poisonous plants, with dr. michael balick of nybg

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.

Margaret: Yes.

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.

Margaret: [Laughter.]

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].

Mike: Yeah.

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.

Margaret: [Laughter.]

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!)

No answer or feeling shy? Just say something like “Count me in,” and I will, but a reply is even better. I’ll pick a random winner after entries close at midnight Tuesday, August 4, 2020. Good luck to all.

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 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).

  1. Barbara says:

    I do try and avoid planting plants that might be poisonous to my dogs in any area they can get into but I do have some in other areas of my yard.

  2. gretchenmgh says:

    I have had an interest in this topic. I’m get a rash similar to poison ivy from clematis, unfortunately, I love clematis!

  3. gretchenmgh says:

    I have had an interest in this topic. I get a rash similar to poison ivy from clematis, unfortunately, I love clematis!

  4. norita carlson says:

    When we moved into our house I saw mountain laurel and other poisonous plants and so I have native bees in my yard and not honey bees. I learned a good deal from your interview and I know of Dr. Balick from the NYBG where I volunteer. I would like to know about more of the poisonous parts of plants to share with my adult children and my grandchildren.

  5. Willa Wick says:

    I’ve had allergic reactions to poison ivy for years – and I never know exactly where on the farm I run into it, but every year I avoid where I think I hit it last year only to find a few days later I’ve broken out again (I do spend a lot of time in the woods keeping our trails clear.

    This year I have developed a sensitivity to Sow Thistle. I’ve been pulling it for years and never had trouble before. Right now my arms, neck and nose have tiny pustules and crazy itch similar to poison ivy. It’s amazing how rubbing with the gel of Jewelweed, or spraying alcohol will help.

    So far I haven’t had any trouble with Datura, Castor Bean or digitalis, but I really do wonder what “weed” I’ll grow a sensitivity to next. This book would certainly put me in defensive mode.

  6. About 15 years ago I began to enjoy many aspects of perennial gardening and also learning to identify weeds. There are times when I’m clearing flower beds of vines or weeds that look familiar, so I handle them with gloves but no other precautions, and in the next day or two will get a mild itching rash that is like a minor spot of poison ivy. I’m frustrated that I can’t find a resource to increase my knowledge of this topic and avoid passing an inadvertent rash from an unknown weed/plant allergen to someone in the household.
    I try to read weed identification books and check websites to increase my working knowledge of poisonous weeds and the “Book Regarding Poisonous and Injurious Plants” you are offering as a giveaway sounds just perfect to help me increase my knowledge in this area. COUNT ME IN!

  7. Susan says:

    Yes I worry for my young visiting granddaughters and their cats when we pet sit. I have elderberry bushes and have cloned them for my daughter’s garden too. I knew that the berries were not tasty until ripe and cooked, but not poisonous! I must become more knowledgeable.

    1. Cindy Carpenter says:

      Same here! I live in the city and do my best to avoid anything harmful to children or dogs who may wander in. Elderberry poison is a surprise, and I just planted a bunch of them :-(

  8. Jan Parandhamaia says:

    I enjoy your articles and especially this one on poisonous plants as I do have pets inside and outside and grandchildren and try to research different ones to be on top of it but it’s hard to keep up with. I am a member of the Mountain Laurel Garden Club of Garrett County in Western Maryland and it is through our club that I was introduced to you and your weekly articles. You provide such great information all the time! Thank you and please continue !!

  9. Susan Cox says:

    I am interested in this book! I have been planting fruit trees and shrubs this year and planted elderberry and just picked a few berries yesterday that were ripe. I had no idea until I looked it up that the berries needed to be cooked before they were eaten. Glad I did look it up and this book would be helpful.


  10. Irene Sokol says:

    I was totally surprised to learn that the Poinsettia plant is not poisonous. Really?
    Hopefully this book will teach me what other plants are labeled as such and that are perfectly harmless.

  11. Ursula says:

    I am a longtime garden enthusiast and lots of plants seem to suddenly appear during one or the other year. I am very familiar with poison ivy, and never do any weeding without some newspaper plastic bags with me, so anytime I come a seedling (the smaller the easier) I will cover my gloves with the plastic sleeve and grab the plant as close as possible to its base and pull it through carefully in fact turning it inside out capturing the plant, then tying the bag up and putting it in the trash (NEVER IN MY COMPOST PILE).
    However I have had some skin irritations over the years, but didn’t realize which other plants are the culprit, so, yes, I’d love to win this handy reference book. Thank for your generosity.

  12. Marilynn Evans says:

    Count me in, I have a 4 yr old grandson the loves to roam and explore my backyard.
    I’d love this book so I can identify the dangerous plants. I’m new to this state and confess I’m not aware of the local plants that could be dangerous.

  13. Cathy Maggard says:

    I often wonder what causes the skin issues while gardening. Is it a plant toxin or just irritation from hoary or hirsute leaves and stems. Count me in.

  14. Joan says:

    I only recently found out that Pokeweed was poisonous to dogs. I used to let it grow because the birds like the berries , and it’s such a pretty plant!. Now I only allow it to grow where the dogs can’t get near it. This is reminding me to go out there and cut it back some…very carefully!
    I do have some May berries that a friend gave me too but also not in a dog area, interesting info! I’d love to have that book!

  15. Anne Pounds says:

    Margaret, I had an experience this weekend weeding around some Monkshood, or wolfsbane, and I touched the monkshood with my arm. It was an instant “OUCH!” I sat there and looked it up on my phone, to find out that it can cause paralysis and cardiac arrest–probably if ingested). I ran into the house and ran cold water on my arm, then soap and more cold water. It was painful. Earlier in the season I was clearing an overgrown raised bed which had gone rampant with English Ivy and Virginia Creeper. My arms were swollen like sausages and bright red, even worse than a tangle with poison ivy, and it lasted and kept getting worse for more than two weeks! I had to go on steroids for a month and a half to get it under control. So I have been learning my lessons this gardening season. I would love to win a copy of this book, and it seems I need it! Thank you. I love your emails and so look forward to them.

  16. Margaret says:

    What an interesting topic! I have been expanding my garden this summer and I do wonder and worry about poisonous and/or injurious plants. I just planted a euphorbia, for instance. I then looked it up online, which is what I often do with an unfamiliar plant. I still have the euphorbia, but when I water it I wonder if I want it in my garden. I realize I don’t have the same feeling for it at the moment as I have for other plants, my drift rose bush, for example, which I love wholeheartedly. But I think over time maybe that will change. The reference book sounds great. Count me in.

  17. Diane L. says:

    I’m commenting before I’ve read all the comments and I’m guessing someone else has mentioned stinging nettles? I was in some deep woods and knew I was walking through some low growing nettles, but wow, I forgot how wasp-like the sting is! Thank goodness it does not last long.

  18. Thea says:

    I have a number of friends that worry about their pets. I love lantana As an annual but I understand it’s toxic. It does form these dark berries which can drop into the soil. That is not good if your dog likes to dig or lies under a window box filled with toxic annuals. This books looks like an excellent reference.

  19. Each year when I garden i seem to brush against the same plant that gives me a terrible itchy rash. The plant is coming from my neighbor’s yard and makes its way into my garden. i try to wear long sleeves and gloves but the leaves always seem to hit my skin.

    this plant seems to be the only toxic one around and i can’t find the name of it.

    i love the Website: Away to garden and i also listen to the podcast when busy in my kitchen. Thank you very much – the show/podcast is greatly appreciated for all the knowledge it gives.

  20. Jill says:

    Although I feel comfortable with identifying many common toxic plants – I always wear gloves in the garden to be safe! Stinging nettles got me once when I thought I’d pull a few weeds without my gloves – no more!

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