A SURPRISING NUMBER of people ask me about whether this plant or that plant in my garden or theirs is poisonous. And so when I saw news from the New York Botanical Garden about a just-published, fully updated edition of a reference book on the subject, I thought, “Why don’t I learn more about this?” (Meaning, why don’t we learn more about this together?) To that end, I’ve invited one of its authors, botanist Michael Balick, to talk poisonous plants with us.
In collaboration with Rutgers University medical toxicologist Lewis S. Nelson, MD, Dr. Michael J. Balick, who is Vice President for Botanical Science and Director of the Institute of Economic Botany at the New York Botanical Garden, has written the new third edition of the “Handbook of Poisonous and Injurious Plants.”
I asked him about plant chemistry—why some plants have the toxins they do–and what we’ve learned from, and about, those plants. And how did people figure out which are poisonous in the first place, anyhow?
Read along as you listen to the July 27, 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).
Plus: Enter to win a copy of the updated reference guide.
poisonous and injurious plants, with michael balick
Margaret: So welcome, Mike, it’s nice to have you back again.
Mike: It’s great to be here, Margaret.
Margaret: Yes. Last time we talked on the show, I think we were talking about your book on herbs, so this is a little bit different. Yes? [Laughter.]
Mike: Yeah. Well, similar and different. I think this also is a book that gardeners should have, because when you go into a nursery and you see a wonderful plant that you want to have in your garden, it might be a toxic plant, or it might be a medicinal plant that’s also toxic. This is a good thing to think about, especially if you have children, or you like to explore things by tasting them. It’s good to know what’s going to harm you.
Margaret: In the preface of the book, I believe it is, it says, “Plants produce many diverse chemicals that we have turned into important medications. However, “since dose makes the poison,” these same chemicals when delivered in large quantities are poisonous.”
So that’s kind of what you’re talking about is sometimes a little is O.K., and a lot’s not.
You’ve seen both sort of angles of plants in your career, and maybe you could tell us a little bit about your background. You’ve sort of seen plants as both friend and foe, so to speak.
Mike: Well, actually, an example of that is the may-apple [Podophyllum peltatum]. It’s probably growing all around you, where you are up in the woods. A may-apple has a compound that is used as a cancer medicine, for example, etoposide. But if the plant’s ingested, it can be toxic. It’s truly the garden of good and evil.
Margaret: Yes. A lot of your career has been… you’re an ethnobotanist. Can you explain a little bit about that and sort of just the short version of what you’ve explored about plants?
Mike: Sure. Ethnobotany is the study of the relationship between plants, people, and cultures. It involves training and a lot of disciplines from sociology to anthropology, to botany and pharmacy.
And so we’re a sort of broad ranging, skilled group of folks. Not many of us, but we also call it “the science of survival,” because what we’ve observed from indigenous living and indigenous peoples who live with plants and study them, is that they know a lot about how to live, how to feed themselves, how to heal themselves, how to make their own dwellings to live in. So ethnobotany is the science of survival, very important today.
Now I grew up in Wilmington, Delaware, and I became enchanted with plants at about age 4, as I remember. And I had training at the University of Delaware by Richard Lighty, and then went on to Harvard, where I worked with Richard Evans Schultes, who was the godfather of ethnobotany in this country.
And so since that time in 1980, I came to the New York Botanical Garden and carried out work in 37 countries, starting in the deserts of the Middle East and Central America, in the Amazon. And currently I’m working in the Pacific Islands of Micronesia and Melanesia.
And the lesson that I see everywhere is that people’s fascination with plants is part of our genetic makeup as human. And we’re all desiring to surround ourselves with beautiful and useful plants. It’s a trait shared by all cultures.
Margaret: Yes. Dick Lighty, by the way, is a hero of mine as well. A great native-plant proponent, a wonderful person.
Mike: Yes. He really influenced the trajectory of my life.
Margaret: Yes. So the book, let’s get acquainted with the book a little. In the book, early on in it, there are some statistics, kind of about how…I think there is, it says that there’s 42,500 calls to plant-related poison centers annually in the United States. I think that’s just a fraction, however, of all the exposure calls in total. 60 percent of those involved kids age 5 and under. I bet a lot more went to vets about their pets, people’s pets, but that’s not what the book is about. So a little bit about the scope kind of, of poisonous plants.
Mike: Sure. Well, there are actually millions of calls to poison-control centers annually. I’d like to give that number to your listeners; I have this handy: 1 800-222-1222. And the 42,500 were plant calls a year or two ago.
Mike: And you can get on the phone with someone 24/7 and ask a question, and this is what Lewis Nelson, my co-author, and the esteemed toxicologist, this is the sort of thing that he does and runs. But I imagine some of the top plants that people would know about are hot peppers, for example—a couple of thousand calls with people, especially children, that might’ve accidentally ingested an ornamental pepper or a hot pepper, what to do. Prunus, the cherries, for example. Cherries and apples have cyanide in their pits and in their seeds, and even, too, many of those can be bad for you.
Pokeweed [Phytolacca]–again, common, probably growing all around where you are. All parts are poisonous, and people do harvest the very young shoots in the spring and boil them. But if you harvest and past that very, very, very young stage and don’t boil them enough, they can be pretty toxic. And poison ivy. I could look out my window and see poison ivy.
And inside, things like Dieffenbachia, Spathiphyllum, they have oxalate crystals—little tiny crystals that are in the mucosal membrane. So these are the sorts of things that the people on the Poison Control Center hotline handle.
And most of the calls for plants are children under 5 years of age. Kids are explorers, right? My daughter, she was about 4 or 5 years old, and she walked into someone’s apartment with me and she saw a beautiful, hairy-covered cactus and went to hug it. It was taller than she was at the time. Of course, those spines just got under her skin, and she learned a lesson. Kids are explorers, which is a good thing.
Margaret: Yeah, they are. [Laughter.] They are. Now the title says “Poisonous and Injurious” and poison ivy [above] you just mentioned, for instance, I don’t know if it’s in both categories or in the injurious. There’s a lot that have a reaction on the skin, for instance, they may have stinging hairs or other skin reactions. Is that what injurious is in the title?
Mike: Yeah, that would be sort of physical injuries. So again, imagine a cactus spine or the poison ivy with urushiol, or thorns. Sometimes you get a thorn in you and you notice it gets infected. It could have a small amount of toxin on it.
Margaret: Whenever I have anyone help me in the garden who I don’t know, well, occasionally I’ll have a helper that hasn’t been here before, and I’m always very careful to say, “I’m going to cut this back,” or “We’re going to pull this weed, however, this one may give you dermatitis. So I want you to wear gloves and I don’t want you to touch your face with it.” You know what I mean? I try to be a little proactive.
And especially for instance, things related to Euphorbia. Things in the Euphorbia family, and I know you have a Euphorbia story that’s the annual poinsettia legend [laughter] that maybe you could share with us. Everyone thinks that’s a poisonous plant. But the euphorbias tend to have a sap that affects your skin. Yes?
Mike: Yes. Lots of the Euphorbiaceae have this milky sap that’s quite irritating to the skin. But the legend, the urban legend that you’re talking about is that Euphorbia pulcherrima, the poinsettia, is toxic to humans and animals. And actually there’s never been a case of toxicity in the human from eating that Christmas plant that everyone knows so well.
I looked into this and it turned out that back in 1919, in Honolulu, in Fort Shafter, one of the Army forts in Honolulu, they found a young child, a 2-year-old child, who had died underneath a poinsettia. And it was of course a tree in the tropics. And the rumor began—and it was in the newspaper—that the trial date, the poinsettia, and dropped dead. And then that was picked up in the 1944 edition of “Poisonous Plants of Hawaii,” saying the milky juice and the leaves are poisonous. And then it just took off.
And so every year we try to write a little story in the New York Botanical Garden’s blog, or we answer phone calls. Poinsettias are not toxic. In fact, there was a toxicologist that fed rats 500 to 600 poisonous leaves… I’m sorry: poinsettia leaves.
Mike: And the rats were happy. There was no toxic activity from a rat fed that many leaves over a period of time. Good news for the poinsettia.
Margaret: Absolutely. I was interested to see so many familiar plants in the book. Again, I know to alert people who might come to help me in the garden about certain plants that I know some people can be “allergic” to, have a reaction to, and obviously thorny things and that type of obvious stuff.
There are so many things that I grow that are “poisonous.” Now you just talked about how poinsettia became a legend, sort of a bit of folklore almost, got handed down, a story got handed down, labeling it as poisonous when it’s not even true. Is that how we normally find out a plant is poisonous? Is it often kind of folklore, so to speak, or whatever? Or are there people analyzing plants all the time to find out?
Mike: Sure. When I was a young gardener, maybe I was 10 or 12 years old and I was digging up English ivy [photo above] in my parents’ garden, I noticed that my arms got covered with rash. I learned very quickly that I’m sensitive, or at least I was so many decades ago, to English ivy.
But not only do we hear stories from our gardening friends, or read about them, but there are a lot of plants that we know about that we don’t realize can cause reactions. So the lady slipper orchids, for example, the Cypripedium. They can give you contact dermatitis from the little hairs.
And Ginkgo trees. Everybody sees people harvesting Ginkgo seeds in the fall, and you can get a rash from the seed. Sometimes because they are a food, if you eat too many and they’re not properly prepared, they can cause seizures.
Daffodil bulbs, during the second World War in Europe, people mistook daffodil bulbs for onions. And daffodil bulbs have a compound called lycorine, and they can cause severe GI upset.
And one of my favorites is mountain laurel. Do you have mountain laurel around where you are?
Margaret: Yes. Up in the mountains, up in the hills behind me, yes we do. So the Kalmia, yes. [Photo above.]
Mike: People mistake these for bay leaves sometimes. They’re very toxic. They have something called grayanotoxins, and they even have it in the nectar of the flowers. So bees that make their honey from mountain laurel, Kalmia latifolia, that’s a toxic honey. They call it mad honey, because it can kill you. Grayanotoxins are very, very toxic poisons.
In fact, I believe the Romans weaponized grayanotoxins in honey; they knew which flowers had those toxins, and used them against their enemy.
Even Hydrangea, we’ve got hydrangea in the neighborhood–toxic flower buds containing a cyanide-like compound.
Margaret: You said “weaponized.” And I was thinking of the castor bean plant, which some people like to grow the red-leaf as an ornamental, a big tall annual thing; a tender thing [below].
Margaret: …come from there? I mean, Mike, who figures this out? [Laughter.] I mean how does-
Mike: Well, I would call it trial and error. First of all, over history, back in the Neanderthal days, someone would eat the wrong plant and have a stomach ache and they would teach the other people not to eat that berry.
But toxicologists and botanists and chemists all study these secondary products or these metabolites of plants and build databases and write books. And a lot of the observations for toxins in humans are based on observations of either wild animals or farm animals.
Wild animals will avoid the plant if it gives them a stomach ache or worse. And then you put a farm animal in a sort of unusual habitat that they didn’t grow up in, and sometimes certain weedy things in the pastures will kill the animal. So a lot of the information we have are based on ethnoveterinary observations, or veterinary toxicology observations.
Margaret: Huh. Some of the plants that I found the most interesting, now you, you mentioned Prunus and how there’s within the seed is something poisonous, but others that are “herbs,” for instance, goldenseal, Hydrastis canadensis, a native woodland plant, has some poison element to it. And elderberry I think, was in the book as well. And yet we can make jam and so forth, but there’s a time, I guess, in the period of the fruit’s development, that it’s not good for you to eat—I don’t remember specifically for that one—but there’s sort of this nuance of what part and at what stage of development it’s dangerous, is that correct? And how you use it?
Mike: Sure. Yeah. The elderberry [Sambucus] is a perfect example. All parts of the elderberry are toxic, including the fruits, but when they ripen and you cook the fruits, they’re no longer toxic. So we all enjoy our elderberry jam, but don’t eat any other part, and don’t eat it fresh.
Margaret: Right. Right. Very, very interesting. We were talking before about that sort of milky sap of some plants, like the Euphorbia relatives, and there’s also a sap and I don’t know chemically if it’s similar or not, but looks similar—white, sticky—in the dogbane relatives and in milkweeds and… Tell us a little bit about those, where do those fit in? Those are related to each other too, I think.
Mike: Sure. Milkweeds [Asclepias], for example, they have a group of compounds called cardenolides, and they’re a sort of a toxic steroid. And a recent study showed that monarch butterfly caterpillars feed exclusively on milkweed so that they can accumulate this toxin in their body. They probably evolved with milkweed, so it’s not toxic to them. But then the birds won’t touch a monarch butterfly caterpillar because they know it’s so bitter and toxic.
So it’s just fascinating. These poisons are not only a plant’s defense mechanism—plants can’t run away, so they have to sort of stand and fight.
Mike: They use a chemical weapon. But then animals have learned how to utilize these toxins for their own defense.
Margaret: Right. And so originally of course, in all of this, us saying it’s poison and so forth, or injurious, it’s a little human-centric and it wasn’t invented, the plants didn’t evolve to poison us. They evolved as you’re saying, for anti-predation and to protect themselves, without being able to run away, to protect themselves in place, as deterrents.
In the last few minutes, I wanted to, from your expertise… As you said at the beginning, all around the world, and with looking at plants, both for their ethnobotanical qualities, for their benefits, for their potential as medicine or in traditional indigenous cultures, for their utility—and then also in this work, for instance, as poisons or potentially injurious: How would you tell us gardeners, kind of what’s the gardeners best practices about being in love with plants like we all are and being among plants? Because I just, I don’t eat things that I don’t know are edible. And again, I know which ones cause dermatitis, and that’s about the best I’ve done, but what would you say?
Mike: Well, one of my herbalist friends from overseas was foraging in a backyard of the home I once owned, and they thought they saw ginseng, but it was Virginia creeper [Parthenocissus quinquefolia], and they got an instant stomach ache. And one of the things one has to do is know your plants, know their names, their taxonomy, know what you’re dealing with.
So horticulture has introduced lots and lots of exotic in the last few years and we bring them indoors and we plant them outdoors, but you really need to know what these plants are. And that’s what this book seeks to do. Continuing education about what plants you’re growing—it’s really very, very important.
Margaret: There’s no substitute for a proper identification, as you say, for the correct taxonomic identification of the plant. And sometimes inferences can be drawn if the plant isn’t in the book by closely related plants. I mean, I assume in seeking medicine and other beneficial things that scientists look at related plants to get more of a similar chemical, for instance.
Mike: Sure. So we know that for example, species in the tomato family, like Atropa belladonna, or Datura—anything in those genera are going to be deadly toxic. Some can be medicines.
So these compounds cluster in plant families. It’s so important to heed the work of the systematic botanists who teach us about these relationships—the tree of life, as it were, the relationships between plants. Because if you’re going to use plants on a certain branch of that tree of life and one of them is toxic, you might assume that there could be others that are toxic.
Margaret: Right. Well, the book is “Handbook of Poisonous and Injurious Plants.” We’re going to have a book giveaway. One of my favorite parts, actually, were some of the tables in the front of the book, that’s family by family and genus by genus within the families, doing just what you said, sort of saying, “Hey, these are some to be aware of.” And Mike Balick as always, it’s a pleasure to talk to you. You’ve had quite the fascinating look around the world at plants. So thank you for making the time today.
Mike: Well, thank you so much for inviting me Margaret, and good luck with your garden.
(Photos except of poison ivy from “Handbook of Poisonous and Injurious Plants,” used with permission.)
enter to win the handbook of poisonous plants
I’LL BUY A COPY of the new third edition of the “Handbook of Poisonous and Injurious Plants,” by Lewis S. Nelson, MD, and Dr. Michael J. Balick, for one lucky reader. All you have to do to enter is answer this question in the comments box at the very bottom of the page:
Do you wonder, or worry, about poisonous or injurious plants either indoors or out? (I don’t have pets or kids in the house to worry about, and as I said in the interview, I just assume everything is unsafe to eat except known edibles I grow for food!)
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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 July 27, 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).