EVERY GARDENER has certainly heard the rallying cry each recent autumn to “leave the leaves,” invoking us to go gentler with our cleanup to support a diversity of beneficial invertebrates who call the fallen leaves their home.
Now a recently published research study calculates just what the impacts of leaf removal are to which organisms, and also offers insights into how and where in our landscapes we can leave the leaves to create habitat with the most positive impact.
Max Ferlauto, the Maryland state entomologist, is one of the scientists who conducted the new research, and in March this year, Max and Karin Burghardt, an ecologist and associate professor at the University of Maryland-College Park, published research on the impacts of leaf removal resulting from their two-year study in 20 residential suburban Maryland yards. On the podcast we talked about their takeaways, and how the data they gathered can help inform gardeners seeking to develop their own more ecological fall cleanup plans.
Read along as you listen to the Oct. 20, 2025 edition of my public-radio show and podcast using the player below. You can subscribe to all future editions on Apple Podcasts (iTunes) or Spotify (and browse my archive of podcasts here).
the impacts of leaf removal, with max ferlauto
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Margaret Roach: Yeah. So we did a recent “New York Times” garden column together about your latest research with Karin Burghardt, and I think I said to you at the time, that I’d heard about leaves the leaves, but this was like: leave the leaves, by the numbers. [Laughter.] It’s like you really collected data. It was amazing. So tell us, how did the study take shape? What did you do? What was the protocol, so to speak?
Max Ferlauto: Yeah, sure. So we use these things called emergence traps [above], which are these tents that you basically place over the earth and anything that emerges from that piece of land gets collected. And so in these suburban yards, as you mentioned, we placed those emergence traps over areas where leaves were raked away and where leaves were retained. So we were able to really see everything that was emerging from the ground after the winter, which is the period of time that we’re concerned about during the leaf litter removal period.
Margaret: So there were pairs, so to speak, of these traps. And one had leaves, one area had leaves, and one didn’t in each case. And how big were the spots? How big were the traps?
Max: So we manipulated the leaf litter in one meter square quadrats, and the emergence traps over those quadrats in areas where we removed or retained leaves.
Margaret: So that’s not really very big. And yet when I read the numbers, I mean it’s a little larger than a square yard, yes, a square meter?
Max: Yeah, a square meter.
Margaret: So it’s not miles and miles and miles. And yet when I read the numbers come spring, or in March when you put these emergence traps up through, I guess what, the end of June was it each year?
Max: Yes. March through June, March through the end of June, and we collected almost 2,000 insects emerging insects and spiders emerging from that square meter of yard. [Above. Max with a waved sphinx moth.]
Margaret: In each one that had leaves.
Max: Yes.
Margaret: I mean, that’s a lot of life [laughter]. That’s a lot of life.
Max: You don’t think about your yard producing that much abundance in biodiversity. But that’s what we found.
Margaret: And which organisms were you counting? You weren’t counting like earthworms or-?
Max: So earthworms and decomposing species like springtails, we had even more of those. These were just butterflies and moths, beneficial parasitic wasps, spiders, beetles and flies.
Margaret: O.K.
Max: And basically when you removed leaves, we saw a decrease in a lot of these groups. So when you removed leaves, we found that the average abundance of butterflies and moths was reduced by about 45 percent.
Margaret: Forty-five percent. Oh!
Max: Spider emergence declined by 56 percent, and beetle emergence declined by 24 percent. So we’re seeing that yes, these small sections of our yard, which produce so many insects and beneficial species, if you remove the leaves, we’re seeing declines there.
Margaret: So now people are probably like, well, but what is… You didn’t mention parasitic wasps just then; you had mentioned them earlier. But I’m a gardener—why am I thinking about spiders? A lot of this is about the checks-and-balances systems, right? It’s like the whole system has to be in place. We’ve all heard in the leave the leaves campaign, we’ve especially heard about the butterflies and moths and their caterpillars, which then of course are the primary food source for baby birds, especially songbirds. And that’s so critically important, and that’s been highlighted to us as part of this campaign. But it’s not quite that simple. It’s not just those Lepidopterans. [Above, a marbled orb weaver spider.]
Max: In your yard, you want to have a thriving ecosystem, and that means thinking about the prey and the predators, and that occurs at the insect level, just like it occurs for mammals and birds. So yes, it’s really important to have butterflies and caterpillars to feed birds, but also it’s great to have beneficial biocontrol.
These are natural enemies that we have in our yards that take care of our pest problems. So things like spiders, certain ground beetles, and definitely parasitic wasps, which are tiny wasps; they can’t sting humans, but they will lay their eggs in different pest species and reduce their numbers. So if you have outbreaks of aphids or tobacco hornworms on your tomatoes, there are these parasitic wasps that will control those species.
Margaret: So if we do anything to interrupt their success in our habitat, whether in some of the cases by the numbers you mentioned a couple minutes ago, by not leaving the leaves, we reduced the percentage of emergent individuals of those groups of invertebrates.
Max: Spraying a pesticide, you might get rid of your problem in the short term. But in the long term, if you’re killing both the prey and the predator, eventually that outbreak could come back even worse because you’re not going to have anything to control it. Anything that is going to disrupt those food chains in the long run, it’s going to be damaging.
Margaret: So if we want somebody to take care of our aphids and our tobacco hornworms, we’d better make sure that the food that they want is around [laughter], otherwise they’re not going to find our environment very attractive. I’m oversimplifying, but you know what I mean.
So there were these declines in abundance, I think you told me when we did the Times story, you would call it—the total number of individuals—and also fewer species you saw in some cases.
Max: Yeah. So butterflies and moths had a reduction in species richness or the number of species by about 44 percent.
Margaret: Oh my goodness.
Max: Yeah. And what we’re seeing was it wasn’t just that the species richness was being reduced, but also the composition, the different types of the species that survived had changed. So specifically species that mine leaves, they’re called leafminers, or species that overwinter as larvae or overwinter within the leaves, those are their life-history traits, we say those were the species that were harmed the most by removing leaves.
And on the predator side, we found that the parasitoid wasps that feed on the leafminers and feed on the larvae that are overwintering declined, just like those prey species did as well. So we can see the entire food chain being disrupted by removing leaves. [Above, a mourning cloak butterfly; they overwinter as adults in leaf litter or under bark of trees.]
Margaret: Many years ago, as gardeners—and I’ve been an organic gardener for decades—many, many years ago, there was a big aha moment where it was like, don’t bag your leaves and put them out at the curb; be more sustainable. Keep them there. Utilize those resources, build your soil, use it as mulch, mow over it on the lawn, shred it and let it decompose into the soil of your lawn and blah, blah, blah. In other words, keep it out of the landfill.
And that’s a great idea, that’s a great idea. But it doesn’t do what we’re just talking about: fostering, supporting this food chain and creating habitat for all of these species to live their life cycles out. So I’m going to still say, I believe that compared to sending it in a plastic bag to the landfill—some leaves may have to get maybe end up in the compost, or sometimes we just have too many in certain configurations of our home landscapes or whatever, to let them all lie on every square inch.
But what should we be doing and what can we be doing, and which actions don’t support these organisms that live in the leaves and so forth? How do I know what to do? What are my best and worst options, so to speak [laughter]? Because it’s confusing.
Max: I’m a huge proponent of composting leaves for your garden, but as you mentioned, by doing that, you are going to be killing those overwintering species.
Margaret: Is the heat; is it the heat of the compost? What kills them?
Max: If you have a compost that is running strong and hot, it will kill those species. But also just the depth of that compost pile is going to be disruptive as well.
So really what we’re saying is you don’t have to leave the leaves over your entire yard—as I mentioned, just a small square meter is producing so many insects. So if you have too many, you can compost what you need to. But by leaving leaves and not composting them, you’re allowing for those life cycles of these different insects we’re talking about to complete.
And some other things you can do is take the leaves from an area that you don’t want them, and just move them to a new area. A lot of the insects are overwintering within that leaf material, so by moving it, you’re just basically moving them to a new home over the winter.
Margaret: So moving them whole, not shredded or not put through a machine of some kind, but moving them whole.
Max: We found that shredding leaves was almost as bad as removing them and bagging them. So removing them whole, placing them at the edge of your yard, those are all alternative practices.
Margaret: So moving them around. And in terms of identifying sort of areas of maximum impact, positive impact? I believe when we did the Times story, we talked about thinking about thinking about: so here I’ve made a pollinator garden in my backyard, let’s say. O.K., let me visualize all those creatures that are living in there in the fair-weather months. Where are they going to go and complete their life cycle, procreate, whatever—what’s going to happen next to them? And maybe think about utilizing that area or an adjacent area. Or if I have trees and letting leaves lie under trees and creating a bed under the trees and things like that.
What kind of strategies—is it that kind of thinking? Is that what you’re doing when you look around? [Woolly bear caterpillars, above, the larval stage of the Isabelle tiger moth, overwinter in leaf litter.]
Max: And we did look at this and we found that leaving leaves in areas where they had previously been removed year after year was more beneficial than just leaving them in an area where you always leave them. So that leaf material is a habitat resource. And so by providing it in an otherwise barren area, you’re attracting those insects to that protective habitat, overwintering habitat. And the areas where you have flower gardens, where you have native plant gardens, those are all ideal places to retain your leaves.
Margaret: Yeah, I would think so. I mean, to me, that’s again, when we spoke before for the Times story, that was kind of what I was visualizing when you were sort of walking me through the research and when I read it on my own and so forth, after our conversations, I was kind of visualizing. And the other thing I was visualizing is that, huh, O.K., at the fringe of my property, for example, I have a lot of mature trees, as many people do. The biggest trees sometimes are along the property boundaries or whatever. And I thought, O.K., so those oaks, what if I can make those have their own leaves beneath them? If I can create beds, so to speak and say, these are my safety zones, these are places where I’m going to leave the leaves, because that would seem to be a very natural, ideal kind of situation.
Max: Yeah. There’s a term that’s been discussed called “soft landings,” this idea where you have those areas under trees and you keep them natural. And if you need to move leaves to those areas, as long as you’re not creating huge leaf piles and you’re just kind of moving them from one area to another, you can create those protective habitats, so that all of those species that live on the trees during the summer and spring can move down to its roots to the leaf layer that’s there during the fall and winter. [More about creating soft landings.]
Margaret: And you just said huge leaf piles. So what we’re not talking about is we’re not saying take every leaf whole, pile them on tarps and drag every single leaf on your property onto one bed under one tree, so it’s a 7-foot-deep layer [laughter]. That’s not good.
Max: The insects, they’re queued to seasonal changes, and also energetically, it’s very difficult for them to climb out of a, as you said, a 7-foot pile of leaves. So they want their environment to be as similar as it would be on a forest floor. So imagine walking through the woods on a hike. The leaf layer that you’re seeing around is the depth that we’re talking about, so not deeper than that.
Margaret: So it’s not giant, giant, giant piles of leaves, which also would smother some of the herbaceous plants at that layer.
Max: Yeah, oxygen levels are lower. It’s just not a great habitat to be an overwintering insect at the bottom of a massive pile of leaves.
Margaret: O.K. And similarly, you’re not advocating that. Again, using this kind of walk around and look and try to draw inferences by thinking about where are these creatures and where are they going to go next month and in three months and in six months, in other words, to have a whole year to have their whole life cycle however long it is, and their next generation, next generation survive in this area where I may have attracted them with my planting methods and so forth. I mean, it is a garden after all. So I’m not just going to dump 7 feet deep of leaves behind my garage, either, right? That’s not the whole answer.
Max: And it’s about thinking about the whole life cycle, as you mentioned. So if you’re attracting the insects in the summer with your native plants or your pollinator flowers, you don’t want to remove the habitat that they rely on over the winter. You’re basically just attracting them to kill them later on. So it’s those areas that can be the best places. If you must leave your leaves in only one area, not the whole yard, those are the areas to prioritize.
Margaret: And you just mentioned winter, and so a lot of us, again, it was leave the leaves, don’t clean up so scrupulously in fall, be a little messier, etc., because of overwintering habitat. This is what we’ve learned so far, because again, this is a fairly new idea for a lot of us, and we’re trying to—and we’re not scientists, we don’t have your expertise—we’re trying to take it all in.
So we’ve learned about the overwintering-habitat idea and who’s in that leaf litter in the offseason, but it doesn’t end there, does it? The importance of that habitat doesn’t end there, does it, at the end of winter?
Max: Yeah. Our experiment was to remove the leaves in the fall, and then we placed those emergence traps in the spring to early summer, but we did not notice any declines in the number of bugs emerging. So there was a thought that at some point, maybe when the ground hits 50 degrees or all of these other potential times, there was going to be a cliff, and most of the insects would have emerged, and now we’re O.K. to remove our leaves.
We didn’t see that. We saw that insects kept on emerging throughout the summer, so that in July we were actually getting more insects emerging than we were in March and April. So yeah, I would say that we’re talking about overwintering, and that is what we specifically studied, but it doesn’t look like there’s much evidence for there being a time where everything is emerged and it’s O.K. to clean up. A lot of species actually have two generations, so they’ll have a generation in the spring, and then they’ll also have one in late summer. This leaf habitat is valuable all year round.
Margaret: Right. Well, and if we think about it, we’re in a way emulating or evoking this sort of duff layer. The forest floor you mentioned before is the closest thing we know in habitats. So nobody cleaned up the leaves there at any time [laughter]. Do you know what I mean? It was a forever thing. The detritivores, all the other creatures who you didn’t count in this particular study, but who we know something about, they were doing their work down below on the oldest stuff, which was getting incorporated into the soil; the organic matter was getting incorporated into the soil. So there was a constant, again, food chain.
Max: Yeah. I mean, there are examples in history where humans have removed leaf litter from forests, like in the Middle Ages. They would use that material for bedding for their livestock. And there’s been studies that have shown that those areas where the leaves have been removed for centuries have lower soil carbon levels, even centuries after that practice had stopped. So the removal of this material has long-lasting implications.
Margaret: Let’s talk a little bit more about that, because I think a year prior, I think you published work about earlier research about the soil carbon and about what you were finding. Was it from these same properties, I believe, but earlier inferences?
Max: We looked at soil carbon and decomposition, and we found that areas where leaves have been removed year after year as a historical practice within yards had a reduction of about 24 percent of soil carbon was reduced.
Margaret: And the soil carbon helps in…what’s its role? It helps with-?
Max: Soil carbon, and soil organic matter really are valuable as, I guess you could say, purveyors of different soil qualities. So soil carbon, having higher soil carbon, means your soil is more nutritious. It means it holds water better. It means structurally it is less compacted. And then of course, thinking on the big picture, soil carbon is important for carbon sequestration from our atmosphere. So soil carbon is very important to the systems of earth.
Margaret: So that was some of the earlier takeaways, and that’s not quick. Like you said, in the places where you added leaves, you saw not immediate, but in that same season, you could see creatures were utilizing it. Whereas repairing this loss of soil carbon doesn’t come back the next year.
Max: Right, exactly. We would say that this would be a legacy effect of leaf removal. The carbon is being reduced by 24 percent. It’s not coming back the next year.
Margaret: Or probably anytime soon.
Max: Yeah, exactly.
Margaret: Well, Max Ferlauto, I mean, the numbers really did it for me. It made me want to learn more and more and more about each of the creatures and how to support them, and the intricate relationships between them and so forth. So I just found it fascinating. I’m so glad to speak to you again, and thank you for making time. I know you’ve got lots and lots and lots going on, so thank you.
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MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its 16th year in March 2025. It’s produced at Robin Hood Radio, the smallest NPR station in the nation. Listen locally in the Hudson Valley (NY)-Berkshires (MA)-Litchfield Hills (CT) Mondays at 8:30 AM Eastern, rerun at 8:30 Saturdays. Or play the Oct. 20, 2025 show using the player near the top of this transcript. You can subscribe to all future editions on iTunes/Apple Podcasts or Spotify (and browse my archive of podcasts here).
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