managing stiltgrass and other invasives, with angela sirois-pitel

IF YOU THINK that managing invasive plants in a garden is a challenge, imagine that on a larger scale—a much larger scale, like Angela Sirois-Pitel faces in the name of supporting native habitat on Nature Conservancy land.

From barberries and multiflora rose bushes to the nasty annual grass called stiltgrass (above), Angela and her team have faced them all.

Angela is a conservation biologist who serves as stewardship manager for The Nature Conservancy in the Southern Berkshires of Massachusetts. Her role there ranges from tackling invasive species, to helping save endangered bog turtles.

She talked about how she prioritizes what to work on, and how she looks for clues to how to manage these unwanted plants—clues in their biology, perhaps. We discussed the hard choice whether to use chemical controls, and when it is made and how, and we talked about tackling stiltgrass, too, and even surprising garden-escapees like Primula japonica on conserved land.

Read along as you listen to the July 11, 2022 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 or Stitcher (and browse my archive of podcasts here). (Photo of Angela Sirois-Pitel, below, from The Nature Conservancy. Detail of stiltgrass or Microstegium vimineum, above; photo by Dr. S. Luke Flory.)

stiltgrass and other invasives, with angela sirois-pitel



Margaret Roach: Hi, Angela. We recently collaborated on a “New York Times” garden column about stiltgrass. Oh my. And we’ll get to that. But first I just wanted a little context to know a little bit more about the work you do, because it’s not just plants. It’s habitats. It’s like I just was mentioning, bog turtles, for instance, is one thing, so tell us a little bit about the breadth of the work.

Angela Sirois-Pitel: Yeah. So, the work involved in stewarding conservation lands or preserves is very diverse. I think that’s what I love most about the job. Really, my passion for conservation and wildlife is really what brought me, well, to stay in the Berkshires, that’s where I grew up. But I really enjoy just the diversity of things.

I got into some of this work with my excitement for turtle conservation and bog turtle conservation in particular. But I learned very quickly that when you want to work with wildlife, so much is about the habitat and the plants and the water and the bigger systems. They’re fun, great, cute little creatures to study and protect, but it really comes down to the plants so much. So, I quickly realized how much I had to improve my botanical skills and learn a lot of native species and invasive species and understand how all of the plants work together to support the wildlife that we’re so attached to.

Margaret: Yes. So, words like “weed” versus “invasive.” Tell us a little bit about, as a person involved in this kind of conservation work, how do you regard these words? When you identify an invasive, what are you thinking about? Because a lot of gardeners, we say “weeds,” but then some of them are technically also regarded as invasive plants on a larger scale and so forth.

Angela: Yeah. So, I would say invasive plant management is like weeding on an ecological scale [laughter]. A much bigger garden. [Above, the Drury Preserve in Sheffield, MA; photo by The Nature Conservancy.]

So, I think that’s the way I generally try to frame it and that invasive plants are plants that sometimes they can be native, and often the ones that we’re working with are non-native, meaning they don’t originate in the Northeast, where we’re based or wherever you happen to be managing land. So, invasive is that these are really super-competitor plants. They have a lot of unique reproductive strategies. They’re highly competitive. They’re very aggressive and vigorous.

So, they are actually pretty impressive plants, but when they get a foothold where you don’t really want them, that’s when they become invasive. And they really push out, often pushing out native vegetation or just changing the structure. So, where you maybe had an open understory under a forest, now you have a dense shrub layer. So, those kind of changes that you see similar to in a garden when a plant growing somewhere that you don’t want it, because you want something else to be growing there.

Margaret: And this invader can not only take up territory, but it can prevent the next generation of the desired plants from successfully going through their seedling and growth process. I mean, there’s so many repercussions. Then, as you said at the very beginning, the creatures, the animals from insects on up, who depend on that food web, that the plants are super important in it, uh-oh, that’s all interrupted.

Angela: Exactly. The study of invasive plants, the invasive-plant ecology field has really grown tremendously in the last 10 to 15 years that I’ve been working in conservation. It’s just amazing what they can understand for the impacts of these plants now, and everything from changing the soil and even the synergies between some of the invasive plants together or the invasive insects, like the jumping worm and its connectedness to some of the other invasive plants. Or the barberries’ connectedness to increased tick populations, and how different plants can change fire regimes. They really have these cascading impacts throughout the system.

Margaret: Yes. So, is the first step then… I mean, I’m fascinated by “weeds,” by the power of them, which you just kind of referred to, and their strategies. So, my first question when I encounter a plant I don’t know that seems to be behaving in a weedy way, is I want to know its biology, its life history, how it operates and what’s its special superpower. You know what I mean?

Like field bindweed has roots that go down 20 feet or more, and the much-loathed garlic mustard, with so many tactics: a profuse amount of seeds as well as this allelopathic chemical warfare it conducts in the soil. Oh my goodness. They have so many powers. So, do you, before you start tackling something, getting to know it, is that what you do? Do you do that research?

Angela: Yeah, absolutely. So, when I first encountered stiltgrass, it was lots and lots of research. O.K., what is this plant? Is it an annual? Is it a perennial? What kind of habitat is it going to thrive in? What are its patterns for expansion? Where do I need to really focus in on to be watching for it to show up or for it to spread into?

So, that’s always the first step, especially with some of these new invasives that aren’t part of our normal repertoire that we’ve been managing for the last couple decades. But just like every plant has… A lot of these invasive plants do have a superpower. They usually have some weakness, there’s something… So, that’s where there’s a very huge intersection between practitioners like myself and the research field working together collaboratively to understand how do we find the weak points for these species? And whether that’s a time-of-year control, a specific control method, a biocontrol agent, any of those is a constant, ongoing battle.

Margaret: Right. So, before we get to stiltgrass, I live in the adjacent area to where you work. The Southern Berkshires of Massachusetts is where the northwest bit of Connecticut and part of New York State come together, and I’m on the New York side. So, I know the area that you’re working in, and the kinds of invasives that I see along the roadsides here and experience in my own garden. Things, as I said in the introduction, as big as barberry shrubs and multiflora rose and privet and these woody invaders. In these large properties that you’re managing, you must have a lot of issues with those big guys, yes?

Angela: Yes, absolutely. I think in our specific region, we have two factors that have really contributed to the explosion of invasives that we have here. One is our agricultural history and current experience with most of our forests, which were pastures 70 to 100 years ago. Our forests were cleared for the coal industry, so these are younger forests. So, a lot of these forests are young, and they’ve had more recent disturbance. So, all of those disturbance events have allowed the invasives to definitely get more of a foothold, especially around the edges. [Above, habitat management at one Berkshires project; photo by The Nature Conservancy.]

Then also the Berkshire Mountains have a lot of calcium deposits in the geology here. So, the rich soils that we have surrounding the Berkshire-Taconic range also make it really nutrient-rich for invasives, as well as a lot of the rare native plants, too. That’s why they’re found here as well.

So, yeah, those two factors [laughter], they work together to create a lot of invasives.

So, when I first started, it was pretty daunting to come into managing so many thousands of acres of land that just were already well-invaded. It really was depressing at first. But I think the way that I’ve managed it, and I teach my seasonals and interns and all the people that come and work for us over the summertimes, is just like: We have to be really focused and prioritized and strategic about where we do our invasive-control work, because we can’t do it everywhere.

They are here, those barberry and multiflora rose, bittersweet. They’re here to stay. Eradication is not an option for those species. It’s more about, where can we not have them take over?

Those are going to be in our areas that are most vulnerable and have our most rare species, that’s where we focus a lot of our work.

Then, on the new invaders, my colleague likes to say like, “What if you found the firstever barberry plant or bittersweet vine, wouldn’t you want to make sure you didn’t let it get a foothold in your yard?”

Margaret: Right. You told me, when we spoke for the “New York Times” piece, for instance, I mean, a plant I hadn’t thought of as a potential invasive Primula japonica, I think it was, one of the primulas that a lot of gardeners like, and you’re encountering something, “Oh, it’s so pretty.” But guess what? In the right spot, it’s out of control. So tell us about something like that, because-

Angela: Yeah, that was a new one for us that my colleague, Rene Wendell, he started seeing it everywhere. “What is this flower?” So, it’s like always, we’re pretty used to the stuff we see. So, when he, all of a sudden, starts seeing something a little bit more common than we’re used to, we really jumped on it. And it likely did escape from a lot of gardens, because it is this beautiful flowering plant that does well in wet areas, which is now invading a lot of these rare open wetlands that we have a lot of rare sedges, or rushes, or a lot of these other delicate plants that just wouldn’t be able to compete.

Margaret: To stand up to it. Yeah.

Angela: Exactly. So, it’s so aggressive. I mean, we used to… When I first started, we did a lot of battling against purple loosestrife, and I’ve been so grateful of the success of that biocontrol agent, because now purple loosestrife is just not really on my radar as a species I have to worry about anymore.

Margaret: So, by saying that you mean a biologic control, something that research scientists have found out about that’s a natural enemy of the plant, is that-

Angela: Correct. Yeah. So, there’s a couple species of beetle that they researched extensively. We have really strict protocols now for researching and releasing any bugs that basically don’t belong here, to make sure that they are really highly specific to the invasive plant that we want to control and they’re not going to jump species. Because there are some native loosestrife species. [Above, a wetland invaded with purple loosestrife; Wikimedia Commons image.]

So, this had been tested really aggressively for many years before it’s been released. I would say it’s probably maybe 10 to 15 years ago that I first started to see it show up in the Berkshires, and they’re these little beetles and they just do a fantastic job. They munch on that purple loosestrife. We still have it, but we don’t get those dense, thick stands of it anymore. I remember seeing calendars with fields of purple loosestrife [laughter] and now you don’t see those anymore.

Margaret: No, no. That’s speaking of plants that have a weedy, a strategy for being successful, I’ve read that a large purple loosestrife plant can produce 2.7 million seeds per season. And to make matters worse, they float, so they can float downstream and invade a new area pretty easily, which actually, our main topic, that we want to get to now, stiltgrass, I think its seeds can float, too. I believe that’s possible, they can move in water [laughter].

Angela: Yes. I recall that is a correct fact. I mean, we often see it moving along drainage ditches or stream corridors. It definitely moves well in those watery-type environments.

Margaret: So, stiltgrass, do we say Microstegium, is that correct?

Angela: Yeah. Microstegium.

Margaret: Botanical Latin is such a funny one, we all pronounce it differently.

Angela: I know. That’s why I try to steer clear [laughter], because I know I’m just going to botch the names.

Margaret: So, you’ve been tackling it for more than 10 years where you are, but it’s been in this country for a century or so. It’s from various parts of Asia, Japan, China, India, Korea, Malaysia. It probably came in maybe as packing material in crates of porcelain that was imported a century ago.

But now it’s as far-ranging, as wide-ranging as the Florida Panhandle up into new England, out to Michigan, as far West as Missouri. It’s gained a lot of ground, and it’s an annual grass, a warm-season annual grass. So, tell us a little about when you met it and what you were like, oh my goodness [laughter], and what you tried to do.

Angela: Yeah. I had just come back to working for The Nature Conservancy from a little stint in grad school. I was up on at some preserves that we own in Mount Washington, Massachusetts, and saw this grass along the roadside that I was like, “Oh, I don’t really recognize that one.”

And I happened to be out with a botanist who was doing some work and they’re like, “Oh, that’s stiltgrass.” And I was like, “Tell me about this.” Like, “Oh, it’s an invasive, it’s new, but it’s been here for a couple years.” And I was like, “Wait, what?” I was like, “Well, how do we have a new invasives and I didn’t know about it?”

That was my first shock, and my first realization that I just needed to keep me and our team much more apprised to what’s coming up the pipeline from Connecticut and New York that is likely to show up in our neighborhood next.

Then it was pretty quickly that year, when I first found out about it, I started doing some research. And then I had my seasonals, putting together some information. Then we just started to do an extensive mapping, walking the roads at the town of Mount Washington to see how bad it was, and where it was, and inventorying our properties just to first get that first sense of how bad is it. I was pretty upset to find out how bad it was already, this was 2010-ish, that it was already pretty widespread along the roadsides. [Above, a half-weed whacked stretch of stiltgrass along a Berkshires roadside; Angela Sirois-Pitel photo.]

So then, it was trying to get our understanding of control up as quick as possible. We started cutting, doing just the weed whacking along the roadsides. And a lot of trying to… I was just trying to reach out to all the partners I could about like, “Hey, did you know about this new plant, stiltgrass? We have it here and you should be on the lookout for it.”

It really became something I was very focused and driven on for a couple years, and was able to organize a small symposium workshop, I think in 2012, to get a bunch of practitioners together to just educate people about this, as well as thinking about some of the newer plants that people were dealing with.

Margaret: So, this, as I said, is a warm-season annual, so it’s an annual that doesn’t come up super-early, it waits a little while to germinate for more favorable conditions that suit it. So, generally speaking, with an annual weed–I have many annual weeds that have come in on garden plants or whatever from the nursery, Galinsoga, for instance. So, annual weed, O.K., Margaret will say to herself, “I don’t want that to flower and set seed. So, I’m going to try to interrupt that part of the process.” Because annuals sowing seed means more of them next year, more of that annual next year [laughter].

But it didn’t prove to be quite that simple with the stiltgrass to just weed whack it and prevent it from setting seed. That didn’t seem to do the trick. Did it?

Angela: No. Where we have been doing it for years, we saw success in that it wasn’t taking off. It wasn’t aggressively spreading outside of our focal areas. But we weren’t really seeing a reduction in the effort that we would like to see, that we see with other invasive-control projects, where you might hit it really hard year one and year two, and the next couple years, it’s just touch-up, a little bit of work here and there, instead of a week of time, you’re just spending a day or half a day.

We just weren’t seeing those results that we had hoped for with stiltgrass. So, I think, like how your article talked about is as the science evolved, understanding why that cutting alone wasn’t working, that they can have these different options for setting seed and they’re lower on the stem, or self-pollination, these things that our cutting just couldn’t actually handle.

Margaret: Right. Right. So, some plants, probably a lot of plants, have a backup strategy for success if there’s a fire or if whatever happens in nature (nowadays it’s if we mow it down), there’s a backup strategy. And this one has these semi-hidden self-fertilizing, self-pollinating flowers way down below. It has what’s called a mixed mating system, which you and I learned about from the scientists, we collaborated on that “New York Times” story.

So, it’s like, that’s the backup. So, even if we mow it down, there’s some of those hidden seeds down below, and they’re not getting knocked down [laughter]. So, that may be one of the things that makes it succeed. You were mowing when? You were figuring out your timing for when?

Angela: Generally, we were mowing in August, we would try to hit it late August to early September, because we felt that that was tall enough where we could really see it and get a good cutting down to the ground as much as possible. If we were cutting it too early, it’s really easy to miss into July. The plants are just so small, and we could even go back and be like, “Oh, here’s a plant here. There’s a plant there.” That we just couldn’t see, because the roadsides are usually packed with quite a diversity.

Margaret: Sure. So, you said you were keeping it where it was, but you weren’t reducing it really substantially.

So, I think people are really surprised to learn that sometimes conservation organizations, sort of in the name of the greater good, are making this hard decision to turn to chemical control, which none of us wants to do. But sometimes when you see precious habitat, such as all these acres that you steward in your role or other people at The Nature Conservancy do, sometimes that’s what seems to be the better choice. Yes?

Can you talk a little bit about that, because I think that’s what you’ve done with some of the worst situations with the stiltgrass. [Below, faded stiltgrass late in the season, covering a garden.; Ken Druse photo.]

Angela: Yeah, absolutely. So, I mean, yes, we always want to choose the option to control a plant that’s going to have the least amount of damage and the most amount of benefit. So, we have a suite of different tools we’ll use depending on the plant, the species, and where the target community is: Are we on a roadside or are we in a sensitive wetland or in a forest?

So, we have to look, we look at each all of those factors when we decide what’s going to be our best option to start with. In this case, stiltgrass, because it was an annual grass, and because we know that there’s a lot of people who prefer not to use chemicals, that we wanted to start with that as an option.

So, we tried that out for a long time, and we probably could have switched sooner, but we realized we weren’t getting the results. So, that is when we decided to talk with some of our colleagues in the field and other professionals about what if we did use a chemical application, what would be the best option for us available based on using the least amount of chemical and having the most direct impacts and the least amount of negative impacts, all of that? So, that is generally how we’ll go.

You can read labels and you can talk to a lot of different experts to figure out what your best approach is going to be. It’s always such a learning experience with these control techniques, because you start off using the least amount possible. And how good is your control effort going to be with an ounce?

So, I mean, when we’re looking at our chemical applications, we’re using extremely small amounts, we’re talking about a couple ounces in a several-gallon tank. So, it’s very diluted and watered-down. They’re chemicals that we’re very comfortable and very safe working with and have been successfully for years. I have the benefit of seeing how wonderful these control techniques can work in sensitive habitats, where we have been using them to do target applications for decades, and they’re thriving with all of these rare species.

Margaret: That are coming back. Right.

Angela: Exactly. Yes. So, there’s not a dead zone.

Margaret: No, no, no. Right. So, we have just a couple minutes left. You made such a good point in the “New York Times” story, which was you don’t just go to the store and buy the familiar brand-name thing at the big box store and go on a mad spraying campaign at your home.

You hire a consultant who has an ecological orientation, who knows all this stuff, and have them evaluate your situation and help you identify the right product. I think that’s super-important, because it’s not one size fits all, and people are random spraying when it’s not required, and they’re using the wrong things and so forth in the wrong habitats.

But I just want to leave a little time to say some of the good news [laughter], because you’re not always fighting invasives, that people can come and visit these incredible properties. I was just looking at even where we are, in the Connecticut-New York-Massachusetts area, there’s so many incredible preserves and so forth that are managed by The Nature Conservancy. So, is visitorship, is that a lot of the mission, too?

Angela: Yeah. So, I mean, The Nature Conservancy is somewhat of a decentralized organization, in that each state has its own chapter and kind of has some flexibility to decide and focus what they want to focus on. So, some states do have more of a focus on recreation and trails and visitor experiences than others do. So, you have a mix of experiences. In Western Massachusetts, we only have one or two trails that we really focus our time on, because we spend a lot of our time managing rare species habitat. But yeah, people can visit nature.org and look for places we protect.

So, I think that the connectedness between nature and people is very much part of the mission of The Nature Conservancy, but our roots are definitely in the science and protecting species and places and habitat. We haven’t lost that. That’s one of the great things about The Nature Conservancy, which is trying to bring all of those elements together in conservation.

There’s so many resources for people on the internet if they want to learn more about invasives or if they… Checking with their extension programs for their major universities, they usually have a lot of great resources. A lot of the states have lists of invasive plants.

Margaret: So, we’re out of time, but there’s so much more to talk about. Of course, I’m over here right near me as my first experience with stiltgrass. And I’m starting with what you advised me in our “New York Times” story, too. I’m starting with the hand-pulling, because it’s a small spot and see what happens. Then looking to maybe mow or weed whack it late in the summer. So, we’ll see. But I appreciate your making time today. Thank you so much.

Angela: Yes. Thank you. I wish you the best of luck with that stiltgrass. You can succeed if you stay with it.

Margaret: It’s small. So, I’m alerted to it, thanks to you.

more help with weeds and invasives

prefer the podcast version of the show?

MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its 13th year in March 2022. In 2016, the show won three silver medals for excellence from the Garden Writers Association. 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 July 11, 2022 show using the player near the top of this transcript. You can subscribe to all future editions on iTunes/Apple Podcasts or Spotify or Stitcher (and browse my archive of podcasts here).

  1. Mary says:

    Good to know there’s some success in controlling stiltgrass– and it’s possible to eliminate altogether if we keep at it.

  2. raymond says:

    am experimenting with corn gluten as a pre emergent against stilt grass. in other areas wood chip mulch over amazon’s cardboard. another area a section of heavily planted buckwheat… and constant weed whacking or yanking it elsewhere.

    corn gluten is expensive…..

    slowly but surely.

  3. Bridget says:

    I have recently bought six acres and have discovered just how many invasives there are in that land! We have a stand of Japanese knotweed, tons of floribunda rose (it’s starting to gain ground in the woods) so much garlic mustard and just recently learned that Dame’s Rocket is an invasive and not the woodland phlox I thought it was! We also have the beautiful and invasive yellow flag iris. That is really tenacious! I am working on removing what I can and not getting overwhelmed.

  4. Carol says:

    After reading your interview with Angela discussing invasives, in particular stiltgrass, I feel utterly defeated. I’ve fought rabbits, chipmunks, ground hogs and deer. I lost those battles. But to be beaten by a weed! Nonetheless, while I wait for a beetle that loves stiltgrass to come along I will continue weeding it from my 60 foot perennial garden. If there is a plus it’s that stiltgrass pulls out easily.

  5. Angela Cipriani says:

    I really appreciate this article. Living in Wellsburg WV I am dealing with bamboo that is invading my neighbors property. Can you help? I have copious amounts of bamboo on my property as well but it is in an area behind a garage. Hoping you can direct me to resources who can help eliminate the growth of bamboo on my neighbors property. Thank you so much

    1. margaret says:

      The University of MD has a guide to the different types of bamboo and the possible control methods. This is a very big multi-year project…and will also require the creation of an underground barrier between you and the source, or the situation will simply repeat itself. The guide.

  6. Jacquelyn says:

    One quibble, although it’s an important point. There is a legal definition of “invasive” as per the US Government, which requires that the plant in question be from OUTSIDE the habitat under consideration. Thus, NATIVE plants cannot, by definition be called “invasive.” Native plants can be aggressive, but it is incorrect usage of the term to call them invasive. I’m particularly annoyed when professionals in botany, horticulture, ecology, etc. use this term incorrectly.

  7. Margaret Manzke says:

    So far I think I’m stilt-grass free, but I’ll keep my eyes wide open! I really love the types of programs that focus on teaching more on book selling. Thank you!

  8. The advantages come from the lack of checks and balances that just don’t exist for these newcomers. Having evolved alongside native plants, insect and mammal herbivores either don’t recognize plants from other continents as a food source, or they find them unpalatable or can even be harmed by them. Stiltgrass is especially well adapted to low-light conditions. It is a threat to native plants and natural habitats in open-to-shady and moist-to-dry locations.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.