When I was 10, I was regularly visited and teased on the playground by a scowling bully and his side-kick, who would subject me to all kinds of verbal abuse. I found his behavior utterly bewildering, since I’d never done anything to him. I was not the kind of kid who was taught to fight back, so I just took what he dished and continued to be the nice girl I’d been taught to be. One day, the bully and side-kick came up to me on the playground, in one of my favorite spots under the trees’ shade, where I liked to pick mulberries and eat honeysuckle. “Watch out,” I said, pointing at his feet, “You’re standing right by some poison ivy.”
“THAT’S not poison ivy,” said the bully, “You’re STUPID.” And, he picked the indicated plant and began rubbing it all over his arms and face. Side-kick followed suit. I watched, stunned. Why would anyone even risk it? What did he gain from rubbing a potentially poisonous plant all over himself? I walked away with my friend Brielle. I shrugged, “Just wait 10 days.”
Even then, I was a pretty decent playground botanist. A couple weeks later, the bully and side-kick team were temporarily hospitalized and put on steroids for a terrible case of poison ivy. When they came back to school covered in a horrible rash, they scowled and avoided eye contact with us while Brielle and I danced around them singing the song “Poison Ivy”. They never bothered us again. And so, I unwittingly tasted the sweetness of vengeance, sealed my fate as a botanist, and discovered that poison ivy was, in fact, a useful plant.
Which leads me to the question for this post: How is poison ivy used, both by humans and by other organisms?
I found out that lots of organisms use poison ivy. Humans are the only organisms allergic to it, so many animals and fungi eat poison ivy. In fact, poison ivy depends on a variety of pollinators for flower fertilization and also on birds that eat its berries to disperse its seeds. A few organisms even specialize on poison ivy. For example, the poison ivy sawfly (Arge humeralis) feeds solely on plants in poison ivy’s genus Toxicodendron. Also, a parasitic fungus called Pileolaria brevipes has only been found infecting this genus. Poison ivy plants infected with this fungus are less likely to flower than uninfected plants. White-tailed deer eat poison ivy, as do cattle and goats. In particular, goats are great at controlling poison ivy (if that is your goal), known to reduce flowering and area covered by poison ivy. One study found that black walnut and sassafras may inhibit poison ivy germination, so planting those trees may help keep the noxious ivy at bay.
Since poison ivy causes allergic reactions in humans only, do any animals take advantage of the potential avoidance of the plant by humans? Few people have studied this. The only suggestive account I found was a study that found that almost half of all red-shouldered hawk nests occurred in trees with dense growth of poison ivy. Whether this could be for visual obstruction or to discourage humans from coming near obviously would be difficult to test directly.
Native Americans and, later, Euro-Americans had a variety of usages for poison ivy. Some uses are more speculative than others, but poison ivy’s properties have been listed as medicinal, culinary, staining, and poisonous. Medicinal properties of poison ivy are difficult to prove, since the potential allergy-inducing side effects of the plant pose an obvious hurdle for clinical trials in humans. Those familiar with the herbal remedy world may have heard that eating small amounts of poison ivy will reduce or alleviate allergic reaction to the plant. In the few studies that have tested this hypothesis, either with oral or skin exposure, the outcome is not predictable, with most people tested unaffected by the exposure to poison ivy, some successfully reduced in allergic reaction, and others having adverse reactions. Historically, poison ivy was used to treat a variety of skin ailments. While poison ivy may have anti-cancer, painkilling, and immunity boosting effects, the lack of studies and conflicting results of the few studies performed do not indicate anything sure.
People also used poison ivy as a dye. Since poison ivy sap oxidizes to a dark color, poison ivy was once used for tattooing and dying baskets and cloth. Since the staining effect is caused by the same compound that causes allergic reactions, urushiol, stained items might also cause allergic reactions. (A great and subversive way to wreak vengeance on your enemies…) Some Native American tribes purportedly dipped the ends of their arrows in poison ivy resin, but whether for practical or religious reasons (or both) is unknown. Clearly, they also had some bullies to deal with, and saw the utility of this plant with a vengeance.