Poison oak or western poison oak, Toxicodendron diversilobum, is native to western North America, with a distribution extending from British Columbia south to the Baja California peninsula. In Washington and Oregon, poison oak is found mainly in the western regions of the states. In California it is widespread and grows in a wide range of habitats from sea level to the 5000-foot elevation, including open woodland, grassy hillsides, coniferous forests, and open chaparral.
Poison oak is a deciduous (loses leaves in winter), woody plant that can have a shrub or vine form. In open areas under full sunlight, poison oak forms a dense leafy shrub usually 1' — 6' in height. In shaded areas, such as in coastal redwoods and oak woodlands, it becomes a much taller climbing vine, supporting itself on other vegetation or upright objects by means of aerial roots.
Leaves normally consist of three leaflets with the stalk of the central leaflet being longer than those of the other two; however, occasionally leaves are composed of five, seven, or nine leaflets. Leaves of true oaks, which are superficially similar, grow singly, not in groups. Poison oak leaves are alternate on the stem. Each leaflet is 1" — 4" inches long and smooth with toothed or somewhat lobed edges. The diversity in leaf size and shape accounts for the Latin term diversilobum in the species name.
The surface of the leaves can be glossy or dull, sometimes even somewhat hairy, especially on the lower surface. In spring, poison oak produces small, white-green flowers at the point where leaves attach to the stem. Whitish-green, round fruit form in late summer. In early spring the young leaves are green or sometimes light red. In late spring and summer the foliage is glossy green, and later turns attractive shades of orange and red.