Poison oak

Poison oak is a deciduous shrub native to North America. Its leaves contain a compound that causes a rash on human skin. Poison oak can specifically refer to:

• Toxicodendron diversilobum, which grows in Western Coast of North America
• Toxicodendron pubescens, which grows in Eastern North America

Poison oak (Toxicodendron diversilobum) and its eastern counterpart poison ivy (T. radicans) are two of the North American plants most painful to humans. Note: These species were formerly placed in the genus Rhus. Poison oak and a related, look-alike shrub, Rhus trilobata belong to the sumac family (Anacardiaceae).

Poison oak is widespread throughout the mountains and valleys of California. It thrives 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.

Rocky Mountain poison oak (Toxicodendron rydbergii) occurs in canyons throughout the western United States and Canada. Because the two species of western poison oak look like a vine as they develop, some authors list poison oak as a subspecies of eastern poison ivy. Poison oak often grows like a climbing vine.

The cautionary rhyme "leaves of three, let it be" applies to poison oak, as well as to poison ivy (Toxicodendron radicans).