To avoid the itch of poison ivy, ‘Leaves of three, let it be’
Afew years ago I was leading a group of Merrimack College ecology students on a nature walk, when one of them pointed to a vine with five leaflets per leaf, and asked if it was poison ivy. “No,” I replied. “It’s Virginia creeper.” Students are often afraid of poison ivy because of the rash it can cause, so I always tell them how to identify the common plant. Poison ivy has compound leaves — leaves that are composed of several distinct leaflets connected to a single stem. Poison ivy leaves have three leaflets per leaf, with the middle leaflet having a longer stalk than the two side leaflets. There’s even a rhyme to remind people what poison ivy leaves look like — “Leaves of three, let it be.”
I also tell students that poison ivy leaves are often shiny.
But not always, explained Brendan Keegan, a horticulturist at Harvard University’s Arnold Arboretum in Boston.
“There’s a lot of variability with poison ivy leaves,” said Keegan. “The leaves are often glossy, but they could be dark or light green. And some leaves have smooth edges, while some have jagged edges. One thing most poison ivy leaves have in common is a red mark in the middle of the leaflets, where they meet on the [stalk].”
Another way poison ivy varies is in its growth forms, Keegan explained.
“It’s a vine, so it likes to climb,” said Keegan. “It climbs vertically, directly up tree trunks, clinging to the tree’s bark with small, hairy roots.”
But if there’s nothing to climb, poison ivy will form a mat, carpeting a large area on the ground, or it will become shrubby, like a bush, Keegan said.
But no matter what form it takes, poison ivy can cause a red, bumpy, itchy, allergic rash with fluid-filled blisters on most people who touch it, according to the Massachusetts General Hospital webpage on poison ivy, oak, and sumac rash. The cause of the rash is an oil called urushiol.
Contrary to popular belief, the poison ivy rash does not spread from one person to another by touching the blisters, or the fluid that oozes from the blisters, said MGH. Rather, urushiol can remain on garden tools, clothing, shoes, or other surfaces, where it can remain active for up to a year, and can be spread to other people. Likewise, urushiol can be on the fur of dogs and cats, and spread to people from contact with their pets.
A Smithsonian article on poison ivy said, “Humans and possibly a few other primates are the only animals that get a rash from poison ivy.”
Treatment usually consists of measures to decrease itching, said MGH, such as calamine lotion, steroid creams, or in severe cases, oral or injectable steroids. Urgent treatment may be needed if a person experiences a severe reaction and swelling.
It’s not just poison ivy leaves that harbor urushiol. Stems, roots, and even smoke from burning poison ivy can cause a reaction, at any time of year, Keegan said.
The Mass Audubon webpage on poison ivy recommends washing with soap and water after possible contact with poison ivy, as a way to remove the urushiol and hopefully avoid a rash.
Keegan said in addition to trying to avoid poison ivy and washing equipment after use, the horticulturists at the Arnold Arboretum use a product called Tecnu, a lotion that breaks down poison ivy oils and reduces the chance of getting a reaction from exposure.
Poison ivy plants can be removed mechanically or chemically if they’re in an area where people are likely to come in contact with them, said Keegan, but he recommends leaving them alone in ecosystems like forested areas.
Poison ivy is something to appreciate in the landscape, Keegan explained. It doesn’t kill trees it climbs on, and it’s important in ecosystems as a food source for animals such as some bird species, which eat the clusters of small white poison ivy berries, especially in winter when other food sources are scarce.
“I’ve seen downy woodpeckers and yellow-rumped warblers eating them regularly,” said Wayne Petersen, Mass Audubon’s director of the Massachusetts Important Bird Areas Program, “and a longer list includes some of the following: bobwhite quail, brown thrashers, northern mockingbirds, hairy, pileated, and red-bellied woodpeckers, yellow-bellied sapsuckers, northern flickers, cedar waxwings, black-capped chickadees, blue grosbeaks, eastern bluebirds, and gray catbirds.”
Mass Audubon said some mammals, including cottontails, deer, and goats, eat poison ivy leaves.
Richard Primack, a biology professor at Boston University, said the small greenish-white poison ivy flowers are pollinated by honeybees, small solitary bees, and flies.
Keegan said he likes poison ivy, despite currently having a poison ivy rash from work.
“It’s a fascinating species whose defense just requires respect,” said Keegan. “It also has underappreciated red, orange, and yellow fall colors, and wildlife value. Just because it’s hazardous for people doesn’t make poison ivy inherently bad or evil, as people often view it.”