Leaves of three, let it be!

By Elizabeth A. Gjieli

Jul 23 2021

If you enjoy spending time outdoors, you surely have heard the age-old adage “leaves of three, let it be.” Poison ivy, or Toxicodendron radicans, with its characteristic three-leaved appearance (more accurately described as three-leaflet, as the plant has compound leaves), causes an irritating rash when touched. This noxious plant pops up in early spring and tends to stick around until winter and is quite versatile - the liana can climb up trees and vertical substrates, may take the form of a shrub, or can grow in clusters on the ground. Poison ivy tends to exploit disrupted habitats, which is why the plant is often found lurking along paths in the woods or around edges of parks or developments. In North America, poison ivy affects an estimated 25-40 million people yearly (Weisburg, 2017).

The itch-inducing chemical in the sap of poison ivy is called urushiol (pronounced: oo-roo-shee-ohl), stemming from the Japanese word for lacquer (Wilson, 2005). Rikō Majima, the Japanese chemist who first identified the irritant of poison ivy, found that urushiol is not a single substance but a mixture of closely related compounds called alkyl catechols, with molecules that have a greasy, water-repellent tail. This tail allows urushiol to penetrate skin and remain potent on clothing and other substances for months to years (Boyd, 2013). Urushiol is also what poison oak and poison sumac plants produce to trigger allergic reactions.

Historically, botanists, physicians, and chemists have been intrigued by toxic plants, believing that medicinal treasures may be unlocked from various parts. Early European explorers of the New World, such as Captain John Smith, believed poison ivy to be an interesting exotic plant, and around the 1640s travelers had carefully gathered and sent the plant and its seeds to Europe to be cultivated for research at Kew Gardens and a few other locations. Around 1780, French army physician and medical professor André-Ignace-Joseph Dufresnoy created a distillation of poison ivy leaves for internal consumption. This concoction was prescribed to people suffering from a variety of skin ailments and to those with leg paralysis, and was said to have mild side effects such as slight stomach discomfort, and increased urination and perspiration (Boyd, 2013). The ingestion of poison ivy was not found to have significant impacts in relieving ailments, and needless to say human consumption of the plant did not persist.

Although humans have an adverse reaction to this plant, many animals benefit from poison ivy. The plant can be a valuable food resource for over sixty bird species, especially during the colder months when food is scarce. Poison ivy produces a white berry in the fall, which is a great source of fat for migratory and over-wintering birds. Downy and red-bellied woodpeckers, wild turkey, and eastern bluebirds are all known to consume the fruit of this plant when over-wintering in the Northeast (Brandywine, 2011). White-tailed deer, black bear, racoons, and goats are also known to graze on poison ivy leaves, berries, and stems (TARA, 2016). The plant is also known to provide cover for small mammals, amphibians, and reptiles; and a variety of insects feed on the flowers of the plant. 

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Boyd, Jane E., and Joseph Rucker. 2013. No Ill Nature: The Surprising History and Science of Poison Ivy and Its Relatives. Distillations: Using Stories from Science’s Past to Understand Our World. Science History Institute. Philadelphia. 2 June 2013. Available at: https://www.sciencehistory.org/distillations/no-ill-nature-the-surprising-history-and-science-of-poison-ivy-and-its-relatives (accessed: 16 July 2021).

Brandywine Conservancy. 2011. Leaves of Three Let it Be. Brandywine Conservancy Blog. 22 September 2011. Available at: https://www.brandywine.org/conservancy/blog/leaves-three-let-it-be (accessed: 15 July 2021).

TARA Wildlife. 2016. Wildlife Value of Poison Ivy. 21 December 2016. Available at: https://www.tarawildlife.com/wildlife-value-poison-ivy/ (accessed: 16 July 2021).

Weisberg, A. et al., 2017. Sequencing and De Novo Assembly of the Toxicodendron radicans (Poison Ivy) Transcriptome. Genes, 8(11), p.317. Available at: http://dx.doi.org/10.3390/genes8110317 (accessed 15 July 2021).

Wilson, Stephanie. 2005. How Poison Ivy Works. HowStuffWorks.com. 23 September 2005. Available at: https://science.howstuffworks.com/life/botany/poison-ivy.htm (accessed: 15 July 2021).