So what exactly is "peak bloom" anyway? When does it happen? And does it matter?
News reports and cherry blossom watchers often talk about the “peak bloom” date. But that’s not exactly the same thing as what we might commonly refer to as “full bloom” or the “blooming period.”
Often the news reports themselves get confused. If the National Park Service issues a prediction that peak bloom will be April 8-12, it doesn’t mean that the flowers will start blooming on April 8 and stop on April 12, which is how some (but not all) news reports wrongly interpret it. It also doesn’t mean that they’ll even be in full bloom for that entire time.
So it’s useful to know what that means if you’re planning your own visit. The peak bloom date is an important milestone in the process, but it’s not the only day when you can see beautiful cherry blossoms in bloom.
The official forecast for the peak bloom period is issued by the National Park Service. It is their horticulturalists that look after these remarkable trees. Their predictions are based on a mix of historical data, weather observations and forecasts, long experience in working hands-on with the trees (some of the NPS horticulturalists are at least second generation cherry blossom carers), and direct observations of a specific group trees. In particular, they look at the maximum temperatures in the weeks leading up to the bloom and convert those into a kind of point system–they’ve worked out that the blossoms need a certain number of these heat points to push them from their winter dormancy to full bloom.
The first, provisional NPS prediction of the season is generally issued around the beginning of March.
More recently, another group has joined the cherry blossom peak bloom prediction fray: the Washington Post‘s Capital Weather Gang. They rely heavily on detailed analysis of historical weather data and weather forecasting models.
When they issue their predictions, both groups put forward a date range. In 2013, for instance, one of the official NPS predictions was for April 3 to April 6. What that means is that they predict that at some point during the period April 3 to April 6 a threshold will be crossed of 70 percent of the blossoms being open. The day that the NPS arborists judge that that threshold has been crossed with then become the “peak bloom” day, and they announce that retrospectively either on that day or a day or two after. That usually falls within 4 to 5 day stretch when we might consider the blossoms to be in full bloom (and looking beautiful).
It’s important to keep a few things in mind about “peak bloom”:
If “peak bloom” refers to a specific day, the NPS also sometimes refers to a “blooming period,” which is probably much more useful for visitors. Because, after all, you just want to know when you can see the trees looking beautiful.
The “blooming period” is a span of several days, perhaps even a week or so, starting when 20 percent of the blossoms are open. It ends when the leaves take over and the flower petals have all fallen. In short, the blooming period is when you can expect to see the flowers. Unfortunately, the focus on the more technical “peak bloom” in news reports and forecasts has meant that “blooming period” hasn’t yet gotten much traction.
On average, the blooms come out around the last week of March through the first week of April, and that’s typically a good time to aim for if you’re planning on visiting. But precisely when peak bloom occurs depends on the weather in the weeks and months leading up to it. And there’s no guarantee it will even fall within that period–sometimes it’s earlier, and sometimes it’s later.
That’s because the date of the peak is heavily dependent on local weather conditions in the months leading up to it. Warmer, sunnier conditions through the late-winter and early spring bring an earlier bloom. Sustained cold, wintry weather delays it. Unseasonably warm and sunny conditions in 1990 helped bring an early peak on March 15, and very cold conditions in 1958 delayed the peak bloom until April 18, the earliest and latest blooms on record.
The average peak bloom since 1921 is April 3, but in the past decade or so the peak bloom has trended earlier than the overall average. But to give an idea of how variable it can be, the peak bloom date for 2012 was March 20. The following year it was April 9.1
Here are the official peak bloom dates for the past decade:
The National Cherry Blossom Festival is typically scheduled over about 3 1/2 weeks from late-March to mid-April. The hope is that the blossoms will come out at some point during that period, but it’s very unlikely they’ll be out for the duration.
Average temperatures aren’t the whole story when it comes to when the cherry blossoms will come out, but they are a big part of it. Sustained warmer-than-average temperatures will bring them out sooner, while sustained colder-than-average temperatures will be delay them.
Here’s a breakdown of how much the average monthly temperature varied from the historical average for the months leading up to that year’s bloom.
|December||January||February||March||Peak Bloom Date|
Unfortunately, it’s not as simple as averaging the averages to predict the bloom date. For the 2014 and 2015 blooms, for example, the peak bloom date was the same, but the averages across January and February are different for those years, as are the averages across January, February, and March. And the December temperatures don’t figure as much as February-March ones.
This chart shows how many times peak bloom has fallen on a particular day since records started being kept in 1921. The earliest recorded peak bloom was March 18 (2000). The latest was April 18 (1958). The most common dates for peak bloom are April 2, 6, 7, and 9. Since 1921, the overall average peak bloom date has been April 3.
And here’s the same thing but looking only at the past two decades. As you can see, the dates are weighted a little earlier in recent years. Over the past two decades, the average peak bloom date has been March 31.2
Here are the official peak bloom dates since 1921.2
If you'd like to help support the care and upkeep of the cherry trees around the Tidal Basin, the Trust for The National Mall has launched an Endow a Cherry Tree Campaign. Donations go to the official Cherry Tree Endowment, which will give the National Park Service additional resources to fund the care, maintenance, and possible replacement of the cherry trees. You can find more information here.
The Trust is dedicated to marshaling private support for maintaining and improving the history National Mall area. I'm not affiliated with the Trust--just an admirer of their efforts.
Last updated April 12, 2017 7:39 am
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