Short Wave - Predicting spring bloom is an art and a science

Episode Date: March 31, 2026

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 [e-mail] 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.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.See 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

Hey, shortwaivers, Emily Kwong here with a quick favor to ask, can you send us a voice memo to Shortwave at npr.org with your questions about science and specifically about your local environment. That would help a lot. Include your name, where your home is, and your question, and we might consider it for a future episode. Thank you so much. You're listening to Shortwave from NPR. Hey, everyone, Emily Kwong here with someone very special to our team, Shortwave intern, Arru Nyers.

Hey, Emily. What's up? You and I are at NPR headquarters in Washington, D.C. And it's your first time living in the district. How's it going? It is. I'm originally from Wyoming and I moved to D.C. last fall. My favorite part has definitely been the public transit. It's very magical to me. It's a joy to ride. But there is one particular event that happens every spring. That's a huge deal. Emily, have you heard of the Cherry Blossom Festival? You can't escape it. This time of you. 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. Yeah, there's so many trees in bloom right now. Yeah. So we have 20,000 trees parkwide, but of the cherry trees in particular, we have right around 3,700. 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.

It's like a flower garden where the flowers are 30 feet tall. It's really what it is. 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. It does feel like Pennsylvania has Groundhog Day and we have this. Like this is D.C.'s homegrown sign-up spring? 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. We've got a dozen different varieties of trees, but the Yoshinos are far in a way the most prevalent. So when 70% of those trees have blossomed, we say it's peak bloom. Okay, so when 70% of the Yoshino cherry trees are in bloom, that is peak bloom. 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. It's a wild-ass guess. There is some science about it.

Today on the show, the art and science of guessing peak bloom. What are the stages of cherry tree blossoming? And how do scientists make that big prediction? You're listening to Shortwave, the science podcast from NPR. Okay, shortwaivers, 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 shortwave intern Arru Nyer. So what goes into a cherry tree bloom, Aru? 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. That sounds like D.C. and winter. Yeah. We went there to talk to Matthew. I've been an arborist since 1979. I oversee everything to do with the trees.

He told us there's six stages of cherry tree blossoming. I love a numbered list. Okay, walk me through these stages. 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. The scales will peel away, and then you'll have the first. first stage of flower development, and we call that green bud. Okay, so stage one green bud, where a flower bud emerges, but it's protected by green leaves. What is stage two? So stage two is when the florets are visible. And that just means that the bud is starting to open up. 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 flowerette. You can see the pink or the white in there. And stage three is when the florets extend. The florets are just a little bit more exposed. I kind of think of like a mouse in a nest that kind of open its eyes for the first time and looking around. And it's still protected, but it's aware. Yes, protected but aware.

So the florets are peeking out. What then happens in stage four? You're definitely not going to guess this one. It's called peduncle elongation. Yeah, I would never guess that. What is that? Yeah, I was also a little bit befuddled. So the peduncle elongation is when that flower, the petals are still closed, but it's elongated.

You don't just have those little tips. It would be as if that mouse that I just said is an analogy, if you saw the entire length of its torso, it's more presented. and that's the peduncle elongation. Regrettable name, but please continue. Regrettable name for sure. And then there's the fifth stage, which is puffy flower. 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 when we have full bloom. 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? 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 Wolcovic about this.

She's an associate professor of forest and conservation sciences at the University of British Columbia. 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. Wait, what does she mean by a bucket of spring warmth? Like a physical bucket? So not a physical bucket. Think of it more like a threshold of warmth that the trees need to reach in order to bloom. 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. Oh, okay. Compared to something that blooms later in the spring, which may have a larger bucket of warmth. Exactly. Okay. Yeah. But it is a little bit more complicated than that, even for cherry.

blossoms. So for most woody plants, we think there's actually a two bucket system. And cherry trees have this as well. Two buckets. What is the second bucket? 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. 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. 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.

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. Exactly. They need both parts. Yeah. 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. 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. Okay, 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. Yeah, exactly. You know, we have had a really strange winter in D.C. 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? Yeah, so basically this up and down weather pattern can totally shake up their predictions. And there's a big range of possibilities.

D.C.'s peak bloom has been recorded as early as March 15th and as late as April 18th. What is Mike's office's track record of accuracy for predicting peak bloom? Well, sometimes they're really accurate. 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, I'll do this every year. I've never been right again. Oh, wow. 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. Because in making the predictions, they're also gathering what the trees are actually doing every year. 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. Cherry blossoms are effectively like our longest written record on Earth. They go back over a thousand years in Asia. Which she says is really useful. full for scientists like her, who want to study how the timing of those natural events shifts. 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. 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. Across the globe, I would say,

whether it's grasses starting to germinate, whether it's the leaf out of beach 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. Two to four weeks doesn't sound like a lot, but in the life of the environment

that is a big difference. Yeah, and Elizabeth and our 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 U.S., in Japan, in Switzerland, in Canada. 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 the forecasting competition to try to get people to help us understand this mystery. That is a good way to get community scientists involved. Make it a competition. Exactly. And they're hoping that turns into better forecasting models that scientists can use in the future. So to bring this back home to D.C., when is peak bloom supposed to happen for us this spring?

It's happening right now. So I was thinking maybe we could go out and see them. We are here together looking at the cherry blossoms right now. No. They're so pretty. Very, very beautiful. And so many people are out today. So many people. And you talk to a lot of them. It's a symbol of spring. Oh, I enjoy the sunshine and the blossom. Like, I have never seen it this beautiful. Like, I've never seen it this, like, crisp and peaky. Philly, I am in another country. I'm in Japan. I don't know.

Somehow it still feels like magic every time you get to peak bloom. Arroo Nair, thank you so much for bringing us this glorious nature quest. Oh, thanks so much, Emily. This episode was reported and produced by Arunzithi Nair and Hannah Chin. It was edited by our showrunner, Rebecca Ramirez. Early McCoy, Angela Zhang, and Tyler Jones, check the facts. Robert Rodriguez was the audio engineer. I'm Emily Kwong.

Thanks for listening to Shortwave, the science podcast from NPR. 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.