Growing Greener Podcast – Phenology

Exploring—and Updating—Nature’s Calendar

Theresa Crimmins, Director of the USA National Phenology Network, explains why knowing the timing of natural events such as the annual bloom of different flowers and the emergences of their insect pollinators is essential to good gardening and shares how climate climate change is disrupting natural collaborations that stretch back over millennia.

August 12, 2026

Subscribe to Growing Greener on Apple Podcasts | Spotify | Amazon

Transcript

Growing Greener Host Tom Christopher

Welcome to Growing Greener, your guide to environmentally informed gardening. Produced in partnership with the Perfect Earth Project, this program brings you each week some expert who puts you in touch with a different aspect of gardening in partnership with nature. Our goal is to make your personal landscape healthier, more beautiful, more sustainable, and more fun.

This is your host, Tom Christopher, and today I will be talking to Dr. Theresa Crimmins, an associate professor at the University of Arizona and the director of the USA National Phenology Network. She has written a wonderful book with the title Phenology that was published in 2025 by the MIT Press. Dr. Crimmins explains in language easily understood by non-scientists such as me the basics of this essential branch of science.

Phenology is the study of nature’s calendar. It records the scheduling of events such as the return to your area of various migratory songbird species in the spring, the emergence of insect pollinators, and when various plants flower. For our ecosystems and our gardens to flourish, all the events must coordinate.

The appropriate insect pollinators must have emerged, for example, when your native phloxes bloom if those plants are to set seeds successfully and so reproduce. However, as Dr. Crimmins explains in our conversation, climate change is disrupting this natural calendar with far-reaching impacts on our natural environment and our gardens.

Before we go to the conversation, though, I want to thank Growing Greener’s sponsor, the Garden Conservancy, a national nonprofit dedicated to preserving, sharing, and celebrating America’s gardens and diverse gardening traditions. Through preservation, education, and public access, the Conservancy helps protect these living works of art for future generations. Learn more and discover gardens near you at GardenConservancy.org.

Welcome to Growing Greener, Theresa!

Theresa Crimmins

Thank you so much! I’m really excited to be here.

Tom Christopher

I want to begin by thanking you for your book [Phenology, MIT 2025], which is fabulous. It’s just 155 pages of text, although this is also followed by 39 pages of glossary, species lists, and endnotes for those who want to pursue their study of the subject further. It’s a fascinating look into a subject that’s essential for any ecologically inclined gardener.

Theresa Crimmins

Yes, thank you! Thanks for encapsulating it, and that really was my intention, was to bring this topic forward to folks that aren’t necessarily embedded in the science, because it really is something that touches our everyday lives, but is sometimes kind of invisible to us.

Tom Christopher

Let’s start with the definition. What is phenology?

Theresa Crimmins

Yeah, sure! So, it’s not the study of lumps on your head, which sometimes, actually frequently, it gets confused with. That’s phrenology. That’s a pseudoscience. Phenology is the study of when seasonal things happen in plants and animals. So, formally, it’s things like when do the plants in your yards put on their flowers in the spring, or when do those migratory birds first show back up? And then, really, it’s events that occur all through the whole growing season, though, not just early season, but when do fruits ripen, and when do the leaves change color and drop? When do the migratory species depart again? So, it’s just understanding when those things happen, and then the second piece of it is why. So many of the seasonal events are a function of what local conditions are like. We experience this, and I think intuit it, even if we don’t think about it consciously. We know that if we are experiencing an extra warm spring, stuff happens earlier.

We are usually very familiar with the fact that the snow melts earlier. If you’re in a place where there’s snow cover, temperatures start increasing earlier in the year. Then we also see those early season plants and animals start to exhibit their activity earlier in the year than in a year that would be cooler. So, phenology kind of brings that piece of it into the definition as well.

Tom Christopher

Yeah, I was fascinated throughout the book by the why, I have to say. How did you become interested in this topic?

Theresa Crimmins

You know, it was being at the right place at the right time. I had just finished up my graduate studies here at the University of Arizona, and it was really serendipitous. There were some forward-thinking scientists based here at the U of A, and actually even in the building that I was in, that had been active for several years in this country, pushing for the establishment of a formal effort to track phenology in many different species across this large country that we have. And right about the time that I was finishing up and seeking employment opportunities, that network, the USA National Phenology Network, was formalized here with the headquarters, literally right in the room in the building where I had an office. And I just kind of drifted over there. And serendipitously, I had been doing research in this field anyways. Funny enough, I had been working with a local fellow who had been tracking when he saw a whole lot of different species in flower in the mountains north of town, and I had been helping him analyze his data and document patterns and that kind of thing. And so, it was in the forefront of my mind. And then also, right, being at the right place at the right time, it became a far greater interest of mine.

Tom Christopher

Uh-huh. I think you discussed the fellow whose observations you were working with in the book.

Theresa Crimmins

I do, yes. It’s Dave Bertelsen, and honestly, it’s a pretty remarkable story. So, if you don’t mind, I will…

Tom Christopher

No, please.

Theresa Crimmins

He was, at the time, a parole officer, and he had a very stressful job, and so took to the mountains as frequently as he could to release stress. He enjoyed hiking, enjoyed paying attention to the plants. And then, what started as a real simple hobby, he said he wanted to know what the plants were that he was seeing, turned into an absolute passion. He, starting in the early 1980s, established a routine of hiking the same trail in the Santa Catalina Mountains, north of Tucson, on a usually twice-a-week basis. And this particular trail that he hiked starts in the low desert, where we have cacti and scrubby stuff, and goes all the way up into pine forest at the top. It’s five miles one way and more than 4,000 feet elevation gain from bottom to top. So, it’s a really significant hike in and of itself. But then, every time he hiked that trail, he kept track of every single species of plant that he saw in flower in each of the mile-long segments of the trail.

And so, he captured an elevation gradient there, and by documenting what he saw every single time, he built this incredibly comprehensive record of when things were in flower. And he kept that up, up until very recently. And so, we have a really impressive record of not only when things have flowered in different times through this multi-decade record now, so you can look for changes over time, but we have it along that elevation gradient too. So, we can look to see, do we see the same kinds of changes or the same rates of changes among plants in those different biotic communities? It’s a really impressive effort, and especially for someone who completely volunteered it. He just did it because he cared a lot. He really, really enjoyed what he did, and was very interested in telling a richer story about what he saw. So, it’s really inspirational, for sure.

Tom Christopher

Yeah, and your book is full of stories of people who took it upon themselves to keep records. And their records, some of them going back to like the 18th century, are really important to the study of phenology. I’ve been interested in phenology at a very basic level for quite a while, because it’s helped me to schedule various tasks in my garden. For example, I long ago learned that when the wild chicory starts blooming with its blue roadside flowers in southern New England, where I am, the squash vine moths are becoming active. And I need to make sure my squash plants are covered with floating row covers to keep the moths from laying eggs of their borers on the vines. And traditionally, phenology was used as a sort of calendar to alert gardeners to when they should perform certain tasks, such as planting various crops. But in your book, you discuss several other ways that phenology can inform our understanding of plants and wildlife. Could you outline some of these other aspects of the field?

Theresa Crimmins

Oh, sure. I’d be happy to. So, you did a really awesome job there of describing indicator species. And I think indicators are actually still very much a part of both backyard gardeners and professional horticulture and species management, too, where if you’re able to identify that when a particular visible event occurs in one species, it means it’s time to take this action. That’s actually a really valuable thing to understand. And that’s been in practice for millennia, actually. But yes, in addition, in the last couple of decades, there became this recognition that indeed, our planet is getting warmer, we are really starting to see the effect of all the additional greenhouse gases that have been pumped into the atmosphere for the last almost 200 years.

And there are clear consequences. And changes in phenology, changes in the timing of when events are occurring, are one of the most visible ways that we see how plants and animals are being affected. And so it really was just in the last few decades that there was a pretty clear shift from not just utilizing phenology in the way that you described, which is very applied, and can be very helpful for identifying when to undertake actions, but also to understand how species and ecosystems are being affected by rapidly changing conditions.

It was really by seeing the fingerprint of climate change present in long term phenology records that have been collected uninterrupted in other parts of the world that that was the large motivation for establishing the National Phenology Network here in the U.S. Because up until the early 2000s, we didn’t really have any large-scale monitoring of when stuff is happening across our country, with the exception of one network that tracked lilacs. But beyond that, we really didn’t have much. And so it’s a big gap. And that’s what my network aims to fill, actually.

Tom Christopher

The changes have been, in some cases, fairly dramatic. I read in your book that globally birds have moved up the beginning of their breeding period by 40 to 60 days since the early 19th century in response to global warming.

Theresa Crimmins

Yeah, in some species and in some locations. It’s really tricky is that things aren’t just shifting all together neatly. Ecosystems are really complex webs, interdependencies among predator and prey and pollinators and their flowers and the other things that they rely upon, and especially among migratory species. Having food availability at the right time in the right place is so crucial. And so the challenge now is that species are being affected in different ways and to different degrees by increasing temperatures and all of the other consequences to the way that humans change the environment.

And so there’s a lot of concern about the links in those complex webs being broken. And then that being one of the many ways that ecosystems and species suffer from all the changes that are afoot.

Tom Christopher

Douglas Talamy, an entomologist at the University of Delaware, has spoken a lot in recent years about how caterpillars are so important to songbirds in raising their chicks. They’re a nutritious, easily digested food, and an awful lot of species depend on them. And the changes in phenology, well, maybe the chickadees are arriving earlier in the spring because things warm up earlier. But maybe the insects whose caterpillars they depended on haven’t reacted as strongly. And so they start breeding and their chicks hatch. And they had co-evolved with these caterpillars so that they were very closely synchronized those two dates. But now they’re no longer synchronized. And they have great trouble finding food for their chicks. And the populations are dropping in a lot of birds.

Theresa Crimmins

Yeah, that’s exactly right. It’s actually the opposite. It’s largely the caterpillars are coming out earlier in the year, because they are cued to the same conditions as the trees that are leafing out and the leaves that they depend on to snack on. And largely those events in temperate systems are cued by temperatures increasing. And so as temperatures start to increase earlier in the year, those events are occurring earlier in the year now than they used to. Especially when we’re talking about long distance migratory birds. They are typically cued to head north by sun angle, because they have no idea.

They’re tens of thousands of miles away. They don’t know what the conditions are like on the breeding grounds where they’re trying to get to. So, what do they use as a clock? It’s the sun angle. Well, the sun angle isn’t being affected by climate change. It is a function of where the earth is in its revolution around the sun and static from year to year. And so those birds are arriving largely, to oversimplify, largely at the same time as they have in the past.

And now they’re arriving past the peak of food availability. And what we’re seeing too is a lot of times when those birds arrive from their long-distance migration, they would like to have a little bit of time to rest and recover and bring their resources back up before they start breeding and nesting. And they have increasing pressure now to just jump right on it as soon as they arrive because of that declining food resource. So, it’s tough. It’s putting a strain on everyone. And you’re right. A very clear consequence of that can be dramatic drops in species numbers.

Tom Christopher

And different classes of plants have reacted to climate change in their phenology in different ways. For instance, I think you said in the book that annuals have shown a quicker and greater response to climate change than perennials and woody plants. So, it starts creating all kinds of discrepancies or breaking synchronicities all through the food chain. Things like some species of native plants that depend on specific species of pollinators to pollinate their flowers so that they can set seeds. If the plants move up and the insects don’t or vice versa, that creates another difficulty.

Theresa Crimmins

Exactly. Yeah. Yeah, you’re right. The annual plants, the short-lived ones that complete their entire life cycle in a season, by design, are more well equipped to adapt. Adaptation is when particular genes are selected for. And in order to select for particular genes or, you know, traits that might make you more well suited for novel conditions, you have to be generating offspring. And long-lived species are just slower at that whole thing of generating generation after generation. Annual plants are turning that over sometimes multiple times within a year. And in the case of insects, if you have lots of generations, there can be pretty rapid adaptation compared to other groups.

And so, yes, we see some evidence of adaptation among annual plants, although scientists still seem to quantify it as not keeping pace with how rapidly environmental conditions are changing. So that still spells trouble for us. But definitely for the long-lived species like, you know, oak trees and overstory canopy, plants that can live decades or even centuries, that’s a real tough charge for them to ask them to keep pace with changing conditions. So, yeah, it’s tough. And, you know, they’re affected not only in terms of phenology. We see the response of the impacts in terms of their phenology, meaning events are occurring earlier in the spring and later in the fall. But then also, can they even persist in the location where they are? Or do we also expect to see changes spatially, range changes, you know, and largely what that looks like is species in the northern hemisphere moving northward, forward, or upslope and elevation.

And to go back to Dave Berthelsen’s data, the data, the flowering records that he was collecting along the elevation gradient here, he had said, you know, I feel like over the duration of time that I’ve been hiking this trail, I feel like I have seen certain species progressively at higher and higher elevations. So that’s one of the analyses that we pursued with his records, and we were able to verify that indeed.

And we were really conservative with our statistics, and still about 30% of the plants that we evaluated showed clearly that they had been shifting their flowering range upslope. So that’s exactly what we would expect to see under warmer conditions.

Tom Christopher

Theresa, I need to pause here to remind listeners that this radio station is WESU 88.1 FM, and this program is Growing Greener, a weekly conversation about gardening in partnership with nature. This is your host, Tom Christopher, and today I am speaking to Dr. Theresa Crimmins of the University of Arizona about her wonderful book, Phenology, which was published in 2025 by the MIT Press.

One thing I also want to allude to, I mean, this has so many impacts, and the phenology is revealing so many things, but I believe there’s been a general observation that invasive plants often react faster to the climate change in terms of their phenology when they start becoming active in the spring than many of the species, you know, native to that area, and this gives the invasives yet another advantage.

Theresa Crimmins

Yep, you are right. You know, I think in a lot of cases that ability to very take advantage of conditions like very early season warmth with greater vigor than native species is one of the things that enables them to be invasive. Oftentimes what we see is, like, species are introduced to a novel environment, and then is there a niche, some sort of gap in the ecosystem that they can occupy, and if there is, then they can persist, and if it’s something that enables them to kind of get a leg up on local species, native species, then they can really take hold and take over, and so you’re right, and a lot of the species that we identify as invasive are such, at least in part, because they are able to out-compete native species for light and nutrients and other resources that are critical to get going in the early season, and so we see this a lot in understory shrubs where understory shrubs, invasive ones, leaf out earlier, and then they just have an advantage, and they’re able to fill out that tier of the forest more readily than the natives. They’re already starting to shade out their peers, and then it becomes a positive feedback there where they’re successful, and so then they are the ones that have the greatest resources. They produce the flowers and the fruits, and then they have another generation that’s successful, and they just kind of take hold.

Tom Christopher

We spoke a bit earlier about where scientists such as yourself get information about the history of phenology, when, for example, a particular flower bloomed in the 19th century, say, or when a particular bird returned to an area from its migration. Could you talk a little bit about how phenologists have managed to put together a sort of timeline?

Theresa Crimmins

Sure. Other than a large-scale effort started in the 1950s to document largely when common lilacs were in flower, aside from that effort, we really didn’t have any formalized and large-scale effort to track when stuff is happening in the U.S. until the establishment of the USA National Phenology Network in 2007. And if we are really interested in understanding how things have changed, then we’ve got to be able to look back further than 2007. Ideally, we’d like to be able to look back, you know, tens of years and even hundreds of years as possible. And, so, yeah, scientists in this field have gotten really creative in terms of what kinds of resources we can look to as historical records to tell us when things happened, even though those data may not have been collected with that intent in mind, and pretty much largely they were not, because back in the 1800s, people were not thinking about climate change in the way that we are right now.

They were documenting things for different reasons, and so there’s a lot of cool places that folks have scrounged up data. Individuals’ private journals and notebooks and just very meticulous records are wonderful. There’s some really famous examples of that. Some folks might know of Aldo Leopold, who was, you know, the father of conservation wildlife management and based in Wisconsin. He took really meticulous records of when he saw things happening at his shack in Baraboo, Wisconsin.

Henry David Thoreau is another fellow who’s really become famous for, he, in addition to living out at the pond, at Walden Pond, and writing about it and doing so many other really cool scientific experiments, he also took it upon himself in the last almost 10 years of his life to take extremely careful records of when he saw things happening in the area surrounding Concord, Massachusetts. And those records were preserved, but basically not recognized as a data resource in terms of understanding how phenology has changed until just in the early 2000s when some researchers at Boston University were on the hunt for historical records, and they connected with folks in the Thoreau Historical Society, and they recognized, like, oh my gosh, these journals, these journals are a huge treasure trove. And so that has spawned other efforts, and other journals and notebooks have come forward, and every now and then we get phone calls to our office, actually, or emails of people saying, hey, I’ve got some stuff that I kept track of in the past, are you interested? And I never say no. I think it’s really cool, but then there’s other really neat ways that we can kind of mine historical records for these, for this information, too.

Photos, historical photos are a good way, and especially if it’s a photo that is repeatedly taken at the same location on the same date, like, say, graduation photos, or when there are key events, like celebrations and recurring events that, where they are photo documented, you can look back at those, at those pictures and just look at, you know, like, what’s the development of the tree canopy in the background. One example that I thought was so cool is a bike race in, I think it’s France, where they’ve been recording it for tens of years, decades, and researchers went in and looked at the footage of the bicycle race, and were able to document that the leaf out has advanced notably, measurably, over the duration of this race being.

And then one other cool, really cool source that I love is herbarium records, which are pressed plant specimens that were collected, usually for the purpose of identifying plants or documenting where they’re found. And herbarium record specimen collection dates back to Darwin and Alexander von Humboldt, and all the folks that went out and tried to figure out where are species around the globe, and you collect the specimens, and you smoosh them flat, and then you preserve them. And every time that that’s done, you’re documenting also where was this specimen collected and when. And we can now go back and examine those. Typically, specimens are collected when they are in flower or in fruit or both, because you have as many of the structures present as possible, which are really important for identifying what that species is. And so if we have specimens that have flowers open on them, then we know at that location on that date, this particular species was in flower. And that can serve as a little pin point in time that we can now compare today’s timing with to see how has it changed.

So there’s a very active effort underway right now, because there’s enormous quantities of herbarium records that have been collected around the globe, most of which have not been digitized yet, and so most of which have not been analyzed yet, and it’s a huge opportunity.

Tom Christopher

I really enjoyed reading about some of the modern technologies, which inadvertently are providing insights. So doorbell cameras, or surveillance cameras, or even, I guess, weather radar. How does that work?

Theresa Crimmins

It doesn’t work as well for plants, but it’s definitely a really valuable way for picking up migratory species, and especially those that occur in a big pulse. Mayfly emergence is a really good example of that. But bats, bat emergence is as well, and some bats migrate. So you can pick bat migrations up on radar. And other bird migrations, for sure. There is some very cool imagery out there now that is available in real time, where we can see these kind of phenomena occurring.

Tom Christopher

Yeah, that’s fascinating. Well, it seems to me that understanding phenology, how and why various organisms schedule their behaviors, and how the phenology of one organism relates to that of another, is becoming even more important as our climate is rapidly changing. Would you agree?

Theresa Crimmins

Yeah, definitely. And again, that’s why our network was established, was to kind of get at that baseline and information, even now, recognizing that baselines are shifting, they are not static, with the intent to understand as much as we can, and use that information in conservation measures and adaptation to the best of our ability.

Tom Christopher

Well, Theresa, I want to close with another recommendation of your book. The title is Phenology, and the author is Theresa M. Crimmins. The publisher is MIT Press, and Phenology is available from Amazon and other online booksellers, as well as through your local bookstore. So, I strongly urge any gardener listening to me now, I think they would find it very interesting if they go out and track down this book.

Theresa Crimmins

Great, thank you so much. I really appreciate the opportunity to share with you.

Tom Christopher

Oh, Theresa, I have one last request. My partners in this venture have asked me to include a gardening tip from everybody I interview, which I think is a fun idea. Can you suggest a way in which gardeners can use phenology to enhance the cultivation or appreciation of their garden?

Theresa Crimmins

Paying attention, and taking it even a step further, documenting when things are occurring can be really valuable. As you cited earlier in this conversation, if you can begin to identify when particular events are occurring, and what else is happening around that same time, it can really be very practical for understanding, and planning, and anticipating when particular pest insects are likely to be active, and you need to kind of prevent their spread, or take action if you want, or understanding the best times to be planting, or anticipating fruit harvest. So really, record keeping. Paying attention, getting out there, looking at stuff, and keeping records is really the most valuable thing you can do to capitalize on the wisdom that is embedded in phenology.

Tom Christopher

Well, thank you very much. I really appreciate you sharing your time today.

Theresa Crimmins

Thank you so much. I really appreciated the opportunity.

Theresa Crimmins

That’s all for this week’s Growing Greener. Please join us again at the same time next week on this radio station, WESU 88.1 FM for another episode, or listen to Growing Greener as a podcast at www.thomaschristophergardens.com. If you have comments or suggestions, please share them with me, Tom Christopher, via the same website, www.thomaschristophergardens.com. Growing Greener is produced as a community service in partnership with the Perfect Earth Project, a non-profit dedicated to educating, engaging, and inspiring individuals, land care professionals, and decision-makers to adopt toxic-free, nature-based, and climate-responsible landscaping practices.

 Transcribed by TurboScribe.

---

About Growing Greener