Poison Ivy
Toxicodendron radicans
Cashew family (Anacardiaceae)

Description: This perennial plant is either a shrub up to 3' tall or a woody vine up to 60' long. The stems are initially light reddish green, but they eventually become brown and woody. The woody stems are often covered with coarse brown hairs, but sometimes they are hairless. As a vine, aerial rootlets develop along the stems; these rootlets can cling to tree bark, fences, walls, and other objects. The alternate compound leaves are trifoliate and up to 1' long; they have long slender petioles. The leaflets are up to 4" long and 2" across. They are cordate-ovate, ovate, or broadly elliptic and their margins are smooth, slightly undulate, or slightly dentate. When teeth are present, they are few in number and somewhat blunt. The upper surface of each leaf is medium green and hairless, while the lower surface is pale green and either slightly pubescent or hairless. Across from the compound leaves, panicles of yellowish green flowers are sparingly produced. These panicles are up to 4" long and across; they are often irregular in shape. Each flower is about " across, consisting of 5 green petals, 5 stamens, 5 sepals, and an ovary with a stout style. The petals are triangular-shaped and recurved, while the sepals are smaller in size and deciduous. The stamens have slender white filaments with yellowish brown anthers. The peduncles and pedicels of the flowers are often pubescent. An individual plant may have perfect flowers, staminate flowers only, pistillate flowers only, or both staminate and pistillate flowers. The blooming period occurs from late spring to mid-summer and lasts about 2-3 weeks. Each flower is replaced by a drupe that contains a single seed (stone). This drupe is dull white and about " across; it has a smooth waxy surface. The large seed is ovoid in shape and dull white; there are a few grey stripes across its surface. The drupes mature during the fall and can persist through the winter. The root system produces a woody taproot and long rhizomes; the rhizomes produce vegetative offsets.

Cultivation: Poison Ivy can be found in light shade to full sun, moist to dry conditions, and various kinds of soil, including those that contain loam, clay-loam, rocky material, or sand. This plant adapts to a variety of conditions and can be very aggressive. It does not tolerate periods of prolonged flooding.

Range & Habitat: Poison Ivy is a common plant that occurs in all counties of Illinois (see Distribution Map), where it is native. Habitats include moist to dry woodlands, sandy woodlands, woodland openings, areas along woodland paths, thickets, meadows, limestone glades, sand dunes, fence rows, and abandoned fields. This ubiquitous plant can be found in both disturbed and higher quality habitats. It usually doesn't wander far from wooded areas, although sometimes it exists as an understory plant in tallgrass prairies.

Faunal Associations: The nectar and pollen of the flowers attract bees occasionally; Robertson (1929) observed only Andrena crataegi. The caterpillars of the following moths feed on the foliage: Amorbia humerosana (Tortricid Moth sp.), Marathyssa basalis (Light Marathyssa), and Paectes oculatrix (Eyed Paectes). The seeds/fruit are eaten by many species of songbirds and upland gamebirds (see Bird Table); woodpeckers and flickers are especially attracted to the fruit. These birds help to distribute the seeds far and wide. To a minor extent, the foliage, branches, and fruit are eaten by the Black Bear, Cottontail Rabbit, and White-Tailed Deer. These animals appear to be immune to the irritating properties of the chemical agent, urushiol, in the foliage.

Photographic Location: Along an abandoned railroad near Urbana, Illinois, in an area that was partially overgrown with small trees and shrubs.

Comments: The foliage of Poison Ivy can irritate the skin of most people, causing redness and blisters. This is caused by a reaction of the immune system to urushiol. People who are immune to Poison Ivy when they are young, can become sensitive to its irritating effects when they become older. Only primates and hamsters are known to have allergic reactions to Poison Ivy, while other animals appear to be immune to its effects. The growth habit of Poison Ivy and the appearance of its leaflets can be highly variable, which sometimes confuses people regarding its identity. At one extreme, Poison Ivy can assume the form of a low little-branched shrub with trifoliate leaves, while at the other extreme it can assume the form of a long woody vine that can swallow up trees. As a vine, its woody stems have a hairy appearance from the abundant aerial rootlets and no tendrils are produced across from the trifoliate leaves. Other woody vines produce simple leaves (e.g., Wild Grapes and Moonseed) or they have compound leaves with 5 or more leaflets (e.g., Virginia Creeper and Trumpet Creeper).