INCREASINGLY in recent years, my garden weeds include more and more tenacious opponents. And the landscape along the roadsides nearby and pretty much everywhere I drive is one of hedgerows formed of a tangle of non-native shrubs and vines. I’m talking about invasive species, of course (like the Oriental bittersweet, Celastrus orbiculatus, in the Wikimedia image above), and I got in touch with Daniel Weitoish of Cornell Botanic Gardens, to hear how to identify which plants to target as we try to manage our landscapes and how to tackle them most strategically.
Daniel is the Arboriculture Supervisor at Cornell Botanic Gardens in Ithaca, N.Y., where he and his colleagues care for the woody plants at the 30-acre botanic garden, plus 100-acre arboretum in about 3,600 acres of natural areas. You may recall a conversation he and I had about the increasing challenges of gardening in a changing climate, and this topic about managing invasives is likewise one made more complex by our warming world.
Read along as you listen to the Sept. 30, 2024 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).
tackling invasives, with daniel weitoish
Margaret Roach: Remember the good old days when the most dreaded weed was a dandelion? Oh, my goodness [laughter].
Daniel Weitoish: We had specific tools for that singular little plant.
Margaret: I know, right? Right, right.
Daniel: I don’t think we’re going back anytime soon.
Margaret: No. So as I hinted at in the introduction, some of the non-native plants that I’d see 10 years ago and 20 years ago, along the local roadsides where I live in basically everywhere, they’re now occupying every square foot. I mean, it’s like a jungle of invasives. And is this yet another sort of gift that keeps on giving of our changing climate, as I implied in the intro? I mean, is that what you all are thinking is that this is one of the side effects of what’s going on with the climate?
Daniel: Yeah, yeah. It can continue to get more invasive, potentially, a given species. And the additional gift that we get there is that brand new species who previously were very well-behaved, they’re now displaying invasive qualities. So with those changing climates, this is… I don’t think it’s going to make battling invasive plants any easier.
Margaret: No, I know, because I feel like some things that I used to see a little bit of and then a little more of it’s now like… It’s every square inch is covered with the things. Do you know what I mean? It’s amazing. Do you make a distinction between weed and invasive? Do you know what I mean? Is there some kind of cutoff word? How do you kind of figure that out?
Daniel: Sure, sure. There are guidelines that state agencies are using to determine in a weed assessment capacity, is this an invasive plant. They are considering how well does it propagate? Does it crowd out other natives? For us at the botanic gardens, what we tackle is somewhere between that very firm designation, weed versus invasive. Right plant or wrong plant in a given spot is a lot of how we approach it.
Margaret: Now, you have developed a policy for identifying and dealing with invasive plants in the context of this property that you all manage, the botanic gardens and etc. there. And when I read, when you shared with me your policy, and I read it, I thought, well, we all need a policy [laughter]. Each of us, even gardeners need a policy because again, turning our backs or not paying attention is going to really catch up with us even faster than it used to. Something for me, like garlic mustard that used to be a nuisance is now… I mean, it literally would take up every square foot if you turned your back for a minute.
Daniel: Sure.
Margaret: Yeah. So did you use some of those lists, some of that guidance from the state agencies, in developing your policy and figuring out what was over the line in terms of being invasive versus just a weed?
Daniel: Absolutely. Yeah. Again, in the context of climate change, there are New York State-regulated lists there. These are the plants that you do not propagate or sell. As a botanic garden, we shouldn’t be planting those plants as well. But considering climate change, there are plants in Pennsylvania, Maryland that we have in our collections. There’s no guidance from New York State necessarily, but there is in those other states. And so part of our policy as we crafted it, was to look at the states around us. If our climate warms, what could be a challenge in the near future?
Margaret: Right. Because as our frost-free season gets longer, and I use that as a rough frost-free season. I mean, the plants are sort of “awake” [laughter]. And again, that’s in quotes too. Longer, it seems like. I mean, it seems like these… Well, not just the plants, a lot of the pests as well seem to have an opportunity to proliferate over a longer period of time each year and benefit from this additional time, this additional sort of growing season, that we’ve been experiencing.
Daniel: Absolutely. One of the competitive strategies for many of the non-native invasive plants is that they’re leafing out earlier than native flora. They’re holding onto their foliage later into the year. Our lowest temperature for nearby weather station last year was 3 degrees, so several degrees warmer than we would expect for upstate New York. It was amazing this spring, seeing the honeysuckle and the buckthorns, how early they leafed out. So yeah, that longer season is, that’s just more fruit for them at the end of the year, more energy for them to keep spreading around.
Margaret: Right. I think in the policy you have… One of the statements was, and I’ll quote, “As the global climate changes, plants that have not previously been of concern are changing behaviors.” And you just were referring to that a minute ago, and what you’re saying about looking at states nearby, because what’s the next character that’s going to behave in an insidious way where we are as well? I mean, making those sort of predictions and trying to stay ahead of it. I mean, it’s tough, but you’re saying looking at some of these state lists and the lists from the adjacent states, and those are pretty easy to find on Google if we say “invasive species, New York state,” “invasive species, Maryland,” “invasive species, Arizona,” whatever it is, each state has this type of list. So that’s one bit of homework we can do to get acquainted with the thinking.
What are the areas… I mean, with you guys, your protecting your collections is one thing. The garden has the botanic gardens and arboretum and so forth, you’re protecting your collections. But in the context of other environments, not a public garden. I mean, I feel like the property line where different… I think of it as ecotone or where different habitats come together, like the woodland edge, I think of these areas where most of the trouble seems to be happening [laughter]. Do you know what I mean?
Daniel: Yeah.
Margaret: Just like that roadside example I used earlier. It’s like, where should we be looking and where should we be focusing a lot of our attention and so forth?
Daniel: Yeah, that brings up an interesting point. You’re talking about our botanic garden. We have an arboretum, botanic garden and natural areas.
Frequently it is different staff working in those places. So as we’re curating collections in the arboretum, our neighbors are the natural area staff, and we’re excited about this brand new plant that has come in, and they are terrified that it’s potentially going to get bird-vectored right over into their natural area across the street. So I think for somebody at home, it’s important to be a good neighbor, firstly. Think about what you’re bringing into your garden, staying present, paying attention to what’s already there. Part of our policy has a monitoring capacity. And over the last few years, we’ve seen things like Koelreuteria, golden rain tree, that has exploded in invasive capacity.
The Japanese katsura, even katsura is starting to seed in. We’re finding tall, happy individuals in our natural areas, either coming from campus or from botanic collections. So yeah, keeping an eye on what you have. And as you suggested, the best places to look are frequently those hedgerows. Many of these invasive trees, especially, they’re going to be wind-spread or bird-vectored, and those birds, they’ll come to the tree, they’ll eat the fruit, and they head right back to the hedgerow. And as we’ve seen in the arboretum, it’s frequently the hedgerows heading towards the water. So that’s where we monitor for things leaving the collection.
Margaret: They’ve had a little snack, and now they’re on their way to have a drink.
Daniel: Yeah, right? And then grab that waterway and spread those trees right down river. So yeah, it can be concerning.
Margaret: Yeah, I mean, Japanese barberry, which is I think on many states’ invasive lists; multiflora rose, of course; Oriental bittersweet: I mean, these are some of the things when you just described that, I’m thinking that’s what I see at the base of certain trees that birds like to… frugivorous, fruit-eating birds like to sit in and [laughter] then poop out the remains of their meal and so forth. That’s where I’m finding a lot of these seedlings at the base beneath a particularly well-placed branch that they might like to be perched in. And that’s where I’m weeding this stuff out over and over and over and over and over again. But yeah, I think… And when I look along the roadsides I mean, you see just trees just strangled by things like the Oriental bittersweet, for example. [Above, the characteristic orange roots of bittersweet.]
Daniel: It’s heartbreaking.
Margaret: Yeah. That’s one that I think is in the Northeast, I don’t know about elsewhere, but in the Northeast has made a huge impact in the last couple decades, destructive impact.
So we’re thinking about that. And there can be wind-spread, there can be bird-spread. There’s also herbaceous things that just spread [laughter] like mugwort or some that spread underground, like mugwort, the Artemisia vulgaris, or whatever. And then I mentioned earlier, garlic mustard, for instance, that has lots of strategies, but it is a prolific seeder; one of its strategies for succeeding as a weed or invasive is it’s a prolific seeder. What about those? I mean, I know you’re an arboriculture person, but are you also dealing with the herbaceous weed layer, the invasive layer?
Daniel: Oh, constantly. Constantly. We want a healthy, native, intact forest, and currently in that forest is choked with buckthorn and honeysuckle and where it isn’t, there’s maybe a carpet of periwinkle or other kind of ground-spreading things that maybe they’re allelopathic, secreting these chemicals that make establishment of other plants challenging. So yes, absolutely, we do have to manage those spaces.
You mentioned garlic mustard, and again, this isn’t my area of expertise, but what I understand, there’s a researcher at Cornell, Dr. Berndt Blossey, and some of what he suggested is that plant… this is a very simple summary, but it is so allelopathic that as garlic mustard completely overtakes an area, it can actually choke itself out, and you’ll see succession of other things in those places. It’s been a minute since I read that paper, but yeah, that’s one plant that the more we learned, we’re doing less control for garlic mustard, and we’re instead shifting towards pale swallowwort, for instance.
Margaret: So there’s hope [laughter]. You’re telling me there’s hope.
Daniel: Yeah. They’re so aggressive. They’re taking care of themselves. So yeah, a couple victories here and there.
Margaret: They shoot themselves in the foot, huh?
Daniel: Right.
Margaret: O.K. Here stiltgrass has made its first appearance in my area and sort of the Berkshire Mountain area, some of the foothill areas where New York and Massachusetts join. Stiltgrass [above, photo by Dr. S. Luke Flory] is getting an edge the last few years. I mean, there’s just so many possibilities. It’s pretty overwhelming.
But with the woody things, back to the woody things, so I mentioned seedlings, so there’s a lot of privet and along the roadsides around here that comes from old houses that used to be here however long ago and had privet hedges and so forth. And so there’s privet, a supply of privet that makes more seeds every year, and the birds move those around. Again, the barberry, the honeysuckles, the bittersweet, the multiflora rose. So I go after all those. But let’s say you have larger than a seedling, something larger than a seedling. What’s your favorite way to… I mean, it’s a lot of digging to get them out. Do you have any tricks for woody plant removal, woody invasive removal, a favorite tool or anything like that?
Daniel: Yeah, yeah. There’s a couple things that I really like. I can’t speak for the rest of the team but for anything that I cannot stand over and tear out the ground with my own two hands, I will first turn toward a Weed Wrench. There’s another one we have called a Pullerbear, it’s a clamp-like device that you can clamp around the base of the woody plant, and there’s a fulcrum on the back of it. And using that mechanical advantage, you can really easily tear some of these plants right out of the ground. You’re saving on using herbicide. You’re not turning over a big mass of this soil and releasing that seed bank as if you would’ve been using a mattock or something to excavate. I just think they’re great tools.
Margaret: So the Pullerbear, which is a kind of a weed wrench type of-
Daniel: Yeah. I wonder if they still make it. I was on the Weed Wrench website-
Margaret: Yeah, they do.
Daniel: O.K. Awesome.
Margaret: Pullerbear has a website, I know that. That’s interesting because a number of my friends in recent years have invested in these sort of Weed Wrench types of things, because it makes it possible for… Well, it saves a lot of time, as you say, it does less soil disturbance than digging, digging, digging, digging, digging.
Daniel: Is it one of those tools that, as your friends got it, they feel like they have to talk about it with everybody around them because that’s kind of-
Margaret: Yeah, I’m afraid so. I’ve been subjected to a lot of Weed Wrench talk [laughter]. How interesting. But I’m glad to hear that you also find it helpful. I think I’m going to have to invest just for me, for managing along the roadside, the property edge and things like that. I feel like it would be really helpful.
And you just said seed bank, and that’s another big issue, not just, if we dig around and we upturn things, we expose more weed seeds that are in the seed bank in the ground to light. And then they can germinate in favorable conditions for germination, and we end up with more weeds of different kinds.
But the fact that this is never really a one-and-done kind of a situation when we’re working with either weeds or invasives. The times I’ve made the worst mistakes, I’ve put in the effort to try to get rid of something, but then I’ve not left the area… I haven’t waited and gone back a second year and maybe even a third year and cleared it again and again and again. Do you know what I mean? I’ve been in a hurry to, “Oh, O.K., now I can put something in there. Now I can replant.” Right?
Daniel: Yeah. I mean, replanting is important, but just like birds and winds, wind is spreading these seeds, there’s plenty of seed that falls right below the plant. So when we tear it out of the ground or just remove it, that’s a lot of sun getting to the ground now that can help initiate that seed bank. So all of those same invasive plants are pushing up seedlings the next year. So as you suggested, absolutely a revisit, and getting anything that was missed or new seedlings. I try to have some kind of mowing regimen that I’m doing after I clear a large area of the woodies. I want to be able to maintain it with mowing, maybe like just a once-a-year mowing while I’m waiting for my caged trees to get large enough to hold some of that sun on their own.
Margaret: Oh, so you’ve put in the replacement trees, but you want to be able to maintain the ground around them in a way to knock back anything that emerges other than-
Daniel: Yeah. The root zone right around the tree. I have my caging around it for deer protection, the first thing to wipe out the new trees. I can maintain that tree root zone by hand, but if there’s several hundred square feet of area around it, I would rather be able to mow that once a year, and just to ensure that a succession back to an invasive forest isn’t going to happen.
Margaret: Right, right. And with removal of herbaceous invasives, I mean, this is again where I’ve really blown it. I’ve gone through the tedium of taking out things with rhizomatous kind of underground behavior. And then I have replanted too soon, as opposed to wait another… either tarp it or something, or wait until the next year, let whatever comes up, do another removal and another removal after that. You know what I mean? It’s not immediate. The clearing, it looks cleaned up, but it’s not. Right? It’s not.
Daniel: Yeah. Yeah. Taking a little bit of time, I have had several heartbreaks where I’ve cleared an area of invasives, and then the bindweed takes over my tree cages and my new trees. Or the things that I didn’t see on the site when I did the first clearing. So yeah, taking your time, this is a several-years’ process.
Margaret: Right. So I wanted to just ask, you mentioned herbicides briefly close to the beginning of our conversation, and I know people are… I’m an organic gardener, and I don’t want to use herbicides. But I know that conservation organizations use them in the name of the greater good to care for, to clean up and then care for, to be able to replant and restore plant communities that have been overtaken by this. So are they sometimes called for, and if so, what’s the way that they should be used? Because we’re not talking about broadcasts, spraying hundreds of gallons over a large area. Right? I mean, we’re-
Daniel: Yeah, right. This is a really tricky question for institutions and agencies to answer. I can answer this personally, for me. There’s a couple of goals, ideas around ethics. So do the least amount of harm to a site. If you can mechanically remove plants, then do it. If the scale is such that there’s some need for herbicide, do it sparingly. One of the ways that we accomplish that is a cut-and-paint method. So let’s take a honeysuckle, it’s occupied… It’s 8 feet tall, holding a lot of sun and soil space.
Removing that at the base with a chainsaw cut, taking the woody material and maybe future seeds offsite, and then painting herbicide around just the cambium, at the base of the ground. It helps us prevent regeneration from that incredibly established root, and we can start developing the site right away.
Another aspect of the ethics around it is to be educated on what you are doing. I’ve had people call and say, “I want X or Y chemical because I just want to kill this plant.” And it’s important to know the labels and to understand how are you potentially compromising your safety or your pets or the people around you if you’re not reading that label. So really be aware of what you are doing.
Margaret: Right. I mean, like certain types, anywhere near water, for instance, are incredibly lethal. They can do additional harm to many, many, many creatures, for example. And people don’t, as you point out, read the labels. But you’re talking about a cut-and-paint kind of using a dauber or whatever to just get a little bit of that chemical onto the base of the removed plant so that it doesn’t regenerate. I see.
Daniel: Yeah. And doing it at the correct time of year, too. Many of these woody plants, they are going to begin translocating as fall comes. They’re moving energy from the tips of their branches down into their roots. And so if we can time a cut-and-paint with that translocation, we’re going to get really good uptake and we’re not going to have to do repeat applications. Conversely, if you try to do that in the spring, when all the water juice energies go in the other direction, it has a good chance to push the herbicide out of the cut you’ve just put it on. So yeah, good timing.
Margaret: O.K. All in the name of minimizing the use. So just in the last minute or so, are you on any particular campaigns at the moment [laughter] against any particular invasives? Is there anything that’s really you’re on the warpath against at the moment there? I mean, as I’ve said three times or so already, bittersweet, bittersweet, bittersweet, that’s the one here that just won’t relent, so.
Daniel: Yeah, it’s an awesome question because I didn’t think that I was, until you asked the question [laughter], and then there was something that immediately came to mind. So apparently I am. This season I’m battling Acer ginnala [above, photo by Daniel Weitoish]. It’s looking lovely right now. We’ve got nice fruit. The foliage is starting to red up. You also hear it as Acer tataricum subspecies ginnala. This is a common street tree. You still see this street tree in many plant catalogs, but it is powerfully invasive when it moves into an open space. So there are a couple field areas where I’ve been brush mowing to just get access to these trees so that we can begin controlling them this fall.
Margaret: Well, I hope that one isn’t coming to visit me anytime soon, thank you very much [laughter].
Daniel: I have some seed if you’d like. We could send it out.
Margaret: No, no, don’t be so generous.
Daniel: O.K. Cheers.
Margaret: Oh, Daniel, Daniel, Daniel, thank you very much for bringing this up—that we need a strategy, and we need to do a little homework first and know what we’re targeting and when and how. So I appreciate it. It’s nice to speak to you, and I’ll talk to you again soon, I hope.
Daniel: Always a pleasure. Until then, good luck with your day.
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MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its 15th year in March 2024. 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 Sept. 30, 2024 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).
