Technical Documentation: Cherry Blossom Bloom Dates in Washington, D.C.

Cherry Blossom Bloom Dates in Washington, D.C.
Identification
1. Description
This regional feature highlights the peak bloom date (PBD) for the most common species of cherry tree
planted around the Tidal Basin in Washington, D.C., from 1921 to 2020. The PBD provides insight into
how shifting climate patterns may be affecting the timing of cherry blossom blooming in one particular
area, as an example of an event associated with the onset of spring. Shifts in phenological events such as
bloom dates can have important implications for ecosystem processes and could have economic and
cultural consequences. For reference, this feature also shows the start and end dates of the National
Cherry Blossom Festival, which is planned to coincide with the predicted PBD each year.
2. Revision History
May 2014
June 2015
April 2016
April 2021
Feature published.
Updated feature with data through 2015.
Updated feature with data through 2016.
Updated feature with data through 2020.
Data Sources
3. Data Sources
Data were provided by the National Park Service (NPS) within the U.S. Department of the Interior, which
cares for the cherry trees around Washington's Tidal Basin. The NPS has continuously monitored PBD
since 1921 for the cherry trees around the Tidal Basin.
The NPS also records the dates for the National Cherry Blossom Festival, with data spanning 1934-2020.
There was a five-year gap from 1942 to 1946 when the festival was canceled due to World War II.
4. Data Availability
All cherry blossom PBD data, as well as National Cherry Blossom Festival dates, are maintained by the
NPS. PBD data back to the 2000s can be found on the National Cherry Blossom Festival and NPS
websites at: www.nationalcherrvblossomfestival.org/about/bloom-watch and:
www.nps.gov/subiects/cherrvblossom/bloom-watch.htm. Festival dates for 2012-2020 were provided
by the organizers of the festival and posted in annual press releases, the most recent of which are
available at: https://nationalcherrvblossomfestival.org/press. The archive of previous PBDs and festival
dates is not presently available online, but those dates were posted on an older version of the NPS
website that EPA accessed in the past. EPA now collects the most recent year to add to the data set for
the purposes of this feature.
Technical Documentation: Cherry Blossom Bloom Dates in Washington, D. C.
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Methodology
5. Data Collection
NPS horticulturalists carefully monitor approximately 3,800 cherry trees around the Tidal Basin. The
most prevalent species—and the one covered by this feature—is Yoshino (Prunus x yedoensis), which
constitutes about 70 percent of Washington's cherry trees. NPS staff have also monitored another
species, Kwanzan (Prunus serrulata 'Kwanzan'), representing about 13 percent of the trees present, but
the Kwanzan data are missing several years (including all years since 2012), so they were not included in
this regional feature.
NPS horticulturalists examine a specific set of Yoshino trees daily and evaluate them with respect to five
stages of cherry blossom development: green buds, florets visible, extension of florets, peduncle
elongation, and puffy white. They use this approach to determine the official PBD, which is defined as
the day when 70 percent of the Yoshino cherry tree blossoms are open in full bloom. A pictorial
description of the phases of cherry blossom development, as well as other general information about
blooming periods, is available at: www.nps.gov/subiects/cherrvblossom/index.htm.
In 2017, roughly half of the Yoshino blossoms were lost during a vulnerable stage of bud development
due to a late frost in mid-March. NPS horticulturalists based the 2017 PBD on when 70 percent of
surviving blossoms reached peak bloom stage. This approach ensured consistency with other years for
comparison.
6. Derivation
Figure 1 plots the annual PBD for the Yoshino trees from 1921 to 2020, along with the annual start and
end dates of the National Cherry Blossom Festival.
For consistency, EPA converted bloom and festival dates into Julian days to support graphing and
calculations. By this method, January 1 = day 1, etc. The method also accounts for leap years, such that
March 31 = day 90 in a non-leap year and day 91 in a leap year, for example. Figure 1 plots Julian dates,
but the corresponding non-leap year calendar dates have been added to the y-axis to provide a more
familiar frame of reference. This means that a PBD of March 31 in a leap year will be plotted at the same
level as April 1 from a non-leap year, for example, and it will appear to be plotted at April 1 with respect
to the y-axis.
7. Quality Assurance and Quality Control
By monitoring the five different stages of cherry blossom bud development, NPS horticulturalists are
able to forecast, and ultimately pinpoint, PBD with minimal uncertainty.
Technical Documentation: Cherry Blossom Bloom Dates in Washington, D. C.
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Analysis
8. Comparability Over Time and Space
The NPS has recorded PBD annually for Yoshino cherry trees since 1921, using a consistent definition of
PBD, examining the same group of Yoshino cherry trees, and using the same set of bud break criteria
throughout the period of record. These consistent practices allow for comparability over time.
Start and end dates for the National Cherry Blossom Festival have been provided for reference only.
While these dates add an interesting cultural and economic element to this regional feature, they
fundamentally reflect human decisions based on economic and social factors that have changed over
time. In particular, the festival has evolved from a single day or weekend to a multi-week event.
This regional feature is limited to a small geographic area. Methods have been applied consistently
within this area.
9. Data Limitations
Factors that may impact the confidence, application, or conclusions drawn from this feature are as
follows:
1. The timing of PBD for cherry trees can be affected by a variety of weather and climate factors.
This feature does not necessarily pinpoint a single main cause of the observed trends, although
winter and early spring temperatures are believed to play a key role.
2. The PBD does not provide information on non-climate factors that may affect cherry tree
phenology (the timing of key developmental events) or health.
10. Sources of Uncertainty
Because PBD is clearly defined, and NPS horticulturalists have used a single, consistent definition over
time, there is little uncertainty in either the definition or identification of PBD. Uncertainty in the
measurements has not been explicitly quantified, however.
11. Sources of Variability
Because PBD is highly sensitive to changes in temperature, natural variations in seasonal temperatures
contribute to year-to-year variations in PBD. Although the PBD for these cherry trees is primarily
affected by temperature, other aspects of changing climate could also affect the annual blooming date.
Extended growing periods or warmer autumns may indirectly affect PBD by altering other stages of
cherry tree development (Chung et al., 2011).
12. Statistical/Trend Analysis
EPA calculated the long-term trend in PBD for Yoshino cherry trees by ordinary least-squares linear
regression to support a statement in the "Key Points" text. The 1921-2020 trend had a slope of -0.062
days/year, with p = 0.013. Thus, the trend is significant to a 95-percent level.
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References
Chung, U., L. Mack, J.I. Yun, and S. Kim. 2011. Predicting the timing of cherry blossoms in Washington,
D.C. and Mid-Atlantic states in response to climate change. PLOS ONE 6(ll):e27439.
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