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U.S. Forest Service

Plant of the Week

Toxicodendron rydbergii range map. Toxicodendron rydbergii range map. USDA PLANTS Database.

Toxicodendron  rydbergii. Toxicodendron rydbergii in habit. Photo © Al Schneider.

Toxicodendron rydbergii. White flowered inflorescence of Toxicodendron rydbergii. Photo © Al Schneider.

Toxicodendron rydbergii. White berries of Toxicodendron rydbergii, City Creek Canyon, Salt Lake County, Utah. Photo © Bill Gray.

Toxicodendron rydbergii. Beautiful fall colors of Toxicodendron rydbergii, but remember "Leaves of three, let it be." Photo © Al Schneider.

Western Poison-ivy (Toxicodendron rydbergii)

By Walter Fertig

As young children, we learn “leaves of three, let it be” to remind us to avoid the shiny green leaves (technically leaflets) of Poison ivy (Toxicodendron spp.).  Touching or merely brushing against poison ivy (or coming into contact with contaminated clothing or pet fur), can result in painful swelling, itching, blisters, or a rash in susceptible people.  Poison ivy’s toxicity comes from the chemical urushiol, which is found in all parts of the plant but is most abundant in its sap. Numerous folk remedies have been proposed over the years, ranging from extracts of gumweed (Grindelia spp.) to Epsom salts, and greasy ointments.  Preventing contact and thorough (but gentle) washing remain the best cures.  Surprisingly, relatively few wild animals are susceptible to poison ivy and indeed, many game species, rodents, livestock, and birds consume the foliage or white, berry-like fruits without ill effect.

Fifteen species of poison ivy, poison oak, and poison sumac are recognized in the New World and eastern Asia, of which five occur naturally in North America. Western poison ivy (T. rydbergii) is a low-growing shrub (rather than a vine, like its eastern relative, T. radicans) that occurs widely across the western states, Great Plains, and northeast.  The poison ivies are sometimes included in the genus Rhus with sumacs, but more frequently they are placed in their own genus, Toxicodendron (from the Greek for “poisonous tree”).  Both genera belong to the Anacardiaceae, a plant family that includes mango, cashews, pistachios, and a number of other species of edible trees and shrubs that are often notable for the production of resins.

Native Americans and early pioneers used poison ivy for dyes and medicines.  Historical records of Indians purposefully eating poison ivy for food or to induce immunity through self hyposensitization have recently been questioned.  Despite anecdotal reports, there is little clinical evidence that there is a difference in susceptibility to poison ivy between human races, though rare individuals can show surprising immunity.

Poison ivy sufferers may be dismayed to learn of recent research by Dr. Jacqueline Mohan and colleagues from Duke University on the impacts of enhanced carbon dioxide levels on poison ivy.  Plants grown under higher concentrations of this greenhouse gas were found to produce significantly more unsaturated urushiol (the form that is most virulent to humans) and to grow faster.  They conclude that poison ivy will become more widespread, aggressive, and toxic in the projected warmer world of the future.

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