Poison oak and poison ivy are similar in many ways. They both contain a chemical called urishiol, a poisonous sap that causes extreme itching and discomfort. Both plants are members of the Anicardiaceae family. Along with these similarities, there are some notable differences between poison oak and poison ivy.
Poison ivy grows as a shrub or vine. The shrub form of poison ivy has a set of shiny green leaves accompanied by a set of three smaller leaflets. This characteristic popularized the saying "Leaves of three, let it be" as a means of identifying this plant. The center leaflet is more pronounced, possessing a longer stem than the other leaves. The leaves are irregular and oblong in shape. Poison ivy leaves are arranged alternately on the leaf, meaning that each leaf is positioned opposite from another. Leaves are typically two to four inches in length. Poison ivy produces waxy, white berries.
Poison oak shrubs, like poison ivy shrubs, have three leaves. The leaves of the poison oak plant closely resemble the leaves of an oak tree. They are oblong and deeply lobed. Depending on the location and season, the leaves may be bright green to red in color. They produce tan berries which turn darker in coloring in late summer. Leaflets may be hairy on the underside and shiny on top. Poison oak leaves range in size from one to four inches in length.
Poison ivy vines attach to the trunks of trees. They become firmly rooted to the tree with aerial rootlets, which allow them to form a secure attachment and give the vines a hairy appearance. The hair-like rootlets of a poison ivy vine range in color from red to gray.
The vine form of poison oak is similar in appearance to the shrub form. The main difference between the shrub form of poison oak and the vine form is that the vine form clings to trees and nearby foliage.
Poison ivy prefers wet, marshy conditions. Poison oak prefers dry, grassy terrain. Both can be found in a number of different terrains. Poison ivy is distributed across every continent of the world except Antarctica and in every state of the United States except for Alaska and Hawaii. Poison oak is found almost exclusively in the western United States, but plants have occasionally been spotted in the eastern United States. In addition to this marked geographic boundary, poison ivy is rarely found at altitudes above 1,500 feet while poison oak can thrive in altitudes of up to 4,000 feet.