Pop quiz: What’s an ecologically important native flowering plant, found in every one of the continental United States but California, that almost everyone hates?
The qualifier “almost” makes room for the dissenting opinion about poison ivy from Susan K. Pell, a botanist and educator, and the deputy executive director at the United States Botanic Garden in Washington.
Her career-long fascination with poison ivy is not based on a personal immunity to urushiol, the oily resin that is the active compound in all the plant’s parts — from seed and leaf to woody vine — even when it is dormant. Exposure to just 50 micrograms of urushiol, equivalent to less than a grain of table salt, causes a rash in 80 to 90 percent of adults, according to the National Institute for Occupational Safety and Health at the C.D.C.
Ms. Pell, the former director of science at Brooklyn Botanic Garden, does get a mild rash from poison ivy, as she has since childhood. But that did not stop her from choosing the plant and its relatives as her research specialty in graduate school — or examining related plants down to the molecular level, as she has done since then, teasing apart the who’s who.
It’s no surprise that poison ivy (Toxicodendron radicans) is part of the family that contains poison sumac and poison oak, which are in the same genus. But the Anacardiaceae family also includes cashews (its common name is the cashew family), as well as pistachios, mangos and garden-worthy subjects like smoke trees (Cotinus) and Rhus, commonly called sumac (the genus poison ivy was long placed in).
Toxicodendron translates as “poison tree,” and a six-year research project at the Duke Forest of Duke University, published in 2006, forecast that climate change would make it even more so. Plants exposed to more atmospheric carbon dioxide, mimicking climate shifts, grow faster and larger — and the urushiol in them is much more allergenic.
It’s a Pioneering Species That Loves a Disturbance
Poison ivy is an early successional species: It moves in where a disturbance occurs because of human interference (the result of construction, say, or the continuous mowing of a once-overgrown area) or natural causes (where a tree fell along your property line, for example). Taking advantage of the increased light, the seeds germinate or the underground stems advance.
The enormous system of roots and rhizomes that poison ivy puts down seems like bad news if it’s in your backyard. But this inclination toward Manifest Destiny is poison ivy’s environmental gift.
“It’s doing this important underground work,” Ms. Pell said, “holding onto sand and soil — like along dunes, where you see poison ivy on your way to the beach. It’s a major player in preventing the erosion of our Eastern coastlines.”
Speaking of pioneering: Poison ivy has been found growing in a microclimate beside a hot spring in the northern Yukon, presumably from seeds a bird carried there in its gut and excreted. The fat-rich white fruits, which ripen in late summer through fall, provide sustenance to many bird species and some mammals, another of poison ivy’s ecological roles.
It’s a Real Chameleon
The adage “leaflets three, let it be” is a good start in identification. Except in rare, five-leaflet populations in Massachusetts and Texas, poison ivy’s leaflets are arranged in threes. But the plant can otherwise be so morphologically variable that it confounds all but expert observers.
“Its plasticity is really crazy,” Ms. Pell said. And that has led to distinctive-looking phenotypes in different regions being designated as local species.
She begs to differ. As part of her long collaboration with John D. Mitchell, honorary curator at New York Botanical Garden, Ms. Pell has been sorting it out. In their upcoming Anacardiaceae chapter, to be published in 2022 as part of the massive “Flora of North America” project, they have determined that most of these are not distinct species, but fall into five varieties of T. radicans.
“We didn’t recognize some well-loved local species,” she said. And no, that is not sarcasm you hear. Taxonomists often become emotionally attached to a particular species retaining its stature in the systematic hierarchy — even if that species is poison ivy.
It can be a ground cover, or shrubby, or a woody vine (called a liana), achieving nearly treelike proportions. Its leaves can be shiny or matte (even in the same population), in various shades of green (or red-bronze, upon emergence), and they vary in size and shape, with margins from smooth to toothed or deeply lobed.
“In one area in Quebec,” Ms. Pell said, “it has straplike leaflets and looks like a fern. In the Southeast, I have seen leaflets as long and wide as my head, and elsewhere, often very close by, leaflets shorter than my thumb.”
On the East Coast, poison ivy is most often confused with box elder (Acer negundo), Virginia creeper (Parthenocissus quinquefolia) or a bramble like blackberry (Rubus). But box elder’s clusters of leaflets are arranged opposite each other, along the main stem, while poison ivy’s alternate. Mature Virginia creeper has five leaflets; its stems have tendrils with thick pads on the end, but are missing the reddish roots that often cover poison ivy vines. And Rubus has spines, unlike poison ivy, which never does.
It’s Tricky to Subdue (But You Knew That)
If you’re lucky, you’ll catch poison ivy’s opportunistic first moves into your backyard. The best strategy with any weed: Get to know what its seedlings look like. It’s much easier to take a shovel to a cluster of tiny sprouts than to subdue an established liana.
With an infestation that is further along (but not yet massive, or up a tree), digging will require multiple years of repeat attention, as any rhizomes left behind re-sprout. And the bigger the problem, the harder it is to tackle without exposure.
Solarizing poison ivy with plastic or smothering it with cardboard are options in such cases. “Lay down plastic or several layers of cardboard over the plants,” Ms. Pell advised, “and be sure the plant doesn’t come up at the edges.” Expect to be at it for a year, or more likely two, to achieve control, regularly expanding the size of the covering as bits try to escape.
Sometimes, however, the infestation is too far along.
“In my experience talking to lots of people over the years about their efforts, the thing that works most reliably is herbicides like glyphosate,” Ms. Pell said.
She does not recommend wholesale spraying, which uses far more chemical than necessary. Instead, she suggests painting it onto leaves or the cut end of a vine.
Wearing eye protection and gloves, she first severs a big vine carefully with a lopper or a handsaw, not a chain saw. Next, she drills a few shallow holes in the remaining vine and fills them with a little herbicide in an undiluted concentration.
What does not work is the advice sometimes offered on social media: pouring boiling water on poison ivy. “That’s not going to kill the plants,” she said.
Remember that any tools you use and garments you wear will be tainted with urushiol, and unless they’re thoroughly cleaned they can transfer it to your skin. Carefully bag any debris for the trash, but don’t add it to a burn pile: Urushiol can be released in smoke and will damage lungs.
And don’t make the mistake of thinking that it will wear off over time.
“I have heard of rashes caused by contact with tools that hadn’t been used in two years, but still had resin on them,” Ms. Pell said. Researchers referring to old herbarium specimens in scientific collections have likewise had reactions to the dried, pressed plants.
Prevention and Remedies
With so many new gardeners joining the fold this past pandemic year, presumably some are also having their first close encounters with poison ivy. It’s reminiscent of the lament that Ms. Pell hears regularly from recently retired people.
“They say they had never been allergic before,” she said, “but that lately they’ve lost their immunity and had rashes. ‘Are you gardening more than when you were working?’ I ask them, and of course the answer is yes.”
Their immunity didn’t change; their plant contact did.
Some gardeners use lotion containing Bentoquatam (sold as products like Ivy Block), which creates a barrier on the skin and affords some protection, although not 100 percent. It must be applied 15 minutes before contact and allowed to dry fully, then reapplied every four hours or any time its dried film is no longer visible.
Limiting your contact is still best, with lotion or without, as is following up with fastidious aftercare — neutralizing that oily sap. A 2000 study compared the effectiveness of surfactants (specifically Dial soap), the grease-removing hand cleanser Goop and Tecnu, a skin cleanser marketed specifically for use with poison ivy.
“All are way better than doing nothing,” Ms. Pell said, although none was a runaway winner.
Another study she cited compared the traditional remedy of mashed-up jewelweed (native Impatiens capensis) with products containing its extracts, and with Dawn dish soap. The extract products performed worse than the mash or the soap.
Ms. Pell is in the soap-and-water camp — cold water, specifically. “I lather up with cold water at first, so I don’t melt and thin the oils, and spread them over my skin further,” she said.
Margaret Roach is creator of the website and podcast A Way to Garden, and a book of the same name.