Predicting spring bloom is an art and a science

Do you ever wish you could predict the future? The National Park Service in Washington D. C. does it every year when they forecast when the Capitol’s cherry blossoms will reach peak bloom. People travel from all over the world to enjoy the annual National Cherry Blossom Festival and to glimpse these fragile flowers before they are gone. On this month’s Nature Quest, we learn the ins and outs of cherry tree blossoms, how scientists make that big prediction every year — and why all this focus on blooms can help scientists better understand climate change. This episode is part of Nature Quest, our monthly segment from listeners noticing a change in the world around them. To participate, send a voice memo to [email protected] mailto:[email protected] with your name, location and your question about a change you're seeing in nature! Want to learn more about nature’s calendar? Check out our first Nature Quest episode on whether flowers are blooming early https://lnk.to/RwKnWs. Listen to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave http://plus.npr.org/shortwave. See pcm.adswizz.com https://pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences. NPR Privacy Policy https://www.npr.org/about-npr/179878450/privacy-policy

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• 0:14Emily Kwong?

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• 0:38Emily Kwong?

  Hey, everyone. Emily Kwong here with someone very special to our team, Short Wave intern Aru Nayar.

• 0:44Speaker 02

  Hey, Emily.

• 0:45Emily Kwong?

  What's up? You and I are at NPR headquarters in Washington, DC. And it's your first time living in the district. How's it going?

• 0:54Speaker 02

  It is. I'm originally from Wyoming, and I moved to DC last fall. My favorite part has definitely been the public transit. It's very magical to me.

• 1:03Emily Kwong?

  It's a joy to ride.

• 1:05Speaker 02

  But there is one particular event that happens every spring that's a huge deal. Emily, have you heard of the Cherry Blossom Festival?

• 1:12Emily Kwong?

  You can't escape it. This time of year, the cherry blossom marketing is everywhere. But originally, these blossoms came from trees gifted by Japan in 1912. And now they turn the city into this, like, soft, pink wonderland.

• 1:25Speaker 02

  Yeah, there's so many trees in bloom right now.

• 1:28Emily Kwong?

  Yeah.

• 1:28Speaker 03

  So we have 20,000 trees parkwide. But of the cherry trees in particular, we have right around 3,700.

• 1:39Speaker 02

  So this is Matthew Morrison. He's an arborist and urban forester with the National Park Service. And he told me every single year, locals and tourists flood the National Mall for the annual Cherry Blossom Festival to view these millions of flowers.

• 1:52Speaker 03

  It's like a flower garden where the flowers are 30 feet tall. It's really what it is.

• 1:57Speaker 02

  But as you know, Emily, these flowers are really fragile, and they're only around for a couple of weeks. And in some cases, it's been as short as five days. So the festival is ideally planned for when all these trees are in peak bloom.

• 2:11Emily Kwong?

  It does feel like Pennsylvania has Groundhog Day and we have this. Like, this is DC's homegrown sign of spring.

• 2:16Speaker 02

  Right, and I wanted to know, what is peak bloom really? So I asked Mike Litters to define it for me. He's the chief of communications for the National Mall and Memorial Parks.

• 2:25Speaker 04

  We've got a dozen different varieties of trees, but the Yoshinos are far and away the most prevalent. So when 70% of those trees have blossomed, we say it's peak bloom.

• 2:34Emily Kwong?

  OK, so when 70% of the Yoshino cherry trees are in bloom, that is peak bloom.

• 2:40Speaker 02

  Exactly, and the National Parks team, it's actually their job to predict when the peak bloom will arrive every year. And let's just say it's kind of a guesstimate.

• 2:48Speaker 04

  It's a wild-ass guess. There is some science involved.

• 2:53Emily Kwong?

  Today on the show, the art and science of guessing peak bloom.

• 2:58Speaker 02

  What are the stages of cherry tree blossoming? And how do scientists make that big prediction?

• 3:03Emily Kwong?

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• 4:15Emily Kwong?

  OK, short wavers, once again, we are on our monthly nature quest, brought to you by someone who's paying attention to how their local environment is changing. And this month, that person is short wave intern Aru Nair. So what goes into a cherry tree bloom, Aru?

• 4:31Speaker 02

  Well, I didn't really know. So to find out, I went with one of our producers, Hannah Chin, down to the tidal basin. It was damp and chilly. You could hear the jets overhead and the trees looked totally bare.

• 4:42Emily Kwong?

  That sounds like DC in winter.

• 4:43Speaker 02

  Yeah, we went there to talk to Matthew.

• 4:45Speaker 03

  I've been in Arborist since 1979. I oversee everything to do with the trees.

• 4:51Speaker 02

  He told us there's six stages of cherry tree blossoming.

• 4:54Emily Kwong?

  I love a numbered list. OK, walk me through these stages.

• 4:57Speaker 02

  So to set the stage, in winter, the trees are dormant. It's like they're sleeping. And on these dormant trees, the tips of the branches are covered in overlapping bud scales, these modified leaves that protect the bud to be, if you will. To me, they kind of look like little pine cones. But once spring comes and it starts to get warmer.

• 5:18Speaker 03

  The scales will peel away, and then you'll have the first stage of flower development, and we call that green bud.

• 5:26Emily Kwong?

  OK, so stage one, green bud, where a flower bud emerges, but it's protected by green leaves. What is stage two?

• 5:36Speaker 02

  So stage two is when the florets are visible. And that just means that the bud is starting to open up.

• 5:42Speaker 03

  And when that bud opens up, just like the bud scales did, you can see the little tips of the flower in there, the florets. It's like a little floweret. You can see the pink or the white in there.

• 5:55Speaker 02

  And stage three is when the florets extend.

• 5:58Speaker 03

  The florets are just a little bit more exposed. I kind of think of like a mouse in a nest that's kind of opened its eyes for the first time and looking around, and it's still protected, but it's aware.

• 6:11Emily Kwong?

  Yes, protected but aware.

• 6:14Emily Kwong?

  So the florets are peeking out. What then happens in stage four?

• 6:19Speaker 02

  You're definitely not going to guess this one. It's called peduncle elongation.

• 6:23Emily Kwong?

  Yeah, I would never guess that. What is that?

• 6:26Speaker 02

  Yeah, I was also a little bit befuddled.

• 6:28Speaker 03

  So the peduncle elongation is when that flower, the petals are still closed, but it's elongated and you don't just have those little tips. It would be as if that mouse that I just said as an analogy, if you saw like the entire length of its torso, it's more presented and that's the peduncle elongation.

• 6:50Emily Kwong?

  Regrettable name, but please continue.

• 6:52Speaker 02

  Regrettable name for sure.

• 6:53Emily Kwong?

  And then there's the fifth stage, which is puffy flower.

• 6:56Speaker 03

  It's not opened like a regular flower, but the petals are entire. They're still kind of protecting one another. And then after that, it opens up and we have full bloom.

• 7:09Emily Kwong?

  And full bloom is the final stage. This is so cool. So how do folks at the National Park possibly predict this? Like, are they just running around checking all 3,700 cherry trees to see what kind of stage they're at?

• 7:23Speaker 02

  I mean, kind of, because it's not like each stage takes a set amount of time. This bloom cycle is super dependent on the weather and specifically the temperature. So I talked to Elizabeth Wolkovich about this. She is an associate professor of forest and conservation sciences at the University of British Columbia.

• 7:40Speaker 07

  You can think about a bucket of spring warmth that the plant needs to fill before it can produce enough energy to produce the flower.

• 7:49Emily Kwong?

  Wait, what does she mean by a bucket of spring warmth? Like a physical bucket?

• 7:53Speaker 02

  So not a physical bucket. Think of it more like a threshold of warmth that the trees need to reach in order to bloom.

• 7:59Emily Kwong?

  Oh.

• 8:00Speaker 02

  So different plants have different sized buckets to fill. Cherry trees have smaller buckets, which is why they come out so early in the spring.

• 8:07Emily Kwong?

  Oh, OK. Compared to something that blooms later in the spring, which may have a larger bucket of warmth.

• 8:12Speaker 02

  Exactly.

• 8:13Emily Kwong?

  OK.

• 8:13Speaker 02

  Yeah, but it is a little bit more complicated than that, even for cherry blossoms.

• 8:17Speaker 07

  So for most woody plants, we think there's actually a two-bucket system. And cherry trees have this as well.

• 8:24Emily Kwong?

  Two buckets? What is the second bucket?

• 8:26Speaker 07

  It's often called the winter chilling bucket. So it's the idea that the plants need a certain amount of cool weather before they can start to fill that bucket of spring warmth.

• 8:35Speaker 02

  The period of cold that the plants need before they get ready to bloom, it's called winter chilling. And scientists have observed this for a while. You might have observed it, too.

• 8:45Speaker 07

  If you go outside and you take a cherry blossom in in December, it'll take a really long time to bloom. If you go outside right now and you take a cherry blossom in, or if you went three weeks ago or four weeks ago and you brought it inside, it'll bloom right away.

• 8:58Emily Kwong?

  You know, this kind of reminds me of baking. You know how you have to chill dough in order to bake it? The trees have to chill a little bit before they'll bloom in the heat.

• 9:07Speaker 02

  Exactly. They need both parts.

• 9:08Emily Kwong?

  Yeah.

• 9:09Speaker 02

  And just to be clear, Emily, this winter chilling bucket is something that scientists are still studying. They know the plants need the cold weather to bloom on time, but they're still not quite sure exactly how it works. And Elizabeth told me there's also the factor of longer days.

• 9:25Speaker 07

  Additionally, a lot of the plants appear to need a certain number of daylight hours for that spring warming. So it's not just that it's warm. It's that they're also getting a certain amount of sunlight.

• 9:35Emily Kwong?

  OK, so Elizabeth is saying to bloom, there needs to be a combination of triggers. Trees need the winter cold, they need the spring warmth, and they need longer days in order to begin the blooming process.

• 9:46Speaker 02

  Yeah, exactly.

• 9:47Emily Kwong?

  You know, we have had a really strange winter in DC. There's been this pendulum swing between multiple snowstorms and then warm 70, 80 degree days. So did any of the folks who work with the trees describe how this weather is affecting the cherry blossoms?

• 10:01Speaker 02

  Yeah, so basically, this up and down weather pattern can totally shake up their predictions. And there's a big range of possibilities. DC's peak bloom has been recorded as early as March 15th and as late as April 18th.

• 10:15Emily Kwong?

  What is Mike's office's track record of accuracy for predicting peak bloom?

• 10:20Speaker 02

  Well, sometimes they're really accurate.

• 10:23Speaker 03

  I started in 2019. And so they said, all right, Morrison, what's the date? And I gave the date and I hit it on the nose. And I was like, Mike, I'm like, oh, I'll do this every year. I've never been right again.

• 10:36Emily Kwong?

  Oh, wow.

• 10:37Speaker 02

  Yeah, Mike even remembers one particularly chaotic year where they announced their prediction, updated the date, and then they had to change the date back again, back to their original prediction.

• 10:48Emily Kwong?

  Because in making the predictions, they're also gathering what the trees are actually doing every year.

• 10:54Speaker 02

  Yeah, and Matthew and Mike are not the only ones focused on peak bloom and predicting peak bloom. Elizabeth told me humans have been recording cherry blossoms around the world for hundreds of years.

• 11:05Speaker 07

  Cherry blossoms are effectively like our longest written record on Earth. They go back over a thousand years in Asia.

• 11:12Speaker 02

  Which she says is really useful for scientists like her, who want to study how the timing of those natural events shifts.

• 11:19Speaker 07

  In the Kyoto record, they have this long-term record. There's also a record out of China. Cherry blossoms across the world are blooming weeks earlier than they did in the past. They're by far, I would say, the best evidence of anthropogenic climate change shifting our springs earlier.

• 11:34Emily Kwong?

  I didn't realize how much insight cherry blossoms provided in this way. I remember in an earlier Nature Quest episode, we talked about this, this field devoted to the timing of periodic natural events, flowers blooming, birds migrating, animals going into hibernation. It tells us something.

• 11:51Speaker 07

  Across the globe, I would say, whether it's grasses starting to germinate, whether it's the leaf out of beech trees, whether it's flowering on a plum tree or a cherry tree, those events have consistently shifted between two to four weeks, depending on exactly what plant you're looking at and how much that place has warmed.

• 12:10Emily Kwong?

  Two to four weeks doesn't sound like a lot, but in the life of the environment, that is a big difference.

• 12:15Speaker 02

  Yeah, and Elizabeth and her colleagues were like, given all this data, plus the reality of a warming planet, how do we make these predictions more accurate? So they started running a competition, asking people to share their predictions for cherry tree peak bloom in places in the US, in Japan, in Switzerland, in Canada.

• 12:34Speaker 07

  As a gateway to better forecast what forest trees are doing and what every fruit tree, peaches and plums, all these things are doing the same thing as cherries. And so we started a forecasting competition to try to get people to help us understand this mystery.

• 12:49Emily Kwong?

  That is a good way to get community scientists involved. Make it a competition.

• 12:53Speaker 02

  Exactly. And they're hoping that turns into better forecasting models that scientists can use in the future.

• 12:58Emily Kwong?

  So to bring this back home to DC, when is peak bloom supposed to happen for us this spring?

• 13:05Speaker 02

  It's happening right now. So I was thinking, maybe we could go out and see them?

• 13:16Emily Kwong?

  We are here together, looking at the cherry blossoms right now. They're so pretty.

• 13:19Speaker 02

  Very, very beautiful. And so many people are out today.

• 13:22Emily Kwong?

  So many people. And you talk to a lot of them.

• 13:25Speaker 02

  It's a symbol of spring. I enjoy the sunshine and the blossom.

• 13:29Speaker 08

  Like, I've never seen it this beautiful. Like, I've never seen it this crisp and peaky. Please, I am in another country. I'm in Japan. Somehow, it still feels like magic every time you get to peak bloom.

• 13:42Emily Kwong?

  Arun Nayar, thank you so much for bringing us this glorious nature quest.

• 13:46Speaker 02

  Oh, thanks so much, Emily.

• 13:49Emily Kwong?

  This episode was reported and produced by Arun Zetie Nayar and Hannah Chin. It was edited by our showrunner, Rebecca Ramirez. Berly McCoy, Angela Zhang, and Tyler Jones checked the facts. Robert Rodriguez was the audio engineer. I'm Emily Kwong. Thanks for listening to Short Wave, the science podcast from NPR.

• 14:13Speaker 02

  We were here maybe three weeks ago. And they looked dead. Hannah and I, we were like, are the trees alive? I don't know. But now, it's, yeah, it makes a world of difference to see them bloom.

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• 14:53Speaker 09

  Oh, Kevin, it feels good to be back. It sure does, Casey. And what a time. The world is talking about AI and what's going on in Silicon Valley. And we are here with a new show to tell you all about it. That's right, Kevin. On our new show, Machine Gods, twice a week, we'll be bringing you the biggest news in tech and bring you unmatched access to the real titans of this industry. You can find Machine Gods from NPR on the NPR app or wherever you get your podcasts.