Poison ivy grows as a perennial vining weed in the southern, eastern and Midwestern United States, and as a shrub in the Great Plains, upper Midwest and Canada. The plant's leaves usually branch from the stem in clusters of three; hence the cautionary rhyme "leaves of three, let them be." Some specimens, however, will show groupings of five, seven or nine leaves. The foliage may be reddish-green in early spring, but turns a shiny green or yellow-green as the season wears on. As the plant approaches dormancy in autumn, the leaves will turn red or orange and then ultimately drop with the onset of winter. Young plants do not flower as a rule, but older, established ones will produce pale green flower clusters and small, white berries favored by many species of birds. Unlike the leaves, the berries typically cling to the plant through the winter. Another difference between young and older plants is that the leaves of younger specimens are often notched in a saw-toothed pattern along the edges, while older specimens have nearly oval-shaped leaves with uniform edges and pointed tips.

When poison ivy emerges as a vine, it starts as a ground cover and spreads until it finds something solid to climb, such as a tree, fence, trellis or telephone pole. Favored habitats include wooded areas, the outskirts of fields and near fence lines. The plant prefers shade, but can manage quite well in partial or even full sunlight.