Poison Ivy
“Leaves of three, let them be."
Close up of typical compound leaf of poison ivy. Credit: SWMNPoliSciProject. CC BY 3.0 | Many of you will recognize this rhyme, alerting us to avoid leaves of the plant we commonly know as poison ivy. It draws attention to the distinctive three leaflets that make up poison ivy’s compound three-part leaf. Botanists have assigned this plant the scientific name Toxicodendron radicans, a name that itself warns of poison ivy’s toxic effects. The genus, Toxicodendron, translated from Greek, means “poison tree.” It is derived from two words—toxikos, or “poison,” and dendron, meaning “tree.” The species ending, radicans, comes from the Latin radicor, “to take root.” The common name “poison ivy” highlights the potential toxic effects of contact with the plant’s leaves, which typically produce an intensely itchy skin rash and blisters. The name also refers to one of the plant’s common growth habits—that of an ivy-like trailing vine. |
The cashew is a member of the plant family Anacardiaceae, which also includes poison ivy. Credit: Abhishek Jacob. CC BY-SA 3.0 | From a scientific perspective, the comparison with ivy is misleading, as poison ivy is not an ivy at all. True ivies are in the genus Hedera in the plant family Araliaceae, which also includes ginseng. Poison ivy is in the family Anacardiaceae, which includes sumacs, mangos, pistachios, and cashews. Many members of this family, like poison ivy, contain an oily substance known as urushiol, which can provoke allergic skin reactions in many people. Two of poison ivy’s closest relatives are poison oak, Toxicodendron diversilobum, and poison sumac, Toxicodendron vernix, both of which contain urushiol and cause adverse skin reactions. Did you know? In 1609, English Captain John Smith, known for his association with Jamestown and Pocahontas, noted the presence of a plant found in the New World that was similar in appearance to English ivy, but that caused redness, itching, and blisters. |
“Everywhere the devil spits, poison ivy grows.”
—Alan Chadwick, horticulturalist and organic gardening pioneer
Poison ivy is a ubiquitous species that thrives in a variety of habitats from woodlands, to fields, beaches, and gardens and on trees, telephone poles, signs, and fences. The plant has no strong soil preferences and, as long as there is sufficient water and sun, it will grow and flourish.
Poison ivy can fool us. It is as adaptable and variable, as it is pervasive. This plant shows a variety of growth habits depending on its environmental circumstances. It can appear as a low-growing herb, a vine that creeps along the ground, or a thick, hairy woody plant climbing up the trunk of a tree. Its leaves can vary from smooth and round to narrow and sharp. The leaves are often shiny reddish in color when they are young and in fall they turn yellow or deep red like other fall foliage.
Shiny, young reddish poison ivy plant. Credit: Famartin. CC BY-SA 4.0 | 
Woody vine of poison ivy. Credit: NatureFramingham CC BY-NC-ND 2.0 | 
Fall poison ivy plant. Credit: Zen Sutherland. CC BY-NC-SA 2.0 |
Did you know? Poison ivy can be confused with other harmless plants that show some superficial similarities. Virginia Creeper, box elder seedlings, and young jack-in-the-pulpit plants have sometimes been mistaken for poison ivy.
Virginia creeper, Parthenocissus quinquefolia. Credit: Sesamehoneytart. CC BY-NC-SA 3.0 | 
Young box elder, Acer negundo. Credit: Robert Stevenson. CC BY-NC 2.0 |
“One man’s poison ivy is another man’s spinach.”
—George Ade, American writer
The urushiol-induced contact dermatitis provoked by poison ivy typically persists for 10-21 days. Credit: Centers for Disease Control. Public Domain. | As many of us already know, people and poison ivy are not a good combination. After making contact with this plant, most people (85%) will have an allergic response in the form of itchy contact dermatitis (skin rash). This reaction is caused by the plant’s release of an oily substance known as urushiol, which is absorbed by the skin and provokes redness and blistering within 12–48 hours. The severity of the reaction depends both on the size of skin area exposed to urushiol and an individual’s sensitivity to the substance. Did you know? Global climate change is stimulating poison ivy to grow better and is increasing the allergenic properties of urushiol. The rise of carbon dioxide in our atmosphere is enabling poison ivy to grow about 149% faster than normal and rising global temperatures are increasing the fluidity of urushiol, allowing it to be more readily absorbed by human skin. Learn more |
The small white fruits of poison ivy. Credit: Wendell Smith. CC BY 2.0 | From the human perspective, poison ivy might appear to lack any redeeming qualities. At best, it seems like a useless irritant and at worst, a danger to human health. However, we humans are almost unique in our sensitivity to this plant. Many other animals, such as white-tailed deer, eastern cottontails, muskrats, and a variety of insects rely on poison ivy leaves as a food source. Birds, raccoons, and fruit-eating animals feed on the plant's small white fruits and then distribute the poison ivy seeds to new habitats. Did you know? Burning poison ivy is dangerous! Never burn poison ivy, as it will release urushiol into the smoke and allow it to be carried in air. When urushiol is inhaled it can cause a severe, life-threatening allergic response. |
Slideshow Image Credits
Toxicodendron radicans (L.) Kuntze by USDA NRCS PLANTS Database. Public Domain; Toxicodendron radicans vine by Melissa MB Wilkins, Wikimedia Commons. CC BY 3.0; Toxicodendron radicans by tylermasten, iNaturalist.org. CC BY-NC 4.0; Toxicodendron radicans by Judith Lopez Sikora, iNaturalist.org. CC BY-NC
