As sakura season arrives in Tokyo, ‘cherry blossom prof.’ explains science of their appeal

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TOKYO — Everyone has been waiting for it, and now it’s here: the cherry blossom season has begun in Japan’s capital. Everyone loves the pink sakura blossoms’ gently falling petals, but what’s the underlying science?

According to the Japan Weather Association, the cherry blossom season was expected to begin in Tokyo on March 21, arriving a day earlier in Nagoya and Fukuoka, two days later in Osaka and on March 31 in Sendai. The variety of trees used for the predictions is Somei Yoshino, which can be found across the country.

According to Toshio Katsuki, a “cherry blossom professor” with the Forestry and Forest Products Research Institute’s Kyushu Research Center, Somei Yoshino spread nationwide due of its ability to survive in different environments and because of its aesthetic beauty.

Somei cherry trees are a hybrid of two varieties, Prunus itosakura (Edo higan) and Prunus speciosa (Oshima cherry). Somei Yoshino refers to the specific Somei cultivar widely found throughout Japan. The cultivar got its pale pink tone and the attribute of flowers blooming before leaves appear from the Edo higan variety, while the flowers’ large size comes from the Oshima cherry.

“Somei Yoshino is truly a cultivar that grabbed the best of features from its parents,” Katsuki said.

Another characteristic is that all trees are clones with the same genes. Somei Yoshino was spread by grafting. Because of this, they all bloom at nearly the same time in similar environments. If the sight of the flowers blooming and falling around the same time puts you in a festival mood, it’s thanks to their cloned nature. That’s what gave them the form that has appealed to peoples’ senses of beauty and impermanence.

The importance of winter

What determines when cherry blossoms bloom? According to Katsuki, temperatures are deeply involved. After the blossoms fall, flower buds form in summer and the trees go into hibernation from fall through winter. Once it again becomes warmer as spring approaches, the flower buds grow and begin to bloom.

In other words, warm weather alone does not make them bloom early. They must also be exposed to cold during fall and winter, and this stimulation turns the switch to end their hibernation. The trees in parts of Kagoshima Prefecture bloom later than those in neighboring areas because warm winters delay their exit from hibernation.

“The growth process lasts until they fall. Flowers will last longer if the weather stays cool after blooming,” Katsuki explained. While it’s often said the flowers fall when it rains, it’s actually the higher temperatures on sunnier days after it rains that suddenly drives their growth.

There are thought to be over 100 cultivated species and 10 wild varieties of cherry blossom trees, including the Kumano cherry that Katsuki discovered in 2018. As he continues research into increasing the number of trees adapted to different environments, Katsuki said, “Each (variety) has its own good points for shape, color and other aspects of the petals. Look for varieties other than Somei Yoshino, and enjoy cherry blossom viewing.”

(Japanese original by Kouki Matsumoto, Lifestyle, Science & Environment News Department)