Poison ivy

Toxicodendron radicans
Family: Anacardiaceae

Natural History
Fall colors of poison ivy
Photo credit: Steve Dewey, Utah State University, Bugwood.org

Poison ivy grows on a wide range of habitats, from moist and shady to open and dry. Poison ivy is found in most of the southeast, between Arizona and Florida in the south, to Nebraska and Canada in the north.

Poison ivy sap causes an allergic reaction in humans. The active agent is secreted from broken resin ducts. Most plant parts contain a chemical that causes itching, swelling, rashes, and blisters.

Plants are variously poisonous depending on the time of year and plant maturity, and people vary in susceptibility. In winter, theplant is leafless and dead looking but is actually alive and still very toxic.

Animals are generally not susceptible to the toxic chemical in poison ivy. A wide variety of birds consume the fruits. The leaves and fruits are some of the most important foods of white-tailed deer. The sap is used to make indelible ink and the plant is used to prevent erosion on sand dunes due to its tough root system.

Poison ivy grows straight up a tree without winding around it. Poison oak (Toxicodendron pubescens) is similar but only grows as a shrub up to 3' tall, usually in drier soils than poison ivy. Poison oak leaves have more wavy margins than poison ivy.


Identifying Characteristics

Habitat: Poison ivy can be found on a wide variety of sites from moist and shady to open and dry. Across this wide range of habitats it most frequently occurs in shady forests.
Size/Form: It grows as a high-climbing woody vine or a small shrub that grows along the ground. Poison ivy vines climb up trees by aerial roots and grow up to 150' in length. Branches from the vine may deceptively look like branches of the tree. Poison ivy shrubs grow from 1½' to 6½' high.
Stem: New stems are gray-brown and hairy but they gradually turn hairless with age. Older stems can be 2" to 4" wide.
Leaves: The leaves are compound, alternate, and deciduous. They have three leaflets that are usually 2" to 8" long and 1" to 5" wide, and ovate to elliptic in outline. The thin leaflets have entire to serrate to shallowly lobed margins and the lower leaf surface is light green and slightly hairy. The leaves usually have a longer, symmetric, terminal leaflet often with one lobe on each side of the leaflet. The two, asymmetric leaflets on opposite sides of the terminal one have a single lobe so they look like mittens with a thumb.
Fruit: The fruit is a dry, round drupe about ¼" wide, tan or whitish, and grooved. The seeds have an oily covering and are primarily dispersed by animals. Since the covering is buoyant, the fruit is also dispersed by waterways.



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