Climate Change Indicators in the United States 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 2016. 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: Feature published.

June 2015: Updated feature on EPA's website with data through 2015.

April 2016: Updated feature on EPA's website with data through 2016.

Data Sources

3. Data Sources

Data were provided by the National Park Service (NPS) within the U.S. Department of the Interior (DOI),
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 1934-2016.
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 1990s can be found on the National Cherry Blossom Festival website at:
www.nationalcherrvblossomfestival.org/about/bloom-watch. Festival dates for 2012-2016 were
provided by the organizers of the festival (contact information at:

www.nationalcherrvblossomfestival.org). Previous PBDs and festival dates are not presently available
online, but they were posted on an older version of the NPS website, which EPA accessed most recently
in spring 2015.

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

-------
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 70 percent of Washington's cherry trees. NPS staff have also monitored another species,
Kwanzan (Prunus serrulata 'Kwanzan'), representing 12.6 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/cherryblossom/bloom-watch.htm.

6. Derivation

Figure 1 plots the annual PBD for the Yoshino trees from 1921 to 2016, 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 actually 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
actually 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.

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

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

-------
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 1921-2016 trend had a slope of -0.052
days/year, with p = 0.0497. Thus, the trend is significant to a 95-percent level.

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.

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

-------