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Prior to Washington, D.C.’s famous National Cherry Blossom Festival each year, the National Park Service announces the projected peak bloom date of the popular flowers.
“It’s really one of the oldest and grandest springtime festivals in the country,” said National Mall and Memorial Parks spokesman Mike Litterst. “Each year, D.C. waits for the cherry tree to signal the arrival of spring.”
Many visitors to the festival, which attracts more than one million people annually, make their travel plans based on the cherry blossoms’ peak bloom date, Litterst said.
The peak bloom date is the point at which 70 percent of the Yoshino cherry trees have blossomed, according to the National Park Service.
Yoshino trees are the most abundant variety of about 3,800 cherry trees that adorn the nation’s capital.
“It’s actually a very scientific way to determine when the peak bloom is going to occur,” Litterst said. “Once the trees reach winter dormancy, we need to collect 220 degree days.”
Cooling and heating degree days are the difference between 65 degrees Fahrenheit and the daily temperature mean, which is the high and low temperatures added together and divided by two.
If the temperature mean is above 65 F, 65 subtracted from the mean will determine cooling degree days, according to the National Weather Service. For a temperature mean below 65 F, heating degree days are calculated by subtracting the mean from 65.
“The warmer a day is, the more degree days you get toward that 220,” Litterst said.
The blooming period, which starts several days before the peak bloom date and lasts for up to two weeks, occurs when 20 percent of the blossoms are open until the petals fall and leaves appear, according to the festival website.
Forecasting the peak bloom date is nearly impossible more than 10 days in advance, according to the National Park Service. To determine the date, National Park Service horticulturalists consider when bloom dates have occurred in previous years, short- and long-range forecasts and what the trees themselves are indicating, said Litterst.
“We have some trees that are known as indicator trees, because they tend to bloom a couple of weeks before the major bloom,” he said. “We keep an eye on them and see where they are in their process.”
Peak bloom is most likely to occur during the final week of March or at the start of April, according to the National Park Service.
How weather impacts peak bloom dates
The average peak bloom date over the previous 100 years is April 4, Litterst said, and weather plays a major role in when the date occurs.
“Part of the excitement around cherry blossoms is that they are so fleeting,” said Litterst. “They’ll usually only last four or five days, depending on what the weather conditions are.”
Unusually warm or cool weather conditions have caused peak bloom dates as early as March 15, which occurred in 1990, or as late as April 18, which happened in 1958.
“Unfavorable weather – cold, rainy or windy – will prevent the blooming period from lasting more than a few days,” said Gena Lorainne, horticulturist and plants expert for gardening and outdoor maintenance company, Fantastic Services.
Cherry trees have a dormant period during the colder months and need their chilling requirements met to activate their blooms, Lorainne said, but the blossoms are susceptible to late frosts.
“Cold snaps may result in trees not blooming at all,” Lorainne added.
In mid-March 2017, a late frost destroyed about half of the Yoshino cherry blossoms, the National Park Service reported.
On the other hand, heat can also accelerate and shorten the blooming process.
Research has shown that warmer conditions as a result of climate change have triggered earlier peak bloom dates in recent years.
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