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Pacific poison oak, Toxicodendron diversilobum (also known as Rhus diversiloba), is an angiosperm of the Anacardiaceae (sumac) family that is confined to western North America and is native to British Columbia, Washington, Oregon, northwestern Nevada, and California, where it grows most prolifically (USDA 2016). It is not a true oak and is closely related to poison ivy (Toxicodendron radicans), its counterpart in eastern North America (NPIN 2007). It is a many stemmed, deciduous shrub or woody vine that can survive and flourish in conditions from part shade to full sun. As a shrub it grows 2-6 ft. tall, and as a vine it climbs trees or other support by adventitious roots and/or wedging stems within grooves or crevices of the support. Vine stems typically grow 10-30 ft., but may be as long as 100 ft. (Gartner 1991). The leaves are bright green (often red in fall) with three or five round to ovate, diversely lobed or toothed leaflets that usually resemble oak leaves (Munz 1974). Leaves are usually glossy and both they and the other parts of the plant are covered with a toxic oil (urushiol) that causes a sometimes severe skin rash, at least in humans, although deer and livestock often browse the plant with impunity (NPIN 2007). Poison oak flowers in the summertime, with clusters of white flowers blooming from leaf axils. The fruits are small green or white berries (Munz 1974). As a perennial, T. diversilobum regrows and expands on its previous branches each year, and it is a hardy plant that is one of the first to regrow after forest fires.
Poison oak thrives in mixed coniferous forests, shaded woodlands, and chaparral, as well as in open fields, growing most thickly near streams or springs where it has unlimited access to water. It is common in most warm-climate woodlands and generally cannot survive at elevations above 5000 feet. Often, poison oak dominates the undergrowth, smothering surrounding trees and ground level vegetation with its sprawling vines. Its hardiness and ability to proliferate, coupled with its danger to humans, contributes to the almost universal perception of poison oak as a pest plant (Howard 1994).