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Posted at 12:45 PM ET, 03/31/2009

Weather, Climate and Blossom Bloom Dates

By Ann Posegate

Wx and the City

* April Showers? Full Forecast | Cherry Blossom Poems *

It's that time of year again: peak bloom time for Yoshino cherry trees around the city. This year, peak bloom is predicted to be April 1-4, after a slight adjustment of dates last week by the National Park Service. The blooming period began Friday and should last through April 11, depending on the weather. Let's cross our fingers that predicted showers don't wash too many blossoms away, although based on past years, that wouldn't be unusual.

As you may have noticed, the famous cherry trees bloom on different days each spring. How does weather affect bloom dates and duration? And how have these dates changed over the years?

Keep reading for the answers...

The National Park Service (NPS) predicts the cherry trees' bloom dates. Much like forecasting weather, predicting these dates is tricky and not very accurate more than 10 days out. But, also like weather, there is a science to predicting bloom dates.

Blooming cherry blossoms with the Jefferson Memorial in the background. Captured Friday by CWG photographer Ian Livingston.

According to NPS, the peak bloom date occurs when approximately 70 percent of the blossoms are open. Horticulturalists at NPS monitor five different stages of bud/blossom development and base their prediction on how long the stages have lasted in the past.

The cherry trees form their initial buds in the fall and lie dormant during winter. During the first few stages of new spring growth (late February to mid-March), buds are more tolerant of extreme weather conditions like wind and rain, since the delicate blossoms have not yet begun to open.

How fast the blossoms develop has a lot to do with temperature; a warm or cold spell can cause them to bloom earlier or later than expected -- the earliest recorded peak bloom date since the trees were gifted to the city in 1912 was March 15, 1990, and the latest was April 18, 1958. Once the flowers begin to open around the end of March, they are sensitive to any weather extreme, especially frost.

Interestingly, Japan's cherry blossom bloom dates are predicted by the Japan Meteorological Agency (JMA). Its predictions of bloom dates are intended to be precise, as a large number of Japanese cultural activities are dependent on blossoms and the trees have been observed for centuries. According to the JMA:

"...prediction of cherry blossom blooming dates is conducted by applying the relationships between temperatures and blooming dates in previous years to the temperatures observed since last fall and those predicted in weekly, monthly and three-monthly forecasts. The predicted blooming date is the first day of the season on which five or six blossoms of a sample tree will bloom, and becomes two to three days later for each 100-meter increase in altitude. The date of full blooming is the first day on which more than 80% of the blossoms of a sample tree have bloomed."

Temperature changes over time can cause long-term shifts in bloom dates. For example, in a Smithsonian study, the District's cherry blossoms were found to be blooming about seven days earlier in 2000 than they were in 1970.

This trend is also occurring in Japan, due to warmer temperatures and increased urbanization. There are many landscapes in Japan, and cherry trees bloom at different times depending on their location -- trees in the southernmost islands begin blooming first and those on the northern most island and in higher elevations bloom last. The "blossoming line" (fun fact: this is also known as a "front") -- the latitude where trees start to flower on a given day -- on April 1 has moved 125 miles north over the past 40 years.

Trends in phenology, or the study of recurring plant and animal life cycle events, are good indications of how human activities are altering the natural environment. To participate in monitoring other plants around the city this spring, check out Project BudBurst.

Related links:

Washington Post's 2009 Cherry Blossom Festival Guide
The Official Website of the National Cherry Blossom Festival
Guide to other flowering cherry trees at the U.S. National Arboretum
Japan Meteorological Agency's cherry blossom forecasts
Climate Change and Cherry Tree Blossom Festivals in Japan (related article)

By Ann Posegate  | March 31, 2009; 12:45 PM ET
Categories:  Cherry Blossoms, Climate Change, Nature, Posegate, Wx and the City  
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