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Species of plant

Toxicodendron diversilobum (syn. Rhus diversiloba), commonly named Pacific poison oak or western poison oak, is a woody vine or shrub in the sumac family, Anacardiaceae. It is widely distributed in western North America, inhabiting conifer and mixed broadleaf forests, woodlands, grasslands, and chaparral biomes. Peak flowering occurs in May. Like other members of the genus Toxicodendron, T. diversilobum causes itching and allergic rashes in most people after contact by touch or smoke inhalation. Despite its name, it is not closely related to oaks, nor is it a true tree.

Toxicodendron diversilobum is extremely variable in growth habit and leaf appearance. It grows as a dense tall shrub in open sunlight, a treelike vine and may be more than 100 feet (30 m) long with an trunk, as dense thickets in shaded areas, or any form in between. It reproduces by spreading rhizomes and by seeds. They generally resemble the lobed leaves of a true oak, though tend to be more glossy. Leaves are typically bronze when first unfolding in February to March, bright green in the spring, yellow green to reddish in the summer, and bright red or pink from late July to October.

Toxicodendron diversilobum is found in California (Los Angeles was built on the site of a village named Yaangna or iyáanga’, meaning "poison oak place"), the Baja California Peninsula, Nevada, Oregon, Washington, and British Columbia. The related T. pubescens (eastern poison oak) is native to the Southeastern United States. T. diversilobum and T. rydbergii (western poison ivy) hybridize in the Columbia River Gorge area.

Black tailed deer, mule deer, California ground squirrels, western gray squirrels, and other indigenous fauna feed on the leaves of the plant. It is usually removed by pruning, herbicides, digging out, or a combination. Poison oak is susceptible to infection by Phytophthora ramorum.

Toxicodendron diversilobum leaves and twigs have a surface oil, urushiol, which causes an allergic reaction. Most, if not all, will become sensitized over time with repeated or more concentrated exposure to urushiol. The active components of urushiol have been determined to be unsaturated congeners of 3 heptadecylcatechol with up to three double bonds in an unbranched C17 side chain. In poison ivy, these components are unique in that they contain a CH2CH2 group in an unbranched alkyl side chain.

Toxicodendron diversilobum skin contact first causes itching; then evolves into dermatitis with inflammation, colorless bumps, severe itching, and blistering. In the dormant deciduous seasons the plant can be difficult to recognize, however contact with leafless branches and twigs also causes allergic reactions. Urushiol volatilizes when burned, and human exposure to T. diversilobum smoke is extremely hazardous, from wildfires, controlled burns, or disposal fires.

Californian Native Americans used the plant's stems and shoots to make baskets, the sap to cure ringworm, and as a poultice of fresh leaves applied to rattlesnake bites. The juice or soot was used as a black dye for sedge basket elements, tattoos, and skin darkening. An infusion of dried roots, or buds eaten in the spring, were taken by some native peoples for an immunity from the plant poisons.

Toxicodendron diversilobum can be a carefully situated component in wildlife gardens, habitat gardens, and natural landscaping. The plant is used in habitat restoration projects. It can be early stage succession where woodlands have been burned or removed, serving as a nurse plant for other species.