Climate Change Makes Cherry Trees Blossom Early – and Puts Them at Risk

The Washington Post / Scott Dance

12:56 JST, March 20, 2023

WASHINGTON – Washington’s cherry trees are on the verge of peak bloom several days earlier than normal, while blossoms are appearing earlier than ever seen in Tokyo. It’s the result of mild winter temperatures sending a signal that it’s safe for the trees to wake from hibernation.

But is it?

As fast-warming winters encourage early blooms, they also risk exposing the trees to damage from cold snaps that remain common into March at the mid-latitudes where the ornamental cherry trees have long thrived. A close call is in the forecast this weekend, with temperatures possibly dropping into the upper 20s in Washington.

It isn’t the only threat climate change presents: Come summer, hot and muggy nights make it hard for the trees to repair themselves. All year round, floods inundate their roots with salty water.

Here is a look at how climate change is affecting the beloved blooms:

Late freezes threaten early blooms

In the spring, it isn’t so much rising average temperatures that threaten the hardiness of cherry trees, according to Christopher Walsh, a horticulture professor at the University of Maryland.

“What worries me more is the erratic temperatures,” Walsh said.

Winter is warming faster than any other season in the northern hemisphere, and that is sending confusing signals to cherry trees and other plant life.

While hibernating for winter, the trees keep track of how long temperatures spend below about 45 degrees. Once the weather starts to warm, though, they start another count, Walsh said, gauging whether it’s safe to begin growing again based on how much temperatures rise.

Increasingly mild winters mean the cherry trees risk blooming so early they could still face damaging hard freezes, as occurred in Washington in 2017.

That year, the Tidal Basin blossoms were on the verge of a mid-March peak bloom, almost two weeks earlier than average, when temperatures dropped into the lower to mid-20s on back-to-back mornings. That wiped out nearly half of Washington’s cherry blossoms.

When temperatures remain that cold for several hours, it harms the blossoms if the buds have begun to flower. Even colder temperatures, as well as sunlight reflecting off snow cover, can also damage cherry trees’ bark and internal tissue, Walsh said. Temperatures are forecast to skirt the danger zone for blossom damage this weekend, with lows expected in the upper 20s to near 30 early Sunday and early Monday mornings.

Both Washington and Japan, where the Yoshino variety of ornamental cherry trees originate, have seen marked trends of earlier blooms. Over the past century, the average peak bloom date in Washington has advanced from around April 5 to March 31 as spring temperatures have warmed.

National Park Service data indicates peak bloom has occurred before April 5 in 15 of the last 20 years; this year’s peak is also expected to be early, probably within the next week.

In Kyoto, Japan, where records go back 1,200 years, the date of peak bloom has rapidly shifted earlier over the past century and a half. Peak bloom arrived earlier than ever in 2021, on March 26.

And this year, the first blooms in Japan arrived in Tokyo this week, 10 days earlier than normal and matching 2020 and 2021 for the record-earliest, according to the Japan Weather Association.

The cherry blossoms are a beloved marker of nyugakushiki, an annual ceremony that marks the beginning of a new school year in early April. But the warming trend could mean the image of school entrance ceremonies under blooming cherry trees may only be a memory one day, weather association forecaster Leona Wada wrote.

Summer brings heat stress

Months after the risk of freezes has passed, cherry trees face risks of similar damage from heat stress. When temperatures surge toward the triple digits, they sometimes must shut down their growing and energy-gathering processes to survive, said Richard Olsen, director of the National Arboretum.

And Olsen said the stress of such high temperatures can compound when heat is slow to dissipate on summer nights, an increasingly common but often overlooked consequence of global warming and climate change.

When that happens, the trees produce so little energy during the day, they don’t have enough to devote to repairing themselves and their regular metabolism to stay alive, he said.

Both heat and freezes can cause damage to the trees’ bark and trunks that produces vertical cracks and gashes that affect their growth and make them vulnerable to further problems, Walsh said.

The issues create a “perennial question” about the survivability of the cherry blossoms in a changing climate, Olsen said. The situation isn’t dire just yet, though.

“It’s more going to be a chronic issue that we can solve with diligent planting and replacement of the species mix we use,” he said.

Washington’s Yoshino cherry trees aren’t going anywhere, of course. But in garden stores and landscape architecture, the Japanese species isn’t among the varieties being planted in the D.C. region and other parts of the country, he said. Instead, popular cherry trees nowadays are descended from species native to Taiwan, at a latitude hundreds of miles south of Japan’s.

Floodwaters add to the challenges

At the same time, another factor of climate change adds stress to the cherry trees that surround the Tidal Basin: sea level rise. Increasing inundation of the soil surrounding the basin exposes the trees to salt water from the brackish Potomac River.

“Enough salt intrusion in the soil can weaken and eventually kill a cherry tree,” according to the National Park Service.

The park service last year launched an effort to restore the area around the Tidal Basin, subject to frequent tidal inundation – it regularly hits one section of its banks.

Sea level rise will continue to be a challenge for the 3,800 or so cherry trees that bring flocks of camera-toting tourists to the Tidal Basin each spring. Waters of the Potomac and Anacostia rivers have surged more than a foot over the past century because of sea level rise, and that rise is forecast to accelerate in the coming decades.