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How Does the National Park Service Know When the Cherry Trees Will Blossom?
The short answer: using lots of educated guesswork. By Michael Gaynor
Photograph by Alison Kitchens.
Comments () | Published March 22, 2012

Of the 12 species of cherry trees sent to Washington 100 years ago, more than half were Prunus x yedoensis, or Yoshino cherry--a flowering tree whose winter hardiness is matched by its springtime beauty. 

The peak bloom, the point at which at least 70 percent of the blossoms are open, isn't easy to predict. Temperature, frost, and soil moisture play a role, making it hard to give an accurate date more than ten days in advance. Since 1991, National Park Service horticulturist Robert DeFeo has been divining bloom dates by monitoring these five stages of development:

1. Green color in buds. DeFeo usually starts noticing the initial stage of blooming in late February or early March. He says the sunlight-rich south side of the Tidal Basin usually begins this process slightly earlier, "but nobody notices that other than me."

2. Florets become visible (on average 16 to 21 days before peak bloom). Small florets, or the beginning stages of flowers, begin to grow in March. DeFeo says the cherry blossoms bloom in concert with many of the plants around them: "There's a shrub, Cornus mas, on my way to work that tells me what I'm in for each day."

3. Extension of florets (12 to 17 days before peak bloom). Today there are more than a dozen species of cherry trees around the Tidal Basin. DeFeo refers to one as his "indicator tree." A hybrid, it always blooms seven to ten days before the rest. He planted one outside the Park Service offices so colleagues could get a better idea of how close they're getting to peaking.

4. Peduncle elongation (five to ten days before peak bloom). This "frost-critical" stage of the blooming process means that a late-March cold snap could be devastating to the flowers. DeFeo says that in his 21 years he's never seen that happen.

5. Puffy white flowering (four to six days before peak bloom). This stage marks the imminent peak bloom. The Cherry Blossom Festival dates are set using the average peak date of April 4. The earliest peak on record was March 15, 1990, the latest April 18, 1958. Despite the recent warm winter, DeFeo pegged this year's peak bloom at between March 20 and 23.

This article appears in the April 2012 issue of The Washingtonian.

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Posted at 11:40 AM/ET, 03/22/2012 RSS | Print | Permalink | Comments () | Washingtonian.com Blogs