temperature or other conditions differ. In the same area, trees with the same genes experience similar temperatures, so their falling times tend to align.
6. Why There Is No Official Date for Petal Fall
The Japan Meteorological Agency has clear rules for the opening date (5 to 6 flowers open) and the full bloom date (about 80% open) [3]. There is no such rule for the falling date. If you were to define it yourself, would you choose the day half the petals are gone, or the day they are almost all gone? Would you track one flower, or the whole tree? The study's clock looks at individual flowers, while the full bloom clock looks at the whole tree. The answer depends on which one you choose. For the official definitions and sample tree descriptions, you can read the "Cherry Blossom Opening and Full Bloom" page on the Japan Meteorological Agency's website [3].