fighting weeds: an all-season approach, with mt. cuba’s duncan himmelman

‘THERE’S ALWAYS a weed out there, no matter when.” That’s what today’s guest, education manager Duncan Himmelman of Mt. Cuba native plant center in Delaware, said the other day on the phone. And that means now, even as we approach the quiet season, weeds are lurking, and we need to know when and how to target our efforts to control them.

Duncan Himmelman, currently the education manager at Mt. Cuba Center, a renown native plant garden and research site, earned his doctorate in ornamental horticulture at Cornell before teaching college for 24 years. He’s also managed a large private estate and designed gardens for private clients, so he knows from weeds and weeding.

We talked about weed ID, and why it matters to know a weed’s name and life cycle (that’s chickweed, Stellaria media, a winter annual, above; photo from Wikimedia). We also discussed removal tactics and why skipping the chemicals makes the best sense, and more.

Read along as you listen to the November 2, 2020 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).

mt. cuba’s 6 new virtual classes on demand

A COURSE on weeds is one of a half-dozen on-demand recorded courses Mt. Cuba is currently offering to the public (more on that below, in the transcript–or browse them now). Enter to win a ticket (on me!) to the course of your choice by commenting in the box at the very bottom of this page.


all-season weed control, with duncan himmelman



Margaret: Seen any weeds yet this morning, Duncan? [Laughter.]

Duncan: Well, no, I’m averting my eyes. No, they are everywhere as you know.

Margaret: So we should say to everybody that what got me alerted to your weed course—well, what got me alerted to the fact that you were into weeds and teaching a course on weeds actually, which is one of the recorded courses at Mt. Cuba that people can buy on demand—was that I got a press release about this whole series of virtual classes.

The classes are a great resource; you guys have great in-person events when the world allows and so forth as well. But I wondered if you could just tell us about this offering of virtual classes on demand.

Duncan: Yeah. As you know, when COVID struck, we hadn’t done any online education. And so we had to quickly pivot to transition our classes that were going to be live and in-person to online. And as a result of that, we record each class and the recordings have been filed and some of them have been kept for resale, as you mentioned, as recordings in our classes-on-demand option. And there are six of them: One is on soils. One is on the best native plants for containers. There’s one on monarch butterflies, a butterfly garden, and groundcovers, and weeds.

Margaret: Yes, it was great because you guys, I mean, I always love talking to anyone on the staff there and I learn so much, so I was so excited to see and to be able to share.

So as I started at getting down to business, because we got to do some weeding, Duncan. As I said at the start quoting you, “There’s always a weed out there, no matter when.” So what’s a weed? Let’s just quickly start there before we get to tactics.

Duncan: Yeah. Generally it’s defined as something that is growing in a place where it’s not wanted; that technically it’s out of place. In that sense, it could be anything from common dandelions, as we all well know, up to even a small shrub that is planted in the wrong location on your property. Not that people would consider that a weed in the sense that we typically do, but yeah, it’s essentially a plant that is growing where it really isn’t wanted.

Margaret: Right. And the problem with these is that they compete against the wanted plants, right?

Duncan: Yeah. I mean, all plants are taking up water and nutrients in the soil, and they can be limiting. So weeds are taking those same water and nutrients for themselves, at the expense of the plants that you really want in your garden. So yeah, competing for resources is one of the ways that weeds are successful.

Clearweedm or Pilea pumilaMargaret: So just to be clear, I mean, Mt. Cuba is this famous and wonderful native plant center—a place of education, a place of research, etc. Not all weeds are aliens, i.e., non-native. There are “native weeds,” quote unquote, and there are sometimes native plants that are in the wrong place; they’re a little too ambitious.

So it’s not “that’s an alien and that makes it a weed,” right? I mean, we have clearweed, Pilea pumila [above], I think it’s called, jewelweed, and pokeweed. They even have weed in their common names, some traditional native plants that have great wildlife value, too, but maybe the gardener sees them as a weed in certain spots.

Duncan: Yeah. The way our gardeners manage the landscape is to edit out plants that are going to be, as you said, too rambunctious or too aggressive. So we do have pokeweed on the property and it can be very… it can seed in fairly prolifically and it is a perennial and it’s difficult to get rid of. But they do leave the odd one here and there because it is native and they do provide ecological services that are pretty important. So yeah, not all weeds in the sense that you would define them are alien.

Margaret: Yeah. I don’t know if you are a field-guide nerd like I am, but I am. I have a whole cabinet full of field guides, and a number of them are about weeds. And I love knowing the names of my weeds. I love being able to ID my weeds. In the class on weeds, do you encourage people to get to know them by their proper names?

Duncan: Yes. I mean, I come from a long line of many years of teaching, and it’s always best to teach the scientific name of the plant because that is the bonafide name that follows that plant oh, from country to country, from state to state, etc. So yeah, I’m always very prone to using the scientific name when I talk about weeds and other plants in general. So I will sometimes stumble over the common names because I’m not as used to using them.

But we give both of them to our students and we emphasize the fact that if they can actually learn the scientific name of these plants then they can do their research online in a more effective manner.

Margaret: And I’m going to give some links, some of which you shared with me and some that I use myself, to some online weed ID resources from various universities around the country and so forth that people may find useful for learning some basic ID, and also to some guide books and so forth [links at bottom of page].

So that gets me thinking, speaking of the proper name of chickweed, Stellaria media is how I say it [photo top of page]; I don’t know if that’s right. But I was outside this week in my garden and at the edge of one large bed, there were lots of little chickweed plants. Now, what the heck is it doing as winter approaches, seeming to start to grow? [Laughter.]

Duncan: Yeah, I know you’re thinking of that weed season is over.

Margaret: What’s going on?

Duncan: It’s heading into winter and we don’t have to worry about weeds anymore, but as you know, weeds don’t take any time off. With Stellaria, chickweed, what you’re seeing is little rosettes or little seedlings coming up at this time of year. And the reason they’re coming up is because it’s cool and moist, and these are the ideal kinds of germinating conditions for what we call winter annuals.

And it kind of gets back to your earlier comment about why are they coming up at this time of year; we’re heading into winter. So they will germinate now. And there’s a number of these winter annuals that will show their heads at this moment and they will remain green in this vegetative state throughout this part of the season, go dormant, and then in early, early spring, when winter is just subsiding, they’ll start to flower. So these winter annuals, like chickweed, will bloom early in the spring. And I think people have seen that, that, wow, here’s this plant, it’s very early, it’s like April, what’s going on, it’s already flowering? But it’s been sitting there, dormant, in the leafy stage over the winter. So yeah, that’s what’s going on. There’s a whole bunch of these things called winter annuals.

Margaret: Yeah. So it seems to me that it’s going to be a lot easier to reckon with those little guys than who they’re going to become by April or later, yes? So, I mean, this gives us some tactical hints, knowing the life cycle, knowing that it’s a winter annual, right?

Duncan: Yeah.

Margaret: So is that a strategy then that I should be dealing with this now, not waiting until it gets worse? I mean, that seems obvious, but in some cases it’s not quite as obvious.

Duncan: Yeah. Since it’s an annual, most of the annual weeds that people will encounter should be eradicated, removed, when they first see them coming up as seedlings or young plants. And it’s no different right now with the winter annuals. You see them coming up and if you recognize them as chickweed, for example, then you should really get rid of them now, before they go into the flowering phase.

Some of the kinds of weed-control methods that we always encourage here at Mt. Cuba Center is to go in and just grub them out, using physical labor, and get them early on. They’re annuals; if you pull them out of the ground and expose the roots, they’ll dry up and die. So yeah, your gut feeling is, “O.K., yeah, I see these little plants. It’s time to get out there and start weeding them out.” So you can hoe them out or whatever you wish to do.

Oriental bittersweet seedling in a sea of ground ivy or GlechomaMargaret: At the edge of another bed, my nemesis is one of my many nemeses… And it’s not really a horrible weed but it’s irritating because it loves to crawl in from the edges of the lawn, into beds, into ornamental beds. And it’s Glechoma hederacea, what do they call it, creeping Charlie or something, ground ivy, is that what they call it? That’s another one that right now it’s like it has advanced again. It seems to have enjoyed some recent rains and who knows what this fall. [Above, ground ivy in the foreground with an Oriental bittersweet seedling and even one of garlic mustard in the middle.]

Duncan: Yeah. Glechoma hederacea is ground ivy. They can call it creeping Charlie. They can call it gill on the ground or gill over the ground as well. And it’s a perennial, yeah, it’s actually a perennial weed that you should really try to eradicate every time you actually see it, try to pull it out and remove it.

It has a fairly dense root system. And it’s the kind of plant that if you can remove it physically manually, the more you can remove it, the more successful you’ll be at preventing it from spreading, because it does spread over the ground, hence its common name, and it will persist. So yeah, it has taken a little bit of time off. It’s gone a little bit dormant in the dry part of the season, and now it’s coming back. So I would definitely recommend you try to get rid of that as a perennial, and try to get the root system out. That’s the problem with perennials of course, is that their root systems hold onto a lot of stored food and keep these plants going.

Margaret: Yeah. And this is one where it seems to touch down in multiple places, it’s like long little runners and they seem to have rooted in, in multiple places, almost vine-like above the ground.

Duncan: Yeah.

Margaret: It’s great when the soil is moist, it’s very satisfying for a crazy person like me to kind of pull it out gently, slowly, so that you get along the whole long extent of it, so to speak. Yeah. So that’s another one I’m going to be tackling, even though again, we have snow forecast late this week, but so what, I’m going to do that; I’m going to do another pass on that one. And you say that’s a perennial, so that makes sense. And then there’s garlic mustard. [Laughter.]

Duncan: Oh God. Yeah. Garlic mustard [Alliaria petiolata, above]. Oh yes. Well, the good news is you can turn it into pesto at some point, but you don’t want to have that much garlic mustard on your property, do you?

Margaret: No. And isn’t that a biennial? We just mentioned a winter annual. We mentioned a perennial. Is that a biennial? I think, is it?

Duncan: Yeah, it is. It’s a biennial. And what you’re probably seeing now is what they call rosettes or small clumps of the plant that are now coming up. They’ve germinated and they’re growing and they’re probably 3 to 4 inches tall and they’re just a series of leaves. And they have that characteristic kind of pungent odor to the foliage if you crush it.

But yeah, you can get out there and get rid of the plant right now because it’s going to go, again, it is going to go to flower and seed very early in the season, come spring. So the more you can get rid of garlic mustard… It’s a biennial, so at this point when you pull it out, it’ll seem like an annual because the root system is just from this year. So pulling it out now is probably well-advised because A, you’re probably not going to get all of them. You’re going to 90% of them and you may be able to catch the remaining few in the spring. But yeah, get them out now because man, they’re up and flowering and seeding pretty early.

Margaret: And this one has a lot of weedy tactics. It’s successful because it’s a prolific self-sower, because it is allelopathic, I think; I think it exudes a chemical in the surrounding soil area that prevents other plants from getting acclimated, getting started. It’s a tricky one. So really to get it out, as soon as you see it is important, I think, before it does those things.

Duncan: Yeah. And not only that, but it’s very tolerant of shade and sun. And so you’re going to see it all over the place. You’ll think, oh, well it’s shady over there, it won’t invade. But no, it does. It takes the shade. The thing about it is that, of course it’s germinating and growing foliage this time of year, and then in the spring, when there aren’t a lot of leaves on the trees above, they will continue to grow and then quickly go to flower. So, it’s a pretty smart weed.

Margaret: It is very, but it’s not smartweed, which is the common name of something else altogether. [Laughter.]

Duncan: I know. I know. All right. Yes. Smartweed. Although many weeds are smart, there is smartweed. Is that Polygonum, I think? Anyway.

Margaret: Yeah.

Duncan: Yeah. I know. And there’s many polygonums as well, so good god. [Laughter.]

Margaret: Yes. So we’ve just talked a little bit about knowing not only … knowing its proper Latin name, ID’ing it, using some of the tools, we’ll give some links to, or having the right book for your region, a field guide for region. ID’ing it, getting to know it by its proper name, doing a little homework. Is it an annual? Is it a winter annual? Is it a biennial? Is it a perennial? That can get you some of the strategy brewing on how to deal with it.

So let’s talk about control measures once we know that. So you were saying Margaret should get out there and grub out those chickweed seedlings and she should be pulling, carefully pulling, some of the perennials. And biennials like the ones we’ve just talked about, the Glechoma and the garlic mustard. So what are some of the other tactics of weed removal that you teach?

Duncan: Yeah. Well generally when we talk about we control, Mt. Cuba Center tends not to rely on a lot of chemical measures. And so in class, we recommend that people really try to exclude weeds from getting into their yard to begin with. And one of the ways that they can do that is that when they go to the garden centers or nurseries, they can look at the pots and containers that the plants are in, and if any of them have weeds in them, then they shouldn’t choose those plants. And they should be working with nurseries that are fairly diligent about keeping weeds out of their containers.

So we try to exclude weeds, but as you know, weeds… Dandelion seeds are blowing in, weeds are, just coming in, in general from surrounding properties.

Another method that we recommend is to keep your tools as clean as possible. I know it sounds mundane, but like you, I’m out there, I’m weeding, I’m using my gardening tools. And then I think, “Oh, well I’ll just put them down and pick up where I left off tomorrow.” But the more you keep them clean, the better you are going to be able to keep weeds from coming into your … from spreading weeds around per se. Does that make sense?

Margaret: So from seed that’s on it or a little bits of plant or seeds in the soil that’s left in it?

Duncan: It can be both. If you’re digging up some mugwort [Artemisia vulgaris], for example, and pulling it out and collecting mugwort [above], then the shovel you use may have little pieces of mugwort roots on it and you’re just moving the spade to another part of your garden. And you’re going to make a new bed, and suddenly a year or two later, you see mugwort cropping up in the new bed. When you think, where did that come from? So yeah, it can be seeds and it can be little pieces of plants as well.

Margaret: Right. Which is also true when you want to transplant a desirable plant from one bed to another bed. If that bed, the first bed, has some infestation of something you are running the risk of… if you don’t wash off that division that you’re making of that. It might be a hosta, for goodness sake, which is not a weed, but if you don’t wash it off, if there’s an adjacent weed in that bed, you may be bringing it along to the new bed where you put that division, yeah?

Duncan: Yeah. It’s so interesting you mentioned that, because I’ve done exactly that where I’m digging up a plant and I know that plant X, the weed is in amongst those roots because the weed tends to spread around. And I would literally, I will sit there and I will discern what is the weed and what is the root of the desired plant?

Margaret: [Laughter.]

Duncan: And I will tease it all apart. Even if I lose all the soil around the plant I’m transplanting—a herbaceous perennial, for example—I will try to be as meticulous as possible at getting all of that little weed root out of there. And so it sort of means you have to know what the weed root looks like and the root of the desirable plant. So it’s not just the leaves and the flowers of the weed, you have to know something about their roots. But yeah, you have to be very diligent. Good point.

Margaret: Yeah. So those are some preventive things. Grubbing out is one removal area. What about can I exhaust some of these plants, like with repeated some of the ones that are more tenacious? If I can’t get them out completely, what are some other tactics?

Duncan: Yeah. I mean preventing our weeds from going to seed as we mentioned earlier, getting the little seedlings out right off the bat. But with perennial weeds, and you can think of things like plantain [below] or dandelions or what else can I think of?

Margaret: Thistles. Those damn thistles. All those thistles.

Duncan: Thistles, yeah, I know. And thistles have roots from here to eternity, but we’ll try to put that on hold for the moment. With things like perennial weeds of that nature, then you really just have to get at getting the foliage out as often as possible. Remove the foliage as often as possible. Mow it down, pull off the leaves, try to dig it out and remove it.

The more you can remove the foliage, the weaker the plant will become, meaning that the leaves are the source or the site of photosynthesis. And that’s the source of the plant’s food. And without plant food, then the roots are not going to continue to grow.

So if you remove the leaves, remove the source of food for the plant itself, then you will ultimately weaken it. And you have to be very diligent about this, because those leaves are going to come popping back, as you well know. But if you really do keep at it within three or so years, you can probably really, really keep that weed down.

Margaret: Right. So no photosynthesis is eventually no plant, right?

Duncan: Yeah. Exactly.

Margaret: So preventing photosynthesis. Yeah.

Duncan: Right, right, right. And speaking of that as well, we do talk about mulching and mulching is-

Margaret: I was going to say, do you mulch?

Duncan: We do mulch here with leaf mulch. And in one of our gardens, we’re mixing up some of our homemade woody plant-sourced mulch with the leaf mulch. So it’s kind of a half-and-half. It’s a very finely shredded woody material, and the leaves that have been shredded as well. So it’s not like a coarse mulch like you would see at the garden centers for sale.

So kind of is a transition area sort of between a compost, kind of thing.

But yeah, we do mulch, and I think the critical thing about mulching your garden is to use something that’s organic obviously, and to layer it up to 2 to 3 inches. I remember years ago there was research done on what’s the depth of mulch that you should use. And generally they came up with 3 inches of depth because that would prevent weed seeds from germinating, etc. Some people say 2 inches. I’m more inclined to say 3 inches because then, you can leave it there alone. As it decomposes, you have to obviously, renew it. So if I layer on around 3 inches of mulch around my garden plants, then I don’t have to constantly reapply mulch. You know what I mean?

Margaret: Right, right. I just want to quickly just shout out that I get a little nervous, and I shared this with you on the phone the other day, I see the big bottles of acetic acid, so-called horticultural vinegar, in the stores—in the garden centers—and I shudder thinking of everybody spraying them without the right proper protection. Do you ever use those, the acetic acid, the horticultural vinegar at all? And what’s it for, if it is going to be used?

Duncan: We aren’t using the horticultural vinegar, no.

We have had some degree of success using some other products here at the garden. Organic chemicals basically work in the same sense as you would apply to removing the top foliage. So in a perennial situation, you want to remove the leaves. Even with an annual, if you remove the leaves, you’re going to really remove the plant.

So some of the products that we’re using here are basically made from pelargonic acid mixed with fatty acids, or citric acid and clove oil. Another product that we’ve used is one that has active chelated iron. And they basically act as foliage burns. They burn the foliage. And in that sense, they’re probably most effective on young plants. So we have used some here on the grounds. I can distinctly smell the clove oil on occasion. But these products are not selective.

And when you said you cringed earlier with horticultural vinegar, even with these products, you’ve got to be very, very specific in targeting the plant you want, because anything green that they’re sprayed on will be damaged.

So I think that the best approach may be to try to do manual removal. Try to plant your garden beds as densely as possible with the plants you want. I mean, weeds are very smart.

Margaret: Right; to outcompete weeds, right.

Duncan: They get into areas that are open and available. So we do recommend that you make your garden beds fairly dense with the plants that you want. We use native plants obviously. And we try not to use the organic compounds, but as I said, they’re very effective on young plants, but they’re not selective.

Margaret: Right. And always read the label and use the protection—eye gear, etc., that it says to use on the label.

Duncan: Absolutely. Yes, absolutely. And don’t think twice as much is better because the-

Margaret: No, no, no, no, no, no, no.

Duncan: Because the companies that have developed these know that this is the concentration you want to use.

Margaret: Well, Duncan, I hope you’ll come back and talk about one of your other classes, the native groundcovers, because I know that would be a giant hit. And like I said, I’ll give links to all of the weed resources [more links below] and the classes that you’re offering on demand now from Mt. Cuba. And I thank you so much for making time today.

Duncan: Yeah. Well thank you, Margaret. It’s been a pleasure talking weeds with you and yeah, groundcovers is another big favorite of mine.

Margaret: Good.

Duncan: And so I look forward to hearing from you again, and wish everybody the best in their winter weed control.

more from mt. cuba

more on weeds and weed control


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I’LL BUY on-demand classes of their choice for two lucky readers, from among Mt. Cuba’s six current offerings: summer weeds, natives for containers, milkweed and monarchs, butterfly gardens, native groundcovers, and one on soil. All you have to do to enter is answer this question in the comments box at the very bottom of the page:

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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 11th year in March 2020. 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 November 2, 2020 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. Brian Hetherington says:

    Technically it’s an invasive shrub and a pretty one: rosa multiflora. It’s lovely in bloom but seeds a lot. From one plant which just appeared in my garden, I now have 4 or 5 and lots more that I’ve removed.

    I cut them to the ground regularly to keep them in check, try to cut off the hips so they don’t seed. The ones that remain are my guilty invasive pleasure!

    1. Judy stacey says:

      Houtannyea weed(aka as chameleon plant) Heart shaped leaves. Varying configurations of pink, green, cream) Loves to hide in other plants(especially Spiderwort) until it is of a size and strength to fight back

    2. Rebecca R. says:

      I have several on my list of worst weeds. Top of the list is Amur honeysuckle, Lonicera maackii. I’m resigned to the fact that it will be a lifelong battle. Also garlic mustard and chickweed. Oh, to have the time back i spend on these! Love your site Margaret. Keep up the good work.

  2. Ann Craig says:

    I am in an eternal battle with bindweed which creeps in from a nearby field. I have tried physical removal, mulch, competitive planting. I do not use herbicides. The other main weed is the Russian thistle -which admittedly is loved by finches and not unattractive when in bloom. I live in Colorado with very alkaline clay soil so have used horticultural vinegar in an area of the yard which was left to scrub oak and sunflowers with some success.

  3. Mary B Schwegler says:

    My absolute worst weed is English/baltic Ivy, Hedera. It grows on trees. It trails on the ground and chokes out everything else including Virginia creeper. It climbs up the house and the utility lines. Then the plants mature, set seed, and so new plants arise in my shaded back lawn. At my old house in Ohio I hired someone to remove it from the brick. Now in Virginia it is crawling up trees and trailing everywhere along the ground. I cut it back and yearly pull it out but don’t want to use herbicides. To think that some people think it’s great and still plant it!

    Bindweed is horrible also. Now my neighbor’s bamboo, an unrelenting and vicious pest, is creeping into my yard although it hasn’t asserted itself strongly yet — one of the few things I’ll spot control wth herbicide.

    But for sheer volume I’d have to say ivy is the worst of the worst.

  4. Renee Gregoire says:

    Creepy Charlie (: o ) – or rather creeping charlie has invaded my strawberry bed and has become a force to be reckoned with. I waged a serious weed pulling campaign and gained some ground this year but just RECENTLY noticed it is regaining ground! Now I know to be more vigilant and not think that 4 inches of snow would be the demise of Creepy Charlie!
    Great podcast Margaret – very much enjoyed this

  5. NJ newbie gardener says:

    Battling garlic mustard. It’s the one I can identify because it’s everywhere and it smells.. And invasive – ajuga, English ivy, etc

  6. Betty Paton says:

    BINDWEED! Could there be anything worse? I’ve tried to help my son remove it from his backyard. It’s in the grass and the gardens. I’ve snapped it off at the surface in the gardens but I can’t go as often as needed and my son works full time. We added a bark mulch in the gardens.

  7. Ellen McCullough says:

    Nut grass is my nemesis! My soil knife will be in up to its handle, and I still don’t get all the nodules out. I keep digging, but I never seem to get it all. I guess we all need an ongoing project!

  8. jan m says:

    for the most part, as long as the flowers are taller than the weeds, I let them be (depending on what they are)…clearweed and some others…don’t get much attention because I have bigger things to worry about. Have been battling Microstegium vimineum (Japanese stiltgrass). Didn’t pay much attention, but then it started appearing EVERYWHERE…

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