get smarter about poison ivy, with dr. susan pell
NO, POISON IVY was not put on the planet to punish mankind, or to boost antihistamine sales. As with every native plant—and yes, Toxicodendron radicans is an American native—it has an important role in the bigger scheme, supporting wildlife and providing erosion control. With help from Dr. Susan Pell of the United States Botanic Garden, we’ll get a much closer look at it from a very safe distance, and hopefully learn to manage around poison ivy better, but also to give it the respect it deserves.
Susan is intimately familiar with poison ivy and its relatives, because she has for years studied them right down to the molecular level. She is Science and Public Programs Manager at the historic United States Botanic Garden in Washington, D.C., a fellow of the National Science Foundation, and former Director of Science at Brooklyn Botanic Garden. (More on visiting the U.S. Botanic Garden, and on upcoming programs there, is in the box at the bottom of this page.)
But I invited Susan to my public-radio show and podcast for another reason that she states on her LinkedIn profile, alongside all those impressive scientific credentials: because, she says, she loves to show people “the coolness of plants.”
So keep an open mind, gardeners, as we explore the “cool” of poison ivy—and of course practical, more obvious matters like what to do to avoid that damn rash. Read along as you listen to the June 22, 2015 edition of my public-radio show and podcast using the player below. You can subscribe to all future editions on iTunes or Stitcher or Spotify(and browse my archive of podcasts here).
read/listen: my poison-ivy q&a
with dr. susan pell
Q. Your area of research concentration has been the cashew family [or Anacardiaceae], Susan, correct?
A. I study the evolution and diversity of the cashew family, which includes poison ivy among its members, and pistachios, mangos, sumacs, smoke trees—a lot of plants gardeners are familiar with.
Q. Smoke trees, meaning Cotinus?
Q. That’s one I didn’t know was related, or it had fled from my mind. Interesting.
A. It’s actually pretty closely related to poison ivy.
Q. Now everyone: Don’t be afraid of your smoke bush!
A. Or sumac—except poison sumac. The genus Rhus is perfectly fine.
Q. The members of the cashew family that most U.S. gardeners are aware of are poison ivy and poison sumac—where are their native ranges?
A. It kind of depends on how you recognize the species of poison ivy. There are some very closely related species—I’ll call them sister species. If you include those under the common name poison ivy, then poison ivy is found in every state in the U.S. except for California. Although you do find poison oak there—so you get your share of the genus Toxicodendron there that way.
The genus that causes the rash is Toxicodendron—which means “poison tree.” Poison ivy, poison oak and poison sumac are all species within that genus. Common names are where we get into the trouble area, but in the strictest sense poison ivy is Toxicodendron radicans. It’s found from the Midwest to the Eastern United States.
But if you include other species, like T. rydbergii, and some other species in that genus that all get called poison ivy, it extends everywhere in the United States except California.
Q. And which one does California have again? I’m sure they don’t escape entirely.
A. They have Toxicodendron diversilobum, poison oak, and it is quite nasty. In fact, I’ve seen some reports suggesting that 15 percent of the State of California’s entire workmen’s comp budget is spent on poison oak-related absences and injuries, and that’s mostly related to firefighters. People fight the wild-land fires that California has, and there is poison oak in it and they’re inhaling that smoke. They can be in the hospital or out of work for weeks.
Q. It’s one of the things we’re warned about: If you do have debris from any of these plants, do not burn it.
A. It’s Toxicodendron vernix. It is found just in the Eastern United States. Its range is a little more restricted than T. radicans; it occurs from Texas and Louisiana eastward, but it doesn’t get deep into the Midwest (or into the West). It’s in wet and very swampy areas.
Q. My garden is surrounded by state park. In two areas by the woodland edges I lost trees last fall, and the conditions changed. This spring, poison ivy has romped quickly in, like it was just poised looking for an opportunity. It that its typical behavior—is it an opportunist?
A. It definitely is. Poison ivy is what is called a successional plant. It really takes over in areas that have been newly cleared—say, by the building of a roadway, or maybe they’re continuously mowed, or in your situation where you have a tree fall down.
What happens is that there is a new resource available in that habitat on the ground level, and that’s light. Either poison ivy seeds will germinate, or the rhizomes sprout—these underground, horizontal stems that are in many cases underfoot where you don’t even know they are. They may sprout stems and come up above ground, and you’ll see them.
There may have been poison ivy there in a very light way, maybe a few plants here and there, and then once that light resource is released by the taking down or falling of a tree, those plants are able to thicken and grow more stems and really take over.
It’s an early successional plant, and grow wells in disturbed areas. Some people would call it weedy.
Q. You mentioned that it can move around by rhizomes. It has fruits, so it can also be moved around by birds and maybe mammals?
A. The fruits are actually an important food for wildlife; a lot of birds eat them. Deer have been known to eat them, and in some cases rodents like mice do. So they get moved around through animals’ guts and deposited in areas away from the plant.
Q. The sort of nursery-rhyme wisdom I was taught decades ago was, “leaves of three run and flee,” but poison ivy isn’t so easily typecast, is it? It can be quite variable in appearance, can’t it?
A. It absolutely can. The leaves can be shiny or not shiny; they can have those red stems or petioles or not (they can be green sometimes). The shade of green is very different in the leaves. The leaves can be quite large—one leaflet can be the size of your head, or the size of one of your fingers, enormous or quite small.
In some areas occasionally they will have five leaflets, not three—though it’s very rare. In one population in Texas, this characteristic has apparently become fixed so the entire population of poison ivy in this one area actually has five leaflets. I have seen occasional collections from other states that have five leaflets. But for the most part: Leaves of three, let it be, or leaves of three, run and flee—those are pretty good rhymes to remember.
Q. They say everything grows bigger in Texas. [Laughter.] The leaf edges can be quite different; you have to look carefully.
A. It’s so true: The margins of the leaves may be smooth, what we call entire, or they may be toothed, or very jagged, or they may be like an oak leaf.
Q. And the habit of the plant—the new plants in my disturbed areas are like a herbaceous groundcover, but along my road there are many big, hairy-looking woody vines wrapping around trunks of very old trees.
A. That’s right. It can have quite a different habit, depending on where it’s growing, and what conditions it’s under. Also depending on probably some of the genetic makeup of the plant as well. Some of these things historically have been treated as separate species or varieties, or sub-species of poison ivy: Sometimes there’s one characteristic was used to recognize that—maybe that it’s freestanding, instead of climbing.
But really when we look across poison ivy we see that it can be a climbing plant, as you said. Sometimes those climbing plants can have quite large trunks of their own and big branches, such that if they’re growing up a tree, the branches of the tree may actually be poison ivy branches, not the branches of the tree. Sometimes the tree it’s on is dead and it looks like it’s the tree.
Q. [Laughter.] “Little Shop of Horrors.”
A. And sometimes they can be small groundcover, and freestanding. I’ve seen in coastal New Jersey, for instance, a shrub of poison ivy that was about 7 feet tall, and had smooth black bark. It was definitely poison ivy—but it was this 7-foot shrub with smooth black bark.
Q. It’s an important plant ecologically—and even a beautiful plant, for instance in fall color. But nobody likes it (except for you!) because it causes an allergic reaction in some people. I have to confess: I’ve been crawling around in the underbrush much of my adult life, and I have never had the rash.
A. About 10 to 15 percent of the human population is immune to poison ivy, and will never have a rash.
I have given lectures around the country about this topic, and someone will come up to me and say, “I’m in my 60s, and I never had poison ivy, and now I get the rash.” And I say: “Has anything changed?” And they answer that they retired and now they garden all the time—they increased their access, their exposure.
So you don’t want to get too far ahead of yourself and assume that you have lifetime immunity. I’d say you’re probably safe in that assumption because you do have so much exposure to it, being an avid gardener. But for people who haven’t had that kind of lifetime exposure and don’t get a rash, I wouldn’t count myself among that 10 to 15 percent who are immune. You can develop a rash later in life, and the more exposure you have, the more likely you are to get the rash—not the reverse.
Sometimes people say, “If I just keep exposing myself I won’t get the rash anymore,” and we don’t really see that to be true.
Q. Why did the plant develop this chemical—certainly not to torture us, right?
A. It’s called urushiol, and basically these compounds essentially only give people a rash, though there are some reports that maybe other primates get it.
These compounds have a lot of protective qualities for the plant—not chasing people off from picking the leaves, but actually fighting off diseases, microbial attacks: fungal, bacterial and viral attacks. There have been quite a few studies that have looked at the effectiveness of the anti-microbial properties of these compounds.
Q. So it developed urushiols as a defense mechanism against disease—not against predation.
A. The compounds don’t seem to have any effect on the animals that eat them. You can see poison ivy leaves that have been almost decimated by various insects eating them. I’ve seen deer grazing on poison ivy, and certainly birds and other other animals eat the fruit.
Q. Does urushiol have any economic uses, or uses in medicine? I think I read it was used in Japanese lacquer.
A. In several countries in Southern Eastern Asia, the sap or the urushiols of the trees of Toxicodendron are harvested—the same genus we have here, but a different species—and also trees from another genus called Gluta that occurs in that area. They’ll slash the bark—put lines down the bark, and put a little bucket there, the same way you’d harvest rubber from a rubber tree, and how you can harvest maple sap from a maple trees, though slightly different.
They’ll harvest that sap, boil it down for a very long time, and either use it to apply to fine woodworking (which is what they do more in Japan) or to these very weak bamboo structures, which is what you see in Burma, or Myanmar. They build these little bamboo structures, and put on 10 or 15 or 20 coats of this lacquer—this boiled-down sap from Toxicodendron. They make these beautiful works of art.
Q. I hear people say they are going to wait to tackle an invasion till the poison ivy leaves fall in autumn—to avoid getting the rash. I say, “I don’t think so.” [Laughter.]
A. That’s a very bad idea. Every single part of the plant, except maybe the actual seed inside the fruit, can cause a rash. They all have these resin canals that have the ururshiols in them. If you look at the fruit in the fall, it’s a beautiful cream color—and I say beautiful because I love poison ivy, but some people don’t.
Q. Because you’re a nutty, botany kind of person. [Laughter.]
A. The fruits have black lines in them, and those are actually these resin canals. They’re present throughout the plant. If you snap off a leaf or cut the stem and see the sap coming out of it, you can watch it turning black as it oxidizes with exposure to the air. That black sap is what is going to give you the rash. That black sap runs in the stems in the wintertime; it’s still present in the leaves that fall off. I’ve heard of people getting rashes from 20-year-old plants that have been collected and dried and pressed in our natural-history collection.
I’ve heard of people getting rashes from gardening equipment they hadn’t used in two years, because it wasn’t washed properly.
Q. People make the connection that because jewelweed sometimes grows near poison ivy, they think if they put the jewelweed on that will solve everything, but that’s not right, is it?
A. No, it’s not. Jewelweed has some properties that bring swelling down, so it would be fine to use jewelweed after you have the rash and have some swollen areas. But it’s not going to do anything to prevent the rash.
Basically these compounds, the urushiols, bind to your skin cells—to the little proteins in your skin cells. So if you’re someone who’s going to get a rash, you’ll get it. The key thing is to prevent the binding from happening—to wash, and remove all those oils as soon as you possibly can after coming into contact with it.
There are some great products out there, like Tecnu, which basically binds to the urushiol before it can bind to your skin cells.
Q. You can’t spread the rash spread to yourself or others, when the blisters break, or if you touch the rash to another part of your body or to someone else?
A. Once you wash yourself, or wash your tools or your dog—if your dog’s been romping in the poison ivy—there’s nothing on you or in that rash, even if it’s oozing, that can cause a rash. What you see on your body is actually your own immune system fighting your own body. The binding of the urushiols to the proteins in your skin cells makes your immune system recognize your body as foreign, and attack them. All of those liquids that come into that rash have nothing to do with the poison ivy plant—it’s your own body fluid.
Q. So what do I do next with my garden invasions of poison ivy? Usually I dig it out and bag it up or dispose of the dug-up plants in the woods.
A. If you have the wherewithal to dig it out, that’s a really good method. I’m a proponent of not using a lot of chemicals in the garden, but if you have a severe allergy to it and have to get rid of it, I would say very selective applications of herbicides—painting it onto the leaves directly, and not doing wholesale spraying—is an appropriate application, in my opinion.
You can do physical removal, but those rhizomes are really strong, and when you break them up even a tiny fragment can resprout. So it’s very hard to remove it all, if that’s what you have to do.
Q. So basically I should just give up, is that what you’re telling me? [Laughter.]
A. And enjoy the beautiful fall color, and know that you’re feeding birds.
Q. I do love it along the outer property edge and always leave it there. I’m just not so happy that it’s now romping in among the perennials.
So in our shifting climate, I imagine this is one of the creatures that’s doing well.
A. It is. There was a study done about 10 years ago out of Duke Forests by Duke University. They have a forest where they are increasing the carbon dioxide levels artificially—really mimicking what we estimate will be the situation with climate change. What they found is that pretty much all plants grow better when they have more carbon dioxide, just like we do when we have more oxygen. But they also found that the urushiols are worse—so what we have to look forward to is more poison ivy, and meaner poison ivy.
Q. Well aren’t you just Miss Perky over there? [Laughter.] Thank you so much for all the information on poison ivy.
- For more on mechanical removal (timing and tips), read the Northwest Coalition for Alternatives to Pesticides poison ivy and oak management pdf
visiting the u.s. botanic garden
THE UNITED STATES Botanic Garden where Dr. Susan Pell is Science and Public Programs Manager is situated at the foot of the Capitol Building, on the Mall, in Washington D.C. There is a historic conservatory, plus outdoor gardens, with exhibits and public programs, and an extensive plant collection with a diversity of habitats represented outdoors and in.
The Garden offers a medicinal and poisonous plants tour pretty regularly (see the events calendar).
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 10th year in March 2019. 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 June 22, 2015 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).
(Poison sumac photo from Wikimedia Commons. Disclosure: Purchases from Amazon affiliate links yield a small commission.)