Poison oak is a widespread deciduous shrub throughout mountains and valleys of California, generally below 5,000 feet elevation. In shady canyons and riparian habitats it commonly grows as a climbing vine with aerial (adventitious) roots that adhere to the trunks of oaks and sycamores. Poison oak also forms dense thickets in chaparral and coastal sage scrub, particularly in central and northern California. It regenerates readily after disturbances such as fire and the clearing of land. Rocky Mountain poison oak (Toxicodendron rydbergii) occurs in canyons throughout the western United States and Canada. Because the two species of western poison oak often exhibit a viny growth form, they are listed as subspecies of eastern poison ivy by some authors.
