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Leaves of Three: A Closer Look at Poison Ivy

By Shirin Kaye

Some enjoy summertime outdoors by soaking in sunshine at the beach, but if you had to choose an activity, you would want to hike in the woods. You especially love hearing crunching twigs beneath your feet, leaves rustling in a breeze, and birdsong from surrounding trees. After an invigorating day of admiring the Pennsylvanian flora, you bend to scratch at odd bumps on your ankle and startle as you spot a glimpse of low green leaves alongside the trailhead. You move to look closer and gasp: it is poison ivy! Spotting the infamous leaves-of-three brings questions to your mind: how did it get here? Will it plague you? How can you prevent that? What is its purpose?

Poison ivy (Toxicodendron radicans) is a plant native to North America. Despite its name, poison ivy is not an ivy; unlike the invasive English ivy, you will not find it climbing the remains of an old building. Poison ivy belongs to the Anacardiaceae family, which also includes mangos, cashews, and pistachios. When in contact with humans, urushiol – a compound in the sap that covers the leaves, stems, and roots – can spark an auto-immune response. This allergic reaction features an itchy, red rash with fluid-filled bumps. A fast-spreading perennial (a plant that returns year after year), poison ivy can reproduce by its clustered white fruits, called drupes.

Despite people’s negative regard toward poison ivy, the plant has a positive impact on its ecosystem. Most animals, such as birds, deer, rodents, and insects, are not allergic to poison ivy and eat it. In particular, poison ivy has a symbiotic relationship with birds. Unlike in other animals, the drupes’ seeds retain the ability to germinate after being eaten and voided by birds. The plant is transported by and therefore able to reproduce thanks to birds. In return, birds turn to poison ivy’s fruits in the winter, once they exhaust the sugary, brightly-colored fruits in the early fall. “They’re really lipid- or fatty-rich,” explains Amber Stanley, a Ph.D. candidate who studies T. radicans at the University of Pittsburgh. Therefore, they make a good energy source, but are “not everybody’s favorite snack, so it’s like that emergency food source for birds.”

Another prominent ecological role of poison ivy is in succession. Succession is a “directional, predictable change in community structure [especially its vegetation] over time,” according to the scientific journal Nature, that is set in motion by disturbance. A disturbance is any natural or man-made event that partially or entirely reduces the plant community in an area — like fire, mowing, or logging. The first plants to arrive after a disturbance are called early successional species. They establish themselves in new areas, and change the microclimate and soil to pave the way for late successional species and habitats. Similarly, animal communities that depend on the plants present differ over time. Successive changes in plant and wildlife populations re-establish a “climax community,” the final stage of succession. The result can be a prairie, mature forest, or other habitats, which can develop through hundreds of years naturally, or be sped up through restoration ecology efforts. Poison ivy is an example of an early successional plant because it is dispersed into and quickly settles disturbed areas; it thrives in sunlight, but flourishes less in the shade of larger plants that come later in succession. Vine, bush, and groundcover forms of poison ivy exist, and are often found near forest edges, clearings, or roadsides, taking advantage of any sunlight accessible.

Although it is native to the East Coast, poison ivy still must compete with other plants for space and resources. It has the ability to spread rapidly in sunny areas via its fruits, vines, and roots. This might seem bad, but aggressive native plants  could be used to “fight back against” invasive, non-native species. Stanley gives the example of kudzu, a non-native vine that was initially introduced in Kentucky for roadside stabilization, but grew out of hand, blanketing buildings and killing trees. She hypothesizes poison ivy may serve as competition for kudzu, helping to control its invasive growth. Native plants are important because they support indigenous animals and pollinators within their ecosystems. For example, John Baird, a researcher in Princeton, New Jersey, observed that many resident and migrant bird species ate berries from dogwood, grapes, and poison ivy during the winter of 1974-75. Notably, poison ivy’s drupes ripened later than those of other species and “may have been the most fully used,” demonstrating the plant’s utility as a cold-weather food source when others are scarce.

The next time you spot poison ivy, do not immediately jump to disgust; rather, admire its pretty fall foliage from a safe distance. Appreciate its role in the continuous cycles of succession and in feeding animals who are already short on resources in urban areas and winter. If you must remove it, look into natural methods such as uprooting individual plants, treating the area with a homemade soap and vinegar spray, pouring boiling water over the roots, or preventing its access to sunlight. Throughout, be careful to avoid skin contact and wash all surfaces that may have touched the plant. Dangerous control methods to be avoided include herbicides – which can destroy the surrounding area if misused – and burning – which releases urushiol into the air and people’s lungs. Remember, although it could harm humans, poison ivy is critical to many animals’ and, by extension, the environment’s health.