Illinois Natural History Survey - University of Illinois

INHS Reports Summer 2001

Species Spotlight: Poison Ivy

by
Susan Post, Center for Economic Entomology

If, as a child, you are going to explore the outdoors, a series of three familiar rhymes must be learned very early--Leaves three, leave it be; Leaflets three, let it be; or Leaves three quickly flee. These rhymes may help you avoid poison ivy, and thus a painful lesson. These folk rhymes probably arose soon after the colonists' first encounter with the plant. Poison ivy is neither poison nor an ivy; instead, it is related to the sumac and belongs to the cashew family--Anacardiaceae. Its scientific name Toxicodendron radicans is intended to spell out the nature of the weed. Toxicodendron is a combination of Greek words meaning poisonous tree or plant, while radicans is Latin for rooting, referring to the roots the stems send out. Its other common names include poison vine, poison creeper, three-leaved ivy, picry, and mercury.

Poison ivy is found in every county in Illinois. With the exception of Alaska, Hawaii, Nevada, and California, it is also found throughout the United States below 5,000 feet in elevation. It follows civilization, cropping up in disturbed sites like cut banks, roadsides, and old fencerows. It prefers woodland borders and clearings, shunning dense forest.


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Poison ivy is a long-lived, vigorous, ropelike, hairy vine that can also grow as a creeper or a medium shrub. The vines can grow 75 feet long and 6 or 7 inches thick. The hairs on the vines are aerial roots that look like reddish brown fuzz on new growth, but darken with age. An old vine can look like a fuzzy rope. These hairs have an adhesive that binds the vine to whatever it climbs on. The vine doesn't spiral but grows straight up, favoring the grooves in rough bark. It has an extensive root system that grows just below ground level.

The plant is actually not three-leaved. Its leaves are compound, consisting of three leaflets to each leaf. Each leaf is attached to the stem of the plant with a long petiole (leaf stem). The leaves grow up the stems alternately, rather than paired. Whitish flowers appear during May and June. The flowers are on slender stems that grow from the angles between the leaves and woody twigs. These flowers yield tiny, berrylike drupes (stone fruit like cherries or peaches). In the winter the plant becomes leafless and dormant.

Found in the tiny subsurface duct glands of poison ivy are resinous oils called urushiol. These oils are found in the plant's stems, roots, leaves, flowers, and fruit year round, and can cause a reaction even when the plants are dormant or a long time after the plant is dead. Urushiol is insoluble in water, resists drying, and stores very well. To get a rash (an allergy attack) you must touch the oil, which usually requires bruising the plant. Many humans (75-80%) are potentially allergic to these oils. The only sure remedy is to recognize and avoid the plants.

For those who are allergic to the plant, its benefits are often overlooked. Poison ivy is an early colonizer, often taking hold in the scars we leave and beginning the slow process of rebuilding the landscape. The plant requires very little nourishment or moisture (less than 10 inches yearly). It is virtually pest free, the roots provide erosion control, and it attracts and sustains wildlife. Though no poison ivy occurs in England, it was imported for its fall color. It was subsequently introduced into Australia and New Zealand where the plants act as a garden backdrop. Perhaps they also help keep the local dermatologist in business.

Charlie Warwick, editor



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