Each spring, large crowds gather on the University of Washington Quad to admire 29 puffy pink cherry trees making their seasonal debut. The trees begin to wake up as the weather warms, and this year, estimates suggest that they will reach “peak bloom” on March 20.
The UW’s iconic cherry trees achieve peak bloom when 70% of the blossoms have opened, but the week before and after still offer visitors an optimal viewing experience.
The cherry blossom visitors’ website provides updates on bloom status as well as details on transportation, activities and amenities. The cherry blossoms also have live video feeds for virtual viewing and their own social media accounts on Instagram and BlueSky.
The cherry trees are both beautiful and ecologically significant. Tracking when the buds burst each year helps researchers predict peak bloom and determine how climate warming is impacting the trees, which were planted in the Washington Park Arboretum in 1936 and then relocated to UW in 1962.
This year, many plants began to emerge early as a mild winter gave way to spring. Recent UW research described how plants rely on both temperature and light cues to time their flowering. Temperature is particularly important to cherry trees, which estimate the arrival of spring based on how cold it has been. They accrue “chilling units” as winter progresses and “heating units” as it yields to spring.
“The buds need to accumulate a specific amount of chilling units before they can start accumulating the heating units. When it is not as cold, the chilling units accumulate much slower, so it takes them longer to wake up from dormancy, which is very counterintuitive,” said Marlee Theil, a UW doctoral student of environmental and forest sciences.
Theil is now overseeing data collection on campus, with the help of approximately 20 undergraduate students. The researchers make observations as the trees begin to wake up and feed the data into a computer model that incorporates weather forecasts to predict peak bloom.
Historically, the onset of peak bloom has fallen between March 12 and April 3, with an average date of March 23. While the weather impacts peak bloom year to year, climate change drives longer term trends over multiple decades.
Research published last summer shows that bloom time has shifted approximately two days earlier each decade since the 1960s. Researchers began monitoring the trees in 2012 and referenced newspaper archives to estimate peak bloom dates for the preceding years.
“With the climate warming more rapidly in the spring, I expected to see the flowers blooming earlier,” said lead author Autumn Maust, a recent doctoral graduate from the UW school of environmental and forest sciences. “But as we dove into the literature and examined the data, we saw a delay in bloom, as a result of winter warming in Seattle.”
The study focused on the Somei-yoshino, or Yoshino, cherry tree cultivar. These trees, sometimes called the Japanese flowering cherry, are found throughout Japan. They also line the National Mall in Washington D.C. and paint many Seattle neighborhoods pink in the springtime.
The bloom delay Maust observed applies only to Yoshino cherry trees in Seattle. In colder climates, such as Washington D.C., the trees have ample time to accrue chilling units. Still, the two populations are quite similar, genetically.
Propagation, or breeding more trees, occurs by grafting one tree onto another. This process limits genetic variability in favor of consistency. Because all Yoshino cherry trees are sterile clones of one another, they do not produce fruits or seeds, but they do reliably bloom in beautiful pink hues each spring.
Even so, there is still enough variation between trees in different places to trace their history. To figure out where the UW’s trees may have come from, UW researchers and students sequenced their DNA. They compared the results to Yoshino cherry trees at sites throughout Japan and found a cluster of close relatives, with approximately 85% genetic overlap, near Shimane University in the city of Matsue.
The work, led by Adam Steinbrenner, a UW associate professor of biology, sheds light on the origin of the trees, some of which may be nearly 100 years old.
For more information on bloom time, contact Theil at mtheil@uw.edu or Maust at amaust@uw.edu. For information about the Yoshino Genome Project, contact Steinbrenner at astein10@uw.edu.