Former HGTV host Paul James explains the science of changing leaf colors

When it's fall, leaf colors change from green to vibrant shades of red, orange, and yellow. But why does that happen?
Paul James, a gardener from Tulsa, Oklahoma known as "The Gardener Guy," and the former host of "Gardening by the Yard" on HGTV explains what happens when the amount of sunlight decreases and temperatures change. He also discusses the variation in fall foliage in different tree species and where you can see some of the best color.
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About the Across the Sky podcast
The weekly weather podcast is hosted on a rotation by the Lee Weather team:
Matt Holiner of Lee Enterprises' Midwest group in Chicago, Kirsten Lang of the Tulsa World in Oklahoma, Joe Martucci of the Press of Atlantic City, N.J., and Sean Sublette of the Richmond Times-Dispatch in Virginia.

Episode transcript
Note: The following transcript was created by Headliner and may contain misspellings and other inaccuracies as it was generated automatically:
Kirsten Lang: Welcome to the across the sky podcast, a weather, climate, and science focused podcast for a general audience. We are so glad that you're here with us today. I'm Meteorologist Kirsten Lang, and I'm joined by my fellow colleagues Matt Holiner in Chicago, Sean Sublette in Richmond, and Joe Martucci in Atlantic City. And together we make up the Across the Sky weather team from Lee Enterprises. Lee is a publication company with over 70 properties in cities across the United States. We're talking today with Paul James about fall foliage and those changing colors. And the interview with him is great. I love listening to what he has to say about that. He’s very knowledgeable in the entire subject.
Matt Holiner: Well, I brought it up in our top Ten Things to know about Fall episode. One of the reasons I like fall so much is because of the fall foliage, which was disappointing growing up in San Antonio. But once I left San Antonio and moved to places like Richmond, Virginia and Cincinnati, Ohio, and now that I'm in Chicago, oh, man, just some beautiful color. And I've been up in the Northeast in October and seen the beautiful fall color there. So if you live in a place that doesn't have good fall color, make it a trip sometime to go to some place that does, because it is one of the joys of fall, for sure. So it was good to chat with Paul all about it.
Joe Martucci: And as Paul said, come to New Jersey for fall foliage. And we'll hear about that, in the pod.
Sean Sublette: Yes, this pod apparently was brought to you by the state of New Jersey, New Jersey, and you perfect together. Just ask. I like hey, I like my colors here in the Commonwealth of Virginia. We get those beautiful Fruity Pebbles colors along Skyline Drive and the Blue Ridge Parkway. And we're getting very close to peak here, in the mountains of Virginia. But the other thing I liked about this, about Paul was so many times we're asked as M meteorologists, well, what. Makes for good fall color? And he's. Like, okay, good. I feel a lot better now.
Kirsten Lang: All right. And with that, let's get onto our interview with Paul James.
Kirsten Lang: Well, today we have on Paul James, an American gardener who may be best known as the gardener guy from his long running show on HGTV Gardening by the Yard. He now lives in Tulsa, Oklahoma, semi-retired, but that's how I got to know him. And he works for Southwood Garden Center, where he continues to educate the public about all things that are related to gardening, flowers, yards, and trees. And that's where we're going to pick up with him today, is the science behind the changing colors and all things fall. So, Paul, welcome on. We are so happy to have you today.
Paul James: Thank you. I'm thrilled to be know we wanted.
Kirsten Lang: To talk a little bit. This is very timely of course, we wanted to talk a little bit about the science behind changing leaves. So I kind of wanted to just jump in with the broad question, as to why it is that a leaf actually changes its color. And why do you see some yellows, and then you see some orange and some red? Why do they all, vary in colors when they change?
Paul James: It all has to do with pigments. They are chemicals that are in the leaves. And what we see primarily throughout the growing season is an abundance of chlorophyll in leaves, which is a pigment that makes them green. But when that chlorophyll production is shut off, and we get into that in a moment, when that's shut off, then the other pigments that are there begin to show themselves. So you have keratinoids is a type of pigment, and those are responsible for orange and yellow. You have xanthaphils that will give you they're not quite as colorful, but they'll still give you some color. And then the anthocyanins, which give you the beautiful reds, sometimes even purplish colors. So that's what happens. The chlorophyll, which is there in abundance, gets shut off and all those other pigments have an opportunity to really show themselves.
Joe Martucci: So Paul and Joe here. So I guess what you're saying is the chlorophyll, right? It's the dominant, I guess, pigment, you could say. And why is that?
Paul James: It's the dominant pigment throughout the growing season by virtue of photosynthesis. Photosynthesis, where plants basically, leaves are eating photons from the sun and producing that abundance of chlorophyll. So that's why virtually all plants that we're familiar with have green leaves throughout the growing season.
Joe Martucci: M, but just to follow up on that, right, so photosynthesis, then you're saying, is the king above. All right, so whatever's happening with the photosynthesis is number one. But I guess I'm trying to dig out maybe I'm getting too deep here. But why is that the primary thing? Why isn't it just a different kind of pigment that's primary?
Paul James: Because that's the pigment that is responsible for growth and producing energy and helping the plants survive. Yeah, got it.
Joe Martucci: Ah, comes with the territory.
Paul James: Yes.
Joe Martucci: Got you. Okay, cool.
Sean Sublette: Yeah, and I get that, on my tennis shoes all the time when I'm cutting grass as well, with all that chlorophyll on the tennis shoes.
Sean Sublette: another question for you about the changing leaves. My understanding is that as the nights get longer, we have longer bits of nighttime. That's kind of the key or the trigger for most plants or most of the, trees to say, okay, well, it's time to shut down for this season. But what other kinds of phenomenon out there, whether it's weather, drought, heat, anything like that, can affect how the leaves look or when they actually change by.
Paul James: A few weeks here or there in the botanical world, we actually explain it in the reverse. We say that days get shorter, rather than the nights get longer. So as the days get shorter, that triggers a chemical response in the trees, wherein the little I don't want to get too technical either, but it's called a pettyol. You got the leaf and then you've got this little stem, that's known as a pettyol and that's attached to the main branch or limb of the tree. So when those days get shorter, that triggers what's known as the obscision layer, which is where that Petio connects to the main part of the tree. It triggers it to just shut off and that prevents any additional chlorophyll production. So the chlorophyll disappears and all those other pigments rise to the surface. Now, there's a gazillion different variables here. It can be somewhat challenging to, actually, there's not a ton of research to substantiate a lot of the claims, such as to what extent does weather play a part? Are, temperatures really that critical to the change? Is moisture level in the soil that critical a factor? I think you have to look at it in its entirety. I think all of these conditions have an effect on m our fall color, whether it's going to be spectacular or ho. but without a doubt, first and foremost, the most important consideration is genetics. Because if the tree does not have the genetics in it, if it's not hardwired to produce good fall color, then it's not going to well, one more.
Sean Sublette: Thing, ah, about those other environmental things. Because I've been working in weather for 30 years, I hear the same thing, like, oh, this obviously affects it and this obviously affects it. I'm like, that's not the way I understand it. I hear there's a bunch of stuff in there that kind of is all wibbly wobbly timey whimy, as they say in Doctor Who. but is it fair to say if a tree is stressed, whether it is drought or maybe overly wet, that might, force it to kind of change a little earlier? Or is that not just fair to say at all?
Paul James: I'd say it's likely. But again, despite the enormous interest in fall color, and by the way, the United States has some of the best fall color anywhere on the planet. Despite the interest in it, the level of research that's been done is really kind of lacking. It's almost too bad, because I don't think it would be that difficult to do experiments that would help us understand better why some years are better than others. It just hasn't been done to any great degree.
Matt Holiner: I guess what also complicates things is all the different species of trees out there and how they are going to respond differently to drought conditions or excessive rain. Because I know growing up in San Antonio and there's a good chunk of Texas, until you get in maybe the far northern parts of Texas, fall color is not really a thing. You go from green to brown, you don't get all those beautiful shades of yellow and orange. So is there anything about specific species of trees that are just well known for their vivid fall colors? Are there some trees that are just extra special when it comes to fall color?
Paul James: To use a San Antonio example, for instance, live oaks don't really have a whole lot of that pigmentation that I was talking about. They have very little. Whereas the red oaks that are abundant in my part of the country are loaded with those pigments. So, yes, even within a particular genus, be it oaks, there can be a tremendous amount of variability in fall color.
Kirsten Lang: Okay, so just paul, where have you seen, in your opinion, the most beautiful color here in the country?
Paul James: New Jersey. I have a particular favorite.
Kirsten Lang: Jersey.
Paul James: Jersey has great fall color.
Joe Martucci: See, I wasn't.
Paul James: No, are you kidding? It's the garden state.
Joe Martucci: Of course.
Paul James: I've seen spectacular examples all over the country, particularly the northeast, also the Carolinas and throughout much of the Midwest. But one of my favorite single images of fall color has to be the quaking aspens in Colorado. I mean, to me, when you see those enormous groves of aspens and they're ablaze in it's just it's astounding to.
Kirsten Lang: Me, yeah, it is beautiful. I lived in Colorado for a couple years, and you better believe, on my days off, I made sure I was going up there and hiking and seeing some of that, because it was beautiful. It was beautiful. Okay, well, we're going to take a short break, and we'll be back after, the break here with Paul James.
Kirsten Lang: All right, we are back with Paul James. Paul, we were talking all about, fall colors, how vibrant they are, in some spots of the country, and then, of course, the contributing, factors to making maybe one season better, than another due to soil moisture, drought. but the other thing, too, that, like I said, it may be an obvious answer, but in case someone's listening and just curious, can you explain why it is that the peak of fall foliage will start north and then just filtrate south through the country throughout the months of October, November?
Paul James: Well, there again, that's, changes in day length, but that's where you almost have to conclude that temperature is playing a role as well, because it's starting to get cooler sooner in the Northeast, typically. And you see that progression from the north to the south and the changes in the know. The drought issue, though, is one that I think is interesting. It's pretty clear to me that during seasons where we've had considerable drought, that we have less spectacular fall color. That's really anecdotal, but I'm sure that at some point, we'll figure it out on a more scientific level. But I've witnessed that myself, and not just here, but throughout the country where even places in the Northeast occasionally have droughts. So that factor again, we go back to all those things. The other thing in the Northeast is that they also typically have a greater variety of hardwood trees than a lot of other parts of the country. So there's so much more species diversity that you'll typically see a lot more color. The ones that stand out, of course, are like maples. The red and maples is just unbelievable. It's so gorgeous. But there are also oaks that produce great reds. Around here we have the Chinese pistache, which can be the color can be highly variable due to its genetics. Some of them are much more colorful than others. One that I've always loved, that is kind of our own aspen is the ginkgo, which is, to me, also an underplanted tree. But the sad thing about the ginkgo is it gives you this enormously, beautiful yellow foliage, but then overnight, typically within one night, all the leaves drop. It's kind of sad.
Matt Holiner: Yeah, Paul, I was going to actually ask about that because I imagine that it does vary by species. How long this process of the leaves changing and falling takes. Is there an average overall that you can look for? Is it really across the gamut? Like how long this process takes for each different species of trees?
Paul James: Wind can certainly play a big role in that. I think we're going to be up to about 40, 45 miles an hour today. So I think, we may lose a few of the leaves that are just slowly beginning to not change color. But that obscision layer I talked about before is starting to weaken. But no, I don't know that. There's a timeline for each species. I'm not aware of one. It may exist, but I'm not aware of one.
Matt Holiner: But I imagine that yeah, as that abssion layer gets a little bit weaker and weaker, it's more likely that a wind, it'll take less and less wind to actually blow them off of the tree so early on, it's going to really take maybe the 40, 45 miles per hour winds. But as you get later into the fall and those leaves are getting a little bit more wilted in the color, perhaps a little bit transitioning from that yellow to red, at least in some species, I guess it's more likely it's not going to take as much wind to blow it off. So that has a big, impact.
Paul James: Sometimes it could be as calm as can be. And you'll still see the leaves, right? Yeah.
Joe Martucci: Hey, Paul, I want to go back to something matt briefly touched on the first half. And as a lifelong Northerner, I still kind of fail.
Paul James: Where are you?
Joe Martucci: You know, some people tell me New Jersey. Every once in a while, they tell me I'm from New Jersey.
Sean Sublette: I like to tease him that he sometimes lives in Central Jersey.
Joe Martucci: Well, that's another story. Right? Paul knows about Central Jersey because his son went to Princeton smack dab in Central Jersey. I mean, he's pretty much an expert on this.
Paul James: Lawrence Township. Elizabeth. Yeah, I know the area well.
Joe Martucci: I grew up in the town next to Elizabeth Union, New Jersey, right down Broad, street over there. But anyway, maybe I'm asking this on behalf of all of our northern listeners, you know, down in the south, and I'm talking like, San Antonio or, maybe know there I've been down there in even January, and I still know leaves on the trees, or I've been there and they haven't. It just seems very finicky down, like far down south to me. How does that process work? Because obviously you're not getting the temperatures that you're getting up in the north, and you still have longer days as you go into October and November compared to places up north. So what determines the foliage process there? And when can those or do those leaves, if those leaves come off the trees?
Paul James: I can't say with certainty, Joe. I can only conclude that, again, it's genetics. Weather certainly may play a role, but I would tend to think that genetics is the principal factor in those,
Joe Martucci: Just because I feel like here in New Jersey and again, I'm not just saying New Jersey because I'm from New Jersey, but in the northeast. By about Christmas, these leaves are off the and then but in some places, I know when I've gone south, they're not off the trees. By maybe, you know, like you said, probably genetics. I don't know. I'm just asking the question. I'm genuinely curious, so I appreciate the answer.
Paul James: We have years where on oaks in particular around here, where the leaves remain for months and months, well past what you would ordinarily expect to be the drop time. To me, that suggests that weather plays a role. I just don't know what that role truly is. What's particularly bad is when a lot of the trees hold their leaves for a longer period of time, and then we have an ice storm.
Joe Martucci: Yeah, those early season ice storms are very impactful. Sure.
Paul James: Yeah. The weight on those trees is just enormous.
Joe Martucci: I mean, I'll even share something. We had, Superstorm Sandy come through in October 29 of 2012. As you could probably imagine, that's pretty close to peak foliage here. And the amount of just leaves I remember the day after it hit or two days after walking on my street, and it was just covered in leaves because everything got ripped off with the wind that we had gusts in the for hours on end.
Paul James: Yes. The yeti brother.
Joe Martucci: I knew I liked it. Paul. I mean New Jersey. We got the Yeti mugs. Life's good.
Sean Sublette: Everybody loves the yeti mug. For sure.
Paul James: they should be sponsoring the Dang podcast.
Joe Martucci: Hey, Yeti, if you're listening, listen, we're all big fans here. And all the people who have New Jersey ties have the Yeti mug, including Sean.
Sean Sublette: That's where I got it. I got it when I was in Jersey and it's lasted this.
Joe Martucci: Know, they do say what Trenton makes the world takes Trenton being right next to yeah, I've seen that. It's a it's a very cool bridge, but I'll digress. I don't want to bore our listeners with Trenton, New Jersey.
Kirsten Lang: This has turned into a Jersey pod, is what this has turned.
Paul James: It. I like know there are places throughout the world where there is no fall color. I mean, if you've ever been to England in the fall, you will rarely see any fall color.
Joe Martucci: Really?
Paul James: in fact, one of the plants this is true one of the plants that is intentionally planted at the base of trees to provide fall color is poison ivy. Because it does produce spectacular fall color.
Kirsten Lang: Really?
Joe Martucci: I guess it better be good for something.
Paul James: So what color does poison ivy turn magenta? It’s really pretty. Wow.
Kirsten Lang: Really? Even here?
Paul James: Yes.
Kirsten Lang: Really?
Sean Sublette: Yes.
Kirsten Lang: That's a dead giveaway because I've always gone by the leaves of three. Let it be or whatever it is. But if it's just turning magenta, well, then that's her giveaway not to touch it.
Paul James: it's quite beautiful. I mean, if you're itching to plant it. Go ahead.
Joe Martucci: I see what you did there.
Kirsten Lang: Pun intended.
Paul James: Sorry, there aren't that many gardening joke opportunities.
Kirsten Lang: I want to ask a little bit. This is not quite as much fall foliage. That word always trips me up, but fall foliage related. But we talked once about leaf scorching here, and with overnight lows this happened when was it? Last summer. Overnight lows in particular were just so warm, and they were staying above 80 degrees here in Tulsa, and we were seeing leaf scorching that was happening. Can you explain a little bit about that and if that would impact the fall color in the coming seasons?
Paul James: Yeah, the scorching can be the result of a couple of things. First of all, just intense sunlight can cause scorching. But what you're referring to is when our overnight temperatures at 11:00 at night, it's still the 90s. That can be devastating for plants because they need a chance to cool off at night. Cellular growth in plants actually takes place at night, so they need an opportunity to rest and chill, so that that cellular growth can take place. when it's that warm, it just doesn't happen. And a lot of people don't understand that. And there's nothing you can do about it, unfortunately. But, it can do a number on plants. We have a lot of plants here that are killed as a direct result of overnight temperatures being excessively warm. And it's really a drag to be out on the patio at 11:00 at night when it's 90 degrees.
Kirsten Lang: I know. It's not like that today, though, in Tulsa, no, feels great. Did that, have any effect then, on the color that following fall?
Paul James: It would almost certainly have to, simply because it is causing a disruption in cellular activity. So things won't be normal with that plant, and it could take it a full year to recover, if it recovers at all. So, yeah, I would think that would have a, negative impact on its ability to produce good but fall color.
Matt Holiner: And Paul for people who have, trees in their yard, and they want to get the best color out of them possible. Have you come across any researcher in your years of gardening? Things people can do to help care for the trees, to perhaps give them a little bit of a boost to help that fall color a little bit?
Paul James: No.
Joe Martucci: We love being honest here.
Kirsten Lang: Poison ivy. Poison ivy.
Paul James: Maintaining good health in the tree means you're going to have to water routinely unless rains do the job for you. Fertilization is another component that is important to maintain good cellular growth, not disturbing the root system. Roots are key. All those things are going to combine to produce a healthier tree. And a healthier tree is more likely to do what you want it to do in fall, and they may produce that great color.
Matt Holiner: I've heard some, talk about putting vitamins in your trees. Like they're things that you can be inserted around the base of the tree that supposedly gives them a boost. In addition to fertilizer, this was something that was occurring in San Antonio when I was growing up, how it improves the health of the trees. These, like, vitamin supplements for trees. Do you know anything about that? And does that really have an impact, or is that just more one of those things, like just something to get money out of people's hands?
Paul James: Vitamins, I would suggest, are pure nonsense. But there is a fungus, a beneficial fungus. When gardeners hear the term fungus, they usually think, oh, no, it's terrible. No, there's some very beneficial fungi out there, in particular the mycorrhiza, of which there are numerous strains. But mycorrhiza are essential to plant growth and can really if you add mycorrhiza to your soil, it can go a long way toward helping any plant that grows, whether it's a tree or shrub or flowers. And what that is, it's a fungus that attaches itself to the roots and enables it's a symbiotic relationship. It helps the roots absorb nutrients more effectively. The vitamins are snake oil.
Matt Holiner: That's good to know. One less thing to spend money on. I'm okay with that.
Paul James: Yeah.
Kirsten Lang: Well, Paul, thanks for joining us, today. We had a great time talking with you about leaves and about Jersey, because we got a lot of that in today. So thanks so much for being on. We really appreciated it.
Paul James: I, thank you so much. This was a lot of.
Kirsten Lang: Right. Paul James, you know, had so much information to give us just on the science behind changing leaves, how weather may or may not contribute to that, as well. But I agree with he's and I know Joe, you're going to say that he favored Jersey, but he did when we asked him the question, he did say what what was his favorite place?
Joe Martucci: Colorado.
Kirsten Lang: Colorado. I know. And it's so pretty out there. It really is.
Joe Martucci: During the is it is. I just think in Colorado, right? Isn't it like mostly the yellow? Like there's not like a lot of oranges and reds, right? Isn't it mostly yellow out there?
Kirsten Lang: There's not a lot of reds. I'm m with mainly I think those yellows from the aspens is what.
Sean Sublette: You that's where my brain immediately goes is to the yellow gold aspens.
Joe Martucci: Yeah. Which is nice.
Kirsten Lang: Joe's getting defensive.
Joe Martucci: I'll just stop it. It's nice. It's nice.
Matt Holiner: I think one of my big takeaways is know, there's still a lot we don't know. There needs to be more research, especially as the climate is changing now we're seeing that temperatures aren't as cold as they used to be in the fall and we're experiencing more excessive rain events and drought events. And how do different species of trees respond to that? Because it seems to be not only different by region, but even the species of trees in those regions are impacted differently. So it's just an area that needs a lot more research. And again, it's one of those things that everybody talks about fall color and there's so much interest in it, but there's still a lot we don't know. So we look forward in the years to come to learning more and understanding more.
Kirsten Lang: All right, and so what we've got coming up, next know, just kind of staying on this whole topic of seasons. We're going to talk a little bit about the winter weather outlook and what you can expect for the cold months coming up. So that's coming up next week. And Joe, if anybody wants to contact us and ask us questions, we would love to answer them. Joe, can you give them that contact information?
Joe Martucci: Totally. So you can drop us a voicemail at 609-272-7099. That's 609-272-7099. And in the break offline, Matt was telling us that his mom is listening to the podcast. So hello to, I don't know, Renee.
Matt Holiner: Renee.
Joe Martucci: Renee. So hello to Renee. So, Renee, if you're listening and you want to give us a call 609-272-7099. It doesn't have to be Renee, it can be anybody. But we will.
Sean Sublette: By the way, Renee, Matt is doing just the best job. We're so proud of him.
Joe Martucci: Yes, we are.
Matt Holiner: Thanks every day.
Kirsten Lang: Such a smart kid. You did great. Is there an email too, Joe, in case someone feels like they want to email instead?
Joe Martucci: Yeah, sure. That would be, podcasts@lee.net again. Podcasts, at lee L-E-E net.
Kirsten Lang: Well, guys, thanks for joining us this week on across the sky. We loved having you, and we hope to see you back here next week.See omnystudio.com/listener for privacy information.