saving (and growing) native lady’s slipper orchids, with longwood’s peter zale

TODAY’S TOPIC is orchids, but not the ones you might be growing as a flowering houseplant. Our subject is native terrestrial types that are more often than not under great pressure in the wild, their numbers dwindling.

Now, thanks to work led by Peter Zale at Longwood Gardens in Pennsylvania, strategies have been developed for propagating one such orchid, the Kentucky lady’s slipper. That Cypripedium and Dr. Zale will travel to the upcoming Chelsea Flower Show in England, where they’ll be part of an international display showcasing efforts in orchid conservation.

Dr. Peter Zale is Associate Director of Conservation Horticulture and Plant Breeding at Longwood Gardens, where he leads a team of scientists and horticulturists focused on conservation, horticulture, plant exploration, breeding and more. He has a longtime special passion for, and focus on, native orchids of the United States.

Read along as you listen to the May 13, 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).

native orchids with peter zale of longwood



Margaret Roach: So orchids: the most diverse of plant families on the planet, I think, but so many of them are in trouble. And so give us a little background of the numbers and the situation of orchids, how many U.S. species and how many worldwide and things like that.

Peter Zale: Sure, sure. So I think one of the reasons that orchids are so engaging is because there are so many of them, and I think scientists would argue about this, but there’s about 24,000 to 30,000 or more worldwide. And so if you think about all of the plants on the planet, that’s about 8 to 10 percent of all the plants on the planet. And so it’s pretty incredible. So I think the only other family that really challenges the orchids would be the Aster family.

Margaret: Wow.

Peter: And here in North America, we have about 220 types of orchids north of Mexico. And a lot of those are in Florida, but many of them are found throughout the temperate and colder parts of the United States. And for example, here in Pennsylvania, at least historically, we used to have about 60 different taxa or types of orchids.

Margaret: Interesting. Wow.

Peter: Yeah. And the thing about orchids is that even though they’re widespread, they’re on every continent except Antarctica, almost everywhere they occur, they’re not ultra-common, and many of them are really quite rare. And it’s estimated that about half of the world’s orchids are vulnerable or threatened or endangered in some way. And that’s the case right here in Pennsylvania as well, and throughout a lot of the United States, that our orchids are rare and declining in numbers.

Margaret: And so I mentioned in the introduction that we’re going to talk about terrestrial species, and there’s also epiphytic orchids, a different group of orchids that I’m just going to over-generalize and say, live in the trees [laughter]. And those are under pressure for a different reason in different regions of the world. Yes. I mean, that’s… Yeah.

Peter: I think most people when they think of orchids, that’s what they’re thinking of. Tropical epiphytic orchids you might see in southern Florida or around the tropics. Certainly we don’t have any of those here in Pennsylvania or in the mid-Atlantic. All of the orchids we have here are terrestrial.

Margaret: So you’re heading to Chelsea, to the flower show, later this month. And when your Longwood colleague contacted me the other day to tell me about you and the Kentucky lady’s slipper, Cypripedium kentuckiense, making this trip, she referred to it as “a Cinderella story.” It was very cute. She made me laugh and she said, “Because like the classic fairy tale, the Chelsea show is like a grand ball of sorts,” and your plant has the word slipper in its name, “and this orchid story is one of transformation sort of against all odds success.” So tell us briefly about how this orchid that’s under so much pressure in its native range in the Southeast that is headed to the Chelsea Flower Show, and what hopefully that says about its future.

Peter: Well, it’s a very interesting case in that there’s probably an estimated 5,000 or fewer plants left in the wild. And when we started working on orchid conservation at Longwood, we specifically wanted to look at how to propagate them, how to grow them from seed. And there’s a lot that goes into that. Orchids have these tiny dust like seeds, they often need to be grown in a laboratory, that sort of thing. And it just so happened that we had an opportunity to work with a local population of our yellow lady’s slipper [below], and we also were able to work with some cultivated Kentucky lady’s slipper. And so we used those two species really as a model to figure out our propagation protocols. And it just so happened that even though Cypripedium kentuckiense is so rare in the wild, it turns out that it’s, using the methodology that we did, very easy to propagate and really quite easy to grow.

In fact, it’s even easier to grow than the large yellow lady’s slipper, which is native just a few miles from Longwood here. And so it’s really interesting where you have this plant that’s really rare in the wild, but seems really adaptable to cultivation. And so some of the plants that we were able to propagate, which are now eight or nine years old, are growing in the gardens, we’re growing them in our research nursery, and we had the opportunity to join this team of orchid experts from around the United States and around the world and showcase some of the orchids that we’ve grown, or some of the Cypripedium we’ve grown, at Chelsea as part of a display.

Margaret: And I think I read that you’ve successfully said grown hundreds of seedlings, I guess, I don’t know how many, but you’ve even sent hundreds back to the U.S. Forest Service to help in conservation plantings. And aren’t a number of them, or one of them, going to be in the collection of the Royal Botanic Gardens at Kew and so forth as well?

Peter: Exactly. And so it turns out we adapted a method of seed propagation that actually Kew Gardens worked on in the 80s and the 90s. And we’ve been able, we’ve very successful with the Kentucky lady’s slipper, and have been able to propagate many hundreds if not thousands of them. And so yes, some of them have gone to the Forest Service. We have other projects where we will propagate them, we’ll keep part of them for our living collections and then send them back for restoration purposes as well. And yes, one of the great things about this is that once the display is over, it’s very likely that the plants, the Cypripedium kentuckiense plants [below] will go into the living collection at Kew Gardens.

Margaret: Oh! So yeah, that must feel… I mean obviously the conservation work is the end game here, but that must feel like a special other… a little something extra.

Peter: It does, it does. And one of the aspects of my job in helping develop plant collections and things like that, I mean, it’s really a great conduit for sharing with other gardens, sharing messages. And I think gardeners will often say the best way to keep a plant is to give it away. That’s really something that I always think about when we’re propagating these rare plants. I mean, if we’re able to have success, it’s important to have them here at Longwood for restoration, but also to get them into the hands of other experts, other gardens, that can keep them going as well.

Margaret: Yeah, you were talking about seed, growing them from seed, I think, and some of what I’ve read, it sounded about some of the native orchids sounded almost counterintuitive like that, I don’t know, that mature seed sometimes fails to germinate or you have to go to all kinds of… Again, not necessarily with this specific species, but it’s not like, “O.K., the seed ripens and I sow it and boom, I’ve got orchids.” Yeah, I mean it’s-

Peter: Right. I wish it were that easy, but…

Margaret: [Laughter.] Tomatoes, it ain’t, huh?

Peter: No, definitely not. And so orchids with these tiny dust-like seeds, they in the wild not only need to fall into the right place, but the right fungi or mycorrhizae in the soil need to be there and infect the seed. And basically the orchid seedling germinates and uses the fungus as a food source as it’s maturing. And so in our lab, we actually can mimic that process using fungi or with the lady’s slippers, we use a nutrient-rich sterile medium that’s specifically for Cypripedium and other types of slipper orchids.

And with the slipper orchids, what we found was in using information from Kew, is that if you harvest the seeds at about 50 days after pollination rather than when they’re physiologically mature at about 100 days after pollination, they actually germinate a lot better, way better. Eighty or 90 percent will germinate as opposed to if you collect the mature seed, it’s kind of a crapshoot. You don’t really know what you’re going to get. It could be 10 percent, it could be 60 percent, and it can change from year to year and between different populations. So it really takes a lot of work.

But using the immature seeds, we have been able to routinely propagate many different types of lady’s slippers here at Longwood, but also take that knowledge and apply it to other native orchids as well that have been really difficult or even impossible to propagate.

Margaret: So you said other lady’s slippers and the Cypripedium are… They’re distinctive-looking flowers. And so the slipper in their common name I guess speaks to that. It’s this pouch-like, I don’t know, modified, what is it, fused petals down below, kind of that form, this lip or pouch or something.

Peter: So the labellum forms the pouch and it’s really meant to… It’s giving all the indicators to a pollinator that it has a nectar reward, but it’s so many of our other native orchids where they have deceptive pollination. So a bee or a syrphid fly lands inside the pouch. There’s these little often red stripes or sort of differently colored stripes, sort of like the lights along a runway when you’re landing, inside of there that attract the pollinator. It lands in there, realizes that there’s no nectar, no reward, gets irritated and it can’t fly out of there. So it sort of crawls up and comes into contact with the parts of the flower that enact pollination, and that’s how it works. So it’s a really fascinating pollination syndrome.

Margaret: Yeah, because I mean, they’re very, very showy just looking at them as a flower. And of course that’s for a reason, having nothing to do with human enjoyment.

Peter: Right, nothing at all.

Margaret: The human aesthetic.

Peter: Nothing at all, yeah.

Margaret: I think there’s quite a few lady’s slipper orchid species in the U.S., what are there, 10 or 12 or I don’t know how many are there, the different Cypripedium?

Peter: There are 10 or 12, and like so many orchids, their taxonomy is often disputed. But yeah.

Margaret: Fluid. It’s fluid [laughter].

Peter: Yeah, fluid, exactly. But yes, there’s about 10 or 12, and there’s a number of them here in Pennsylvania.

Margaret: Do they hail from common types of environments? Are they all woodland plants, for instance? Can we say anything general about all of the lady’s slippers?

Peter: Well, no, I mean I guess you could say that they’re woodland plants, but the one thing about finding them in the wild and where they grow, it’s very hard to generalize their habitat. And I think that often they’re associated with specific geologic features or physiographic provinces or things like that, rather than a particular ecosystem or habitat type.

And so for example, the Kentucky lady’s slipper is found in forested regions, but it often grows along seepage areas. So I know out in the western part of its range, it grows along areas where there’s water seeping out, grows in that kind of habitat in Virginia. But then in the Cumberland Plateau, it grows on these sandy terraces above these fast-moving streams and these coves and hollers and hollows and things like that. So it can be hard to generalize. I mean, I guess they are woodland plants, but when you start to see them in the wild, it really… You get a lot more questions I think in the end than answers.

Margaret: I looked at the range maps, the BONAP range maps for the genus Cypripedium, and it looked like there’s some species most everywhere in the U.S. except for, I think Nevada was the one place where I didn’t see any. Obviously those are not probably 100 percent up to date, or who knows. And then there was one species, parviflorum, that looked like it was so widespread, I mean relatively. So some are just very, very, very limited to a small area, and some are wider spread and so forth. But there seemed to be a Cypripedium in most areas of the country.

Peter: And looking at the yellow lady’s slipper, we spoke about the large yellow, which is the one that we have in this part of Pennsylvania, and the small yellow, they basically stretch from Eastern Canada all the way across to Alaska, and then at many points South. So they’re incredibly widespread. And there’s actually a species in Europe and Russia and the Far East as well, a yellow lady’s slipper, that has a similar range, although it’s a different species. So it’s interesting in orchids is that you can get these orchids that are very, very widespread sort of generalists. And then you have things like the Kentucky lady’s slipper that is much more specialist in its habitat preferences.

Margaret: And where is that? I mean, it says Kentucky, but is that it’s not just limited; it doesn’t recognize the human boundaries probably of the state of Kentucky [laughter]. Where is it located?

Peter: It’s got a very unusual sort of disjunct distribution. And so there’s a population in Virginia, which is widely disjunct from sort of the core of the populations which are in the Cumberland Plateau in Kentucky and Tennessee, maybe even down into Alabama. And then you have populations scattered through Georgia, Alabama, East Texas. And the species actually has its stronghold in the Ouachita Mountains in Arkansas, in sort of West Central Arkansas. So you almost could call it the Arkansas lady’s slipper, but it was first discovered from a population or described from a population in northeastern Kentucky. So that’s-

Margaret: I see, that’s how it got that name. Now, this is not the only native orchid by any means that you’ve been working on in the hopes of helping to restore wild populations. Are there others that you want to just sort of call out and speak about a little bit or…?

Peter: Well, we are working broadly within the genus Cypripedium, but we really want to develop a database for how to propagate all of the 150 or so terrestrial orchids that are native in the United States. And so one genus of great interest that I think also has potential as a garden plan is the genus Platanthera. And these are often called fringed or bog orchids. And some of them, like Platanthera ciliaris [below], are somewhat easy to propagate, and they have these incredible bright orange flowers that occur from let’s say mid-late July through August. There’s some white-flowered species, and then there’s another group of Platanthera, there’s one called the purple fringeless orchid, Platanthera peramoena, and some related species, Platanthera grandiflora and psycodes, and these are beautiful kind of purple-flowered orchids, and they can be two or three feet tall, and they have so much ornamental potential.

They’re great plants as ecosystem indicators, but it turns out they’re really, really hard to grow or propagate. And so that’s actually one group where we’ve taken this idea of using the immature seeds, this sort of horticultural hack, and applying it to that. So we’ve done quite a bit of research on that.

And I think orchids, our native orchids are a great group of plants to really showcase the role of horticulture, that horticulture can play, in plant conservation. And I feel like horticulture is an often underrepresented part of the plant-conservation scheme.

Margaret: So let’s talk about that a little bit. I mean, about potentially growing orchids in our gardens, these terrestrial native orchids in our gardens. I mean, I have to confess, the only orchid I have in my garden is non-native and grows as a weed in my vegetable raised beds, you probably know what it is. How do you say it, Epipactis helleborine or something? Is that a word, Epipactis, did I make that up [laughter]?

Peter: No, that’s it, Epipactis helleborine, which is probably most people… When I was a college student, I worked landscaping one summer and I went to pull out weeds at this lady’s yard. She’s like, “I don’t know what I have growing in my yard.” And we showed up and it was thousands of Epipactis helleborine and that was an extreme case. But it’s an interesting orchid that it’s basically spread from the Eastern U.S. all the way west. And you find it kind of everywhere, but it never makes carpets. It’s not really super noxious, but it is fascinating. We often think of orchids as rare plants. And here’s one that is making its presence known well outside of its native range.

Margaret: And I think it’s not even from North America in the first place, it’s from somewhere else.

Peter: It’s European.

Margaret: Yeah. Yeah. So I looked in the flora, the current flora, the list of species known to be present in my county, which one of the conservation organizations in my area, a nonprofit, keeps up to date and is very active and we’re very lucky. I’m in the Hudson Valley of New York State, in a county called Columbia County adjacent to the Berkshires of Massachusetts. And they list 41 kinds of orchids that they know a location or multiple locations where they’re present. And in some cases these are as varieties of a single species. So there’s such and such variety, blah, blah. It’s not a distinct species, but a lot of orchids. And yet, what do I have? I have the Epipactis helleborine. [Laughter.] I have this funny… Boy, it’s a tenacious thing. It really roots in and it’s charming. I mean, if you look at it closely, its tiny little flowers are beautiful, but it’s not what I want in the vegetable beds necessarily.

Peter: No. And it’s interesting is when I was younger, I tried to transplant it and it doesn’t transplant. So it’s one of these things where it seems like, and maybe others have had different experience, but yeah, it’s one of these things that is highly mycorrhizal. And if you try to move it, the environmental conditions have to be just right. So it’s interesting that it can be sort of such a generalist, but also be kind of particular.

Margaret: Fussy, right.

Peter: But that pretty much sums up orchids.

Margaret: So if we wanted to grow lady’s slipper, and as I said, I noticed on the range maps that there are many species around the country, so one could probably do the research and find out about the appropriate one or ones in your region. You spoke about some that are native, the yellow ones that are native adjacent to where you are. Because we’ve always thought, oh, they’re impossible. I can’t grow. They’re so hard. They’re so precious, they’re so rare. What is the story about growing them in our gardens, do you think?

Peter: I think many orchids, like lady’s slippers and some others as well, make great garden plants. I think part of the difficulty has been in propagating them, because it wasn’t really until about let’s say 25 or 30 years ago that anybody really figured out how to propagate a lot of our native orchids in larger numbers. And it takes a lot of time to get the first flower from something like the Kentucky lady’s slipper or yellow lady’s slipper, it takes about one year in the lab and then another three to five years growing in a nursery or in another setting to get the first flower. And to get a nice big clump of them could be another three to five years.

And so it takes a long time, and they don’t really, I think fit a lot of modern production cycles, and they don’t often do well under sort of the standard nursery conditions and things like that. So I think that’s part of it as well.

Margaret: So it’s kind of like a disincentive for the commercial nursery people to propagate them, to give them space in their greenhouses or their coldframes or whatever, their fields, because it’s a long investment before they get a return. But some specialty people, some native plant specialists, do have them and some other specialists, our mutual friend Tony Avent at Plant Delights has some. There are, I see them in catalogs and so forth. And obviously we should always have to give that disclaimer: We must never wild-collect anything.

Peter: Absolutely.

Margaret: That would be absolutely against the law and ethically incorrect. But so assuming an appropriate one is available, is there any trick to making them at home, the young plant at home if we were able to buy it?

Peter: I think planting time is important. I think lady’s slippers have seasonal root growth. So if we’re talking about lady’s slipper orchids, planting them in the late winter is very good. A lot of times they’re offered in the fall, and that can work, too, but they often sit there dormant. So the planting time can really help, but also just recognizing where they want to grow. And I grow several lady’s slippers in my own garden and have for a long time now and growing them and conditions that suit Epimedium and Polygonatum and hostas and astilbe and things like that are really what they’re looking for. So the north side of your house, perhaps the east side of your house, works really well where they’re protected by other plants, but not crowded, either.

They like well-drained soil. Well, what does that mean? I have well-drained soil at home. What I used to do when I first started growing these was sort of excavate a hole that’s say, I don’t know, 6 inches deep and 12 to 15 inches wide. And then I filled it with that… I got some builder’s sand, some coarse sand, and mixed leaf mold in it, and filled the hole with that and then planted the lady’s slipper there.

And a lot of times if you buy a lady’s slipper, maybe it will come bare root. Maybe it’s in a pot, but their root system is very distinct and it really wants to spread out where it’s planted. And so I think making sure that you spread out the roots correctly and don’t plant them too deep is really helpful as well. Keeping them mulched and making sure they don’t frost heave and stuff like that.

But it’s an investment in time, and they’re incredibly long-lived. We have an accession of the large yellow lady’s slipper in the gardens here at Longwood that is a 1963 accession. So the same plants have been growing and have been divided for the last 60-plus years. I know that some lady’s slippers in the wild, they estimate that they can live for centuries. And so if you get the conditions right, there’s a potential that you have this really long-lived, really rewarding plant.

Margaret: Right. So I’m just going to ask you, I said in the introduction that orchids, native orchids, have been a long fascination or passion of yours. Do you remember what your first orchid was?

Peter: I remember the first time I learned about native orchids, and I will talk about that. When I was young, I became interested in trees at about age 14. And I would go… I’m from Cleveland, Ohio, and my family would take me to the Holden Arboretum to study trees. And I remember walking through there, and they had a pamphlet with a line drawing of a yellow lady’s slipper on it. And I saw it, and I picked it up and started looking at it, and it just seemed like something clicked. I was like, “Oh my gosh.” It was my first… “Orchids grow here?” It was like a revelation. And from that time, I’ve been very interested in all kinds of orchids, but especially hardy orchids and native orchids and that kind of thing.

Margaret: Well, it’s really great to speak to you. I’m so glad we finally connected, and yeah, that I could hear the story. And I hope you have a wonderful adventure, you and the orchids. I hope you have a wonderful adventure at the Chelsea Flower Show coming up at the end, toward the end of May. It sounds very exciting. And of course the best part of all is that it’s in the name of an international effort toward orchid conservation. So thank you for making time today, Peter. It’s wonderful to talk to you.

Peter: Thanks for having me.

(All photos courtesy of Longwood Gardens.)

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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 May 13, 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).

  1. Robert Roggeveen says:

    A sunny morning in Connecticut, coffee, and this wonderful conversation. It is always a thrill to find an orchid when waking in the woods. Do I have a place for one here? Perhaps, a long a path on the north side of my house, the new entry to the back portion of the garden. I wonder, would it share the same general growing conditions as a Jack-in-the-Pulpit? If it can, and I will study it, then I could see it from the window just to my left (next to my desk).

  2. Catherine A Mitchell says:

    I always remember see these when I was a kid in Minnesota and was very intrigued on how something so delicate could be living on the ground in the woods. They are so beautiful and just get you so close to nature! This is a great episode and brought me back to my childhood in Northern Minnesota.

  3. N.Duffey says:

    About fifty years back, when I was a teenager, my uncle showed up at our house, handing my mom a bucket full of pink lady slippers (it was 50 years ago, people didn’t know then!). He’d picked them when out hiking from my grandfather’s cabin, coming across a field full of them. Another time when we were at the cabin my mom found a tiny brown orchid, the plant maybe six or seven inches tall, the flower less than an inch, perhaps as small as half an inch. Beautiful. Lady slippers have been my favorite since.
    I am interested in that growing mix. Is it available?

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