Cherry blossom viewing season—better known as hanami—has peaked in Tokyo and throughout much of Japan. Among all of the beautiful cherry blossoms, it is the Somei Yoshino, which originated in the Edo era, that is regarded as synonymous with the season. Its cherry blossoms are a breathtaking sight with their flower petals simultaneously falling and dancing in the wind.
The Somei Yoshino’s origins are being traced with the most advanced sciences, giving us an opportunity to focus on its genetic diversity and consider how to protect Japan’s spring beauty from the damage caused by pests and disease.
Mystery of Its Origins Lingers
Although there are numerous breeds of cherry trees in Japan, the Somei Yoshino often seen in parks and schoolyards is the most popular variety. At the end of the Edo period, a gardener in the village of Somei—presently Komagome, Tokyo—called the cherry trees Yoshino-zakura and began to market them. The trees flowered well, their leaves did not stand out, and they grew fast, which quickly made them popular across Japan.
With the development of taxonomy as the Meiji period opened, the Yoshino trees were clearly distinguished from the native cherry blossoms of the Yamazakura trees of Nara, and in 1900 “Somei Yoshino” officially became its Japanese name.
By the 1960s, a theory that the variety was a crossbreed was firmly established. Finally, in 1995, a research team from Kyoto University collected individual genes of Somei Yoshino trees throughout Japan and discovered that they were all genetically identical clones.
The Somei Yoshino has been known to be a hybrid cherry with the native Edo higan as its mother plant, but the father plant has long remained undetermined. There are 10 breeds of wild cherry trees in Japan, out of which four are closely related, making it difficult to conduct a generic diagnosis of their parentage.
Finally, in 2014, Mr. Toshio Katsuki, research team leader at the Tama Forest Garden, Forestry and Forest Products Research Institute, traced its father plant to the Oshima-zakura by using a method of combining numerous genetic markers.
However, a suspicion that other species are partially mixed into the breed still remains. Contradictory theories that the trees originated from pollination by chance, or that they were the result of artificial hybridization, suggest that the specifics of the Somei Yoshino’s origins are still a mystery.
“If the mystery is solved, the answer will be of interest to many people”, said Katsuki.
Refuting Theory of South Korean Origins
South Korea has claimed that the Somei Yoshino originated from the Eishu-zakura, or King Cherry, which grows on Jeju Island. In 2017, the theory prompted Katsuki to conduct a paternity test based on bibliographic surveys of the genes and how the flowers formed.
The result was that the mother plant of the Eishu-zakura was indeed the Edo higan, the same as the Somei Yoshino. However, the father plant of the Eishu-zakura turned out to be the Oyama-zakura, which is different from the Oshima-zakura parent of the Somei Yoshino, thus refuting the South Korean origin theory.
In ancient times, the Yamazakura—often depicted in Japanese poetry called waka—was the favored cherry blossom tree for hanami. However, today the blossoms of the Somei Yoshino take center stage during hanami season.
Although many cherry trees have been propagated by using the grafting method, the outspread of cloning is an exceptional case. The Somei Yoshino trees are genetically the same and have uniform characteristics, allowing their blossoms to dramatically bloom and fall at once.
There is also academic merit in their genetics. For instance, the timing of their blooms can be used to accurately forecast the arrival of spring. The “Cherry Blossom Front,” which indicates bloom forecast dates, is also useful in understanding climate changes due to global warming.
Concerns About Trends in Pests and Diseases
On the other hand, cloning has its demerits. There is fear that damage can be caused by a massive infestation of a single type of pest or disease.
The Somei Yoshino is known to be susceptible to a contagious disease called witches’ broom, which causes the formation of a dense mass of small branches, resulting in the leaves becoming more prominent when the tree is in bloom.
In 2005, in order to preserve diversity among Japan’s cherry blossom trees and to control the spread of disease and damage, the Tokyo-based Flower Association of Japan, a public interest foundation, suspended its long-running distribution of Somei Yoshino saplings.
Instead, they are now distributing Jindai-Akebono cherry trees, which are cultivated in the Jindai Botanical Park in Chofu, Tokyo. This variety is less prone to infection from the witches’ broom disease. Their flowers are slightly darker in color, but otherwise similar to the Somei Yoshino in their blooming pattern and timing.
In recent years, the red-necked longhorn beetle has been causing widespread damage to Somei Yoshino trees. The beetle’s larvae are known to eat their way through inside of the trunk, which can result in the sudden death of the tree. The species is said to have invaded Japan through cargo from China, its native region. This past January, the Ministry of Environment officially designated the beetle an invasive alien species.
New sightings of hump-like mutations recently found to grow on the Somei Yoshino’s branches are causing additional concern. The tree experts are calling for removal of the afflicted branches when these appear. The cause is unknown, and it is feared the blight could lead to more serious consequences in the future.
Although the Somei Yoshino has an excellent growth rate. The downside is that its branches grow too long, obstructing traffic and often growing into roads. Cherry trees tend to weaken when their branches are severed, so from a living environment point of view, the Somei Yoshino is falling out of favor.
For instance, a Somei Yoshino cherry blossom-lined street that stretches one kilometer along a Yokohama city bus route is being replanted with another variety, Yoko cherry trees. The branches of the Yoko cherries grow upward and do not cause road obstruction like the Somei Yoshino trees. Roadside cherries are being replaced with different breeds in many other regions as well.
If diversity in cherry blossoms continues to progress, we may witness a transformation in the famous spring scenery of Japan.
Shigeki Harada is a senior staff writer for the Sankei Shimbun Science News Department.
(Click here to read the original article in Japanese.)