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FlowercastingHow do horticulturists know when the cherry blossoms will bloom?

By Ben Harder
Posted Tuesday, March 27, 2007, at 6:24 PM ET
Cherry trees in Washington, DC. Click image to expand.

On March 8, the National Park Service predicted that the Washington, D.C., cherry trees will be in peak bloom during the first week of April. Past predictions have usually been accurate to within a few days, even though the peak varies from year to year by as much as five weeks. How does the agency determine when the trees will blossom?

It—or rather, he—makes an educated guess. Responsibility for the official prediction falls to the Park Service's Chief Horticulturalist Robert DeFeo for the national capital region. Every year, at a press conference in early March, DeFeo announces when peak bloom is most likely to occur around Washington's Tidal Basin. (The peak is technically the date on which 70 percent of the area's Yoshino cherry blossoms are open.) He reads nature's clues, follows the weather forecasts, and then ventures out on a limb with his guess.

Various environmental factors affect when plants open their flowers, including day length, air temperature, and soil moisture. In the Washington area, the number of hours of sunlight and the earth's water content are relatively consistent from one spring to the next. So, air temperature looms large in determining when the cherry blossoms emerge.

Chilly days during the winter and warm or mild days during the spring generally accelerate the maturation of flower buds. By contrast, unseasonable winter warmth and unseasonable springtime chills can slow the process.

As blossoms mature, they go through a sequence of developmental stages. The arrival of each new stage reduces the uncertainty about when the blossoms will peak. On Tuesday, buds entered a stage called peduncle elongation, which means that they're almost certain to blossom at least five days from now but not more than 10. Weather forecasts over the next few days will help DeFeo determine where peak bloom will fall within that window. He sometimes refines the prediction even more by analyzing how quickly other flowering species are maturing.

In 2000, a series of scorching days in early March brought peak bloom on March 17, two weeks ahead of what had been predicted and too soon for it to coincide with the National Cherry Blossom Festival (which brings about 360,000 tourists to the capital each year). This year, on the other hand, a cold snap that smacked the Eastern Seaboard a week and a half ago threatened to postpone peak bloom until after its anticipated date. But a subsequent string of warm days got the blossoms back on track.

Not all kinds of weather can be canceled out, however. Once peduncle elongation occurs, cherry-blossom buds become sensitive to frost, which can damage or destroy them. Rough weather can also cut short the blooming period, which otherwise lasts up to two weeks: When the blossoms are fully open, heavy winds or rain can knock petals off the trees. But by that point, peak bloom has either arrived on schedule or it hasn't.

Bonus Explainer: While DeFeo's bloom prediction applies only to downtown Washington, Japan's meteorological agency issues predictions for that whole country. This year, it made an embarrassing error in calling the blossoms too soon. But one can hardly blame them: Japan's cherry trees have been blooming earlier and earlier, a trend that scientists attribute to global warming.

Got a question about today's news? Ask the Explainer.

Explainer thanks Scott Aker of the U.S. National Arboretum, David Ellis of the American Horticultural Society, and Rob DeFeo.

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Ben Harder is a writer at Science News magazine and acting editor of sciencenews.org.
Photograph of cherry blossoms in Washington, D.C., by Alex Wong/Getty Images.
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