ONE OF THE TALLEST perennials in my garden is New York ironweed, Vernonia noveboracensis. But basically my knowledge of the genus starts and ends there—or at least it did until just recently when Mt. Cuba Center, the renowned native plant garden and research institution, published the results of its four-year trial of a range of ironweeds, powerful plants that pollinators love and deer generally don’t.
Sam Hoadley, the manager of horticultural research at Mt. Cuba Center in Delaware, joined me to talk about what he and the team there learned in their multi-year trial of native ironweeds. I always look forward to visits from Sam, and to the in-depth reports that result from each genus of plants that Mt. Cuba studies, including this latest one. (Sam is also doing a virtual talk on Vernonia on March 5, 2025; information on tickets is at this link.)
Read along as you listen to the Feb. 17, 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).
a deeper look at vernonia, with sam hoadley
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Margaret Roach: Hi, Sam. How’s the winter down there in the mid-Atlantic?
Sam Hoadley: Oh, we’re actually having a bit of a real winter for a change. We’ve had some cold. I know you have as well, and I think we’re gearing up for a little more snow, but the most wintery winter I think we’ve had in a few years at least.
Margaret: So just a quick description of Mt. Cuba Center for those who may not know it and how long you’ve been conducting trials there and so forth. You’re near Wilmington, Delaware, and Zone 7A, I believe.
Sam: That’s correct. Yeah, we’re just north of Wilmington, Delaware, and just south of the Pennsylvania border. And so Mt. Cuba Center is a botanic and public garden focused on native plants in their conservation. We were founded by the DuPont-Copeland family. This was their home. They moved here in the 1930s, and very early on recognized that their passion lay in gardening and gardening with native plants, and the conservation of open space was also very important to them.
So kind of with that visionary outlook on this property and the space, they eventually realized that they wanted to welcome the public to this space, and hopefully have those people that visit Mt. Cuba Center be inspired by the beauty and value of native plants. So that’s everything we do at Mt. Cuba Center is tied back to that intention, which now lives on today as our mission.
And it was really also Mrs. Copeland’s intent for the trial garden to be created here at Mt. Cuba Center. It used to be the cut-flower space. After she passed away it was one of the first pieces of the gardens here to be transitioned to kind of its new role, in 2002. And we’ve been conducting plant trials in that space since then, and have released, I think, a dozen research reports now that are all available on the website.
Margaret: So this time you trialed 45, I believe, different Vernonia, ironweeds—species and cultivars, including hybrids. And frankly, when I saw that number I thought: Wait a minute, I think I only know about two [laughter]. So what gave you the idea to do Vernonia? First of all, it’s not like when you’ve done Echinacea in the past or Carex, the sedges, the things that there’s a lot of, and people are wondering a lot about. What got you onto this? [Above, detail of V. gigantea.]
Sam: Yeah, so I think that there’s kind of two big buckets of trial plants that we look to evaluate. One of them is like a trial like Echinacea where there’s a lot of options out there to consumers, and we kind of help with the where do you start? We kind of try to shed a little more light on some of the bigger questions: Which one of these will maybe attract more pollinators, which might be longer-lived? Just kind of filling in the gaps of plants that are commercially available.
On the other hand, with a trial like ironweed or goldenrods or Pycnanthemum, we feel like this part of our role here is to maybe promote and shine a spotlight on maybe an underutilized or underappreciated group of plants. And there is some availability of Vernonia out there. But I think once we started planning this trial, we realized there was many more out there than we originally anticipated, and we did a lot more of comparison of species type plants.
We did some plants from wild-collected seed compared to commercially grown examples of the same species. There’s just some extra layers embedded into this trial. And again, it really kind of goes towards promoting this group of plants that I think really deserves another look in landscapes throughout the Eastern United States. Ironweeds are wonderful, wonderful plants, and hopefully we’re providing that platform for them.
Margaret: So you just said throughout the Eastern United States, and I sort of looked at range maps of the genus and all the different species and so forth, and I don’t see any in the far West, any species in the far West, I don’t think, but maybe the Eastern, I don’t know, two-thirds of the country or thereabouts, really from all the way north to all the way south. There seems to be some occurrence in many places, different places. And I read also that there are Vernonia on other continents, I believe—Southeast Asia and Africa as well as North and South America. So it’s not just an American genus.
Sam: No, it’s a very widespread genus. And actually kind of a fun horticultural fact: The largest Asteraceae plant in the world is an ironweed, or in the genus Vernonia. And it’s found, I believe, in Southeast Asia in a tree form [laughter], which is totally wild to think about. But most of our native ironweeds and we consider native to be Eastern Temperate Forest region, so roughly the Eastern half of the United States. And those are the plants that we incorporate in the trials. These are herbaceous perennials, clump-forming, and some of them are relatively accessible in terms of size, not terribly large. Others are tremendously large. But one of the traits they all share in common are those kind of beautiful pinkish-purple inflorescences that we all know, and love.
Margaret: So you used the Latin word, but they’re in the Aster family.
Sam: Yes.
Margaret: I have the New York ironweed and I’ve had it for a million years and it blooms late for me, almost into the fall, I believe. I don’t remember when it starts, but are they generally high summer-to-fall bloomers, or is there a big range or…?
Sam: There’s a pretty decent range when we take into account all of those species native to the Eastern U.S. But the plants that are most familiar to us in this part of the world, the mid-Atlantic region and into New England, would be kind of that late-summer-into-fall window, really from August into early September. That’s when our New York ironweeds are going to be blooming.
But there are ironweeds out there, one that’s native to the Midwest called Vernonia fasciculata that starts blooming in early to mid-July. So for us, very early for ironweed. And then there are other species, particularly the ones native to the Southeastern U.S., that tend to bloom a little bit later. We actually had some of those species and Vernonia gigantea, and Vernonia angustifolia, blooming into September, October and one of them even into November a few times. So there is a pretty respectable range for the genus overall.
And if you selectively choose some of the earliest-blooming, and kind of the middle-season bloomers, and the latest bloomers, you can have this really great succession of ironweed flowers in your garden by just strategically picking a few species and selections.
Margaret: So one of the things that you evaluate as part of your trials—and this has been more so in recent years, I believe—is insect interactions. And some insects (and I’m going to completely oversimplify this)… some insects are generalists and they can utilize resources from a range of plants to survive, and others have specialized host relationships with a particular plant or small group of plants, related plants. And in this case, with the Vernonia, I read in the report that you got to see an example of a specialist insect in action.
Sam: Yes. It was absolutely fascinating. So we started evaluating these plants. We planted them in 2019, the first year in 2020, it was established in 2021, we started observing pollinator interactions. And we identified a native bee visiting the ironweeds as Melissodes denticulatus [above], which we learned was an ironweed specialist bee—at least the females collect pollen from ironweed. And if you see white pollen on an insect, that’s generally coming from ironweed, especially if you’re seeing them active in kind of that late-summer-to-fall window.
Those females are collecting that snowy-white pollen, and that pollen is then used to feed the larvae of those insects, and that larvae overwinters and then hatches out the following year. So this is an insect that very heavily and specifically relies on the pollen of Vernonia, and also pollinates the plant. It’s a very mutualistic relationship, I would say, between this insect and ironweed.
So we kind of came up with this idea of, O.K., we know this insect’s here, does it have a favorite? We wanted to track its activity specifically, in addition to doing those overall insect counts, which give you kind of a basic gauge of which of these ironweeds can attract more insects and likely benefit more insects in a cultivated setting. But we have the specialist bee, we have 45 different options for it. Does it have a favorite? [Laughter.] And it’s like this huge menu for it.
Margaret: What flavor of the ice cream at the Baskin-Robbins does it want?
Sam: Yeah, exactly. Yeah, that’s funny. And this is kind of that perfect zoomed-in example of that question of species, cultivars, their value to pollinators. Does it matter? Does it matter that an ironweed might not be locally native? We really expected to see a lot of activity on our locally native species like New York ironweed, which was heavily represented in the trial, because in theory, that’s the ironweed that this bee would be experiencing locally that would-
Margaret: Right, that it would have co-evolved with and so forth over the generations and so on. It would’ve been here, like the plant.
Sam: Exactly. And we had other ironweed from hybrids to plants that are native to other corners of the Eastern Temperate Forest Region. So the question was: Does it matter? Does it have a favorite?
And so we collected these observations specifically regarding this bee for two more years, in 2022 and 2023, thanks to our incredible pollinator watch team or volunteer team who really does the vast majority of our pollinator data collection for us. And what we found is that the Melissodes denticulatus is active during a pretty discreet window of the year, and probably not coincidentally, that window of activity aligns perfectly with the bloom period of New York ironweed, that August to early September window, which makes a lot of sense.
But unfortunately, New York ironweed in the trial garden really just struggled in the trials. It’s not a bad species. And if you have really moist garden soils, you should absolutely grow New York ironweed. But the soils in the trial garden are a little bit drier than probably what it wanted. It’s not getting supplemental water from us. And that species struggled over all.
Margaret: Yeah, because that’s like where we see it in nature would be in moist and wet meadows and ditches by the roadside where there’s moisture collecting and things like that. It’s not a dry, fast-draining-soil kind of a resident.
Sam: Yeah, it seems that cool, moist root zones really seem to be necessary for that species to thrive. If you have those situations at home, by all means plant New York ironweed; it will reward you with beautiful flowers in late summer and fall. You’ll have lots of pollinator activity. But it just doesn’t seem to be very adaptable to the middle-of-the-road garden soils that we have in the trial garden, and certainly not dry soils.
And we often saw a manifestation of this kind of mismatch of soil types in the trial garden compared to where those plants would occur in the wild as disease presence. So these plants were stressed, diseases like powdery mildew and rust would exploit that stress, and as a result you’d have less-vigorous plants, less flowers and less value for pollinators. So the health of the plants a lot of times are very closely tied with the amount of value that they can provide for pollinators.
So it’s important to site your plants correctly.
But what we found is that there was another species in the trial garden that was very adaptable to the average garden conditions and probably can grow a lot drier as well, that bloomed during that same window. And we saw all the Melissodes denticulatus activity on that species, which was Vernonia baldwinii, not locally native; native to more of the Central and even Western U.S. It goes, I believe, just east of the Rockies into Colorado and its farthest western extent. But again, the bloom time was right. That was an adaptable plant to the average garden conditions.
And that’s where the Melissodes were. It was fascinating to see, and they’re such charismatic little insects. If you have ironweeds in your garden during that time when your locally native ironweeds are blooming, or if you go see them out in the wild, you’ll probably see those bees, and picking up on that white pollen on their hind legs is a great way to identify them.
Margaret: Well, silly me, I never knew about that specialist. And so now of course I can’t wait until late this coming season and I want to go look and see if that’s who’s on my plant, and so forth [laughter].
Sam: Oh yeah, I bet you’ll see them. They’re a lot of fun to watch.
Margaret: Yeah, we’ve mentioned New York ironweed a bunch of times, and I said in the introduction it’s one of the tallest perennials in my garden. And I guess for me, I don’t know if it’s around 7 feet, maybe it’s taller, it’s taller than 6, but it’s not as tall as I’ve read it can get to be. But there are some larger ones, and you alluded to this before because you talked about the one that’s like a tree size or something somewhere else in the world, I think. But is it gigantea, the one that sounds like giant?
Sam: Yes, very aptly named Vernonia gigantea. It is an absolutely massive plant, and we had a few accessions of our versions of Vernonia gigantea in the trial garden. We had two selections of the species and then two accessions that were sourced from wild-collected seed in Alabama. And the Alabama forms were very tall, very robust plants, really just considering their size, very sturdy. And they ranged from anywhere between 9 to 12 feet in height, which is-
Margaret: Unbelievable. That’s crazy.
Sam: Substantial, yes, big plants. But we had one selection from Plant Delights that was selected in Arkansas called ‘Jonesboro Giant’ [above center] that it set a record for us in terms of height in the trial garden. It reached 15 feet tall in 2022 in the trial garden, which was just astounding,
Margaret: Fifteen feet for a perennial, from a herbaceous perennial that died to the ground, I guess, in winter.
Sam: And the really kind of fun thing about ironweeds, and it was a little bit jarring for me every spring, is they sometimes are a little bit late to come out of the ground, but once they do, they put on so much growth so quickly, it’s really incredible to watch them. And despite being 15 feet tall, that plant endured some pretty severe weather and stayed upright. It was a head turner. Everyone who came to the trial garden was just drawn to that plant. And I’m happy to say it’ll be back in the trial garden this spring, so we can see it again. But definitely one of the most visually impactful plants that we’ve ever had at the trial garden.
Margaret: That’s kooky. That’s really kooky. Yeah, I mean, that’s tall. [Laughter.] So that’s tall. And like I said, New York ironweed can supposedly get to up to 10 feet. I don’t think I’ve had mine get to 10 feet, but whatever. Are these all in sun? Is sun what they want or what other requirements we’re talking about? Is there any generalization?
Sam: So we evaluated all the ironweeds in the same conditions, in full sun and kind of this average garden soil, that we have in the trial garden. There are some species in the evaluation that can take a little bit more shade. New York ironweed can take some light shade, but it’s not going to be as robust or as floriferous as plants that are grown in full sun.
Vernonia glauca, which is another locally native species that’s a little bit less common both in the commercial world and in the wild than New York ironweed, but it can grow in a little bit more shade than New York ironweed, as well as it can take some drier conditions. Think of it kind of more as like an upland version of New York ironweed.
And then there’s another species which is incredibly obscure, which I hope someday will catch on in the nursery industry and someone will want to grow it. But Vernonia flaccidifolia or the Tennessee Ironweed is another one that I suspect can take a lot more shade than what it was growing in in the trial garden. It wasn’t necessarily a top performer, but maybe would offer some options to people who don’t have full sun in their gardens.
Margaret: So the other one that I had ever heard of besides knowing the New York ironweed, I knew one called I believe, a species, lettermannii. Is that right?
Sam: Yes.
Margaret: And I think that was because there have been some cultivars maybe of that in the nursery trade. I don’t really even know why it’s stuck in my head. But what I was fascinated to see in the report was that you with, I believe one of each of the plants that you had, each type of plant that you had in the trial, I believe one of them, you tried giving it the Chelsea Chop, like the cutback in the springtime to see how it would perform a little fuller, bushier and slightly later to start blooming. That sort of that thing that we use on certain perennials that can sometimes get a little floppy and so forth. I think lettermannii was one of the ones that responded well to it [below]. Is that correct?
Sam: It really did. So Vernonia lettermannii in the wild, it grows in kind of river corridors in the Ozark region, and it’s probably experiencing soils that while they can be periodically flooded, are oftentimes a bit drier and perhaps leaner than the soils that we have in the trial garden. And I think a lot of times we’re just treating that species too nicely in our gardens. I think it wants to be grown a little bit drier, a little bit leaner. Full sun is best. But what we get with that extra moisture, those extra nutrients, as we get leaner growth that can often end up being slightly floppy, especially towards late summer, which is kind of characteristic of that species in kind of common garden settings. You’ll get that splaying a little bit, you’ll see that open center.
But that cutback essentially fixed that issue. It just created a more densely branching, tighter-growing plant that really was very resistant to that flopping. Another way to fix that tendency is to just grow in drier soils. But the Chelsea Chop really seemed to work very well with species that were smaller-growing anyways, had finer foliage, and would produce better branching from those cuts.
Some of the tallest species, like Vernonia gigantea, it did have some results where you’d have some more branches produced on a slightly more compact plant. But often the union of that newer growth onto those older stems that were cut tended to be kind of weak and you would end up with a lot of breaks happening.
So it’s one of those things that’s maybe not the best treatment to apply to all ironweed, but for plants like Vernonia lettermannii and Vernonia angustifolia, it’s a really viable option to kind of keep your plants a little tidier, and you can even prolong your bloom season. So if you had one plant, if you had two of them, for example, two Vernonia lettermannii, if you cut back one and left the other full, you would have again, a succession of blooms for maybe twice the amount of time you would normally have.
Margaret: So you cut it back maybe a third to a half sometime in spring. And is this around late May, or what was your timing?
Sam: Yeah, we tried to time it between late May and early June, and we did cut all the plants in half. So it wasn’t a consistent height, but we just took a measurement of the plant overall and cut that plant halfway.
Margaret: The Chelsea Chop. So let’s take a couple minutes now to talk about some stars. I think one that was the star performer for you, and again, considering the conditions and everything—so it’s not across the board going to be the same for everybody—has a funny name considering that Vernonias really have an almost purplish flower. It was called, I think ‘Plum Peachy,’ but what’s the peachy part? [Laughter.]
Sam: Yeah, it’s kind of that. I’m not really sure, but it is a catchy name. It does have that same kind of beautiful kind of pinky-purple blooms, but this is a Vernonia angustifolia that was again selected by Plant Delights in Georgia from a naturally occurring plant. And it just proved to be a really adaptable garden plant, really accessible for people who don’t have a lot of space.
And we’ve been talking about plants that get 8, 9, 12, 15 feet tall. This one tops in at around maybe 4-1/2 to 5 feet tall and wide. So if you have less space, maybe a good option for you. And this is another what I think of as a three-season perennial. So it has good interest three seasons of the year. Some of the taller ironweeds, they don’t look like a whole lot until they start blooming, and maybe they’re best reserved for the back of a border.
But plants like Vernonia angustifolia, they have beautiful foliage. And in this case, the selection, the foliage has this kind of incredible blush of this purply to almost wine-colored pigmentation and the petioles and the stems. It’s a stunning plant. And one of the nicest features of it is actually after it blooms, the seedheads have that incredible coloration as well [above], that kind of wine blush to them, which are absolutely stunning, especially with some grasses or mixed into a meadow. It’s a beautiful, beautiful plant.
Margaret: Oh, interesting. I just wanted to just ask, are you growing them also on the property elsewhere at Mt. Cuba? Have you incorporated them into any borders or natural areas. Just for some inspiration visually of where. You just talked about some of the taller ones, obviously going in the back of a border. Any other ideas of what they combine well with or…?
Sam: Yeah, we have incorporated some of the species out in the gardens. There’s of course New York ironweed out in our natural lands, which is spectacular out there. And one of my favorite garden companions for ironweeds in general tend to be goldenrods, which—spoiler alert—our next trial report will be on goldenrods next year. But even like Vernonia gigantea or Vernonia noveboracensis [above] mix in with some of the taller and later-blooming or kind of that late-summer and early-fall-blooming goldenrods, it’s an absolutely spectacular combination.
Also, Silphium make a good companion. Some of the taller native grasses are a great companion as well. And that purple color just goes with so much. They’re great, great garden plants. Again, I hope people will be inspired by seeing them and seeing this report, hopefully wanting to plant a few of them in their gardens and try some of them out.
Margaret: And be inspired by the fact that mammals don’t tend to like chewing on them, so they’re not animal magnets, which is good. They’re pollinator magnets, but not animal, other animal, magnets. So that’s good.
Sam: Exactly.
Margaret: Now I see, just real quick, you’re doing a virtual event that people could get a ticket for and attend March 5th, I believe. You’re going to do a webinar talk about ironweeds.
Sam: Yes. It’s going to be an ironweed-specific class, a virtual talk, essentially going over the trial results, what we learned, basically an in-depth dive on the research that was conducted here, and hopefully a few take-home messages, messages that maybe didn’t make it to the report, some extra details there. So I think it’ll be a lot of fun.
Margaret: Sam, I’m always happy to talk to you, and I’m always excited when one of these reports comes in the mail. They’re gorgeous. And I’ll give the links to where people can read the full report online as well, of course.
other native plant conversations with sam hoadley
(Photos from Mt. Cuba Center; used with permission.)
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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 Feb. 17, 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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