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Pacific Poison Oak – Toxicodendron diversilobum

Pacific Poison Oak


Pacific poison-oak, a member of the sumac family, is a deciduous plant that grows throughout many parts of west coast. Urushiol, found on the stems, berries and leaves is the main component of the oily resin that causes rashes and blistering. Poison oak can survive under a wide range of temperatures, elevations, soil types, moisture conditions,


Western poison oak is variable in plant growth and leaf appearance. It can grow as a dense shrub, a tree with a 3” – 8” trunk or as a climbing vine.
When Pacific poison-oak grows as a shrub, it can reach up to 13 feet tall. When poison oak grows as a vine or tree, stems can reach up to 82 feet long. Twigs can be hairless to sparsely hairy and gray to reddish brown.

Leaves, generally resembling the leaves of a true oak, consist of three, and sometimes up to five leaflets but three leaflet leaves are most common. Leaf edges can be smooth, wavy, or have slightly rounded lobes. The upper leaf surface is hairless, or nearly so, and usually slightly glossy. The lower surface usually has sparse, short hairs. Leaves turn bright red in the autumn.

White flowers form in the spring in leaf axils, where the leaf meets or connects to the stalk, and white or tan berries usually form later in the summer

Without leaves, poison oak stems may sometimes be identified by occasional black marks where its milky sap may have oozed and dried.
Location: as the map indicates, western poison oak is found only on the Pacific Coast of the United States and of Canada. southern Canada to the Baja California peninsula.


Urushiol binds to the skin on contact, where it causes severe itching that often develops into a red rash or flesh colored bumps and blistering. Symptoms generally appear 12 to 48 hours later. The rash can be treated with Calamine lotion or other over the counter remedies such as oatmeal baths and baking soda. In severe cases hospitalization may be required or if the plant has been ingested.

Urushiol oil can remain active for several years, so handling dead leaves or vines can cause a reaction. In addition, oil transferred from the plant to other objects (such as pet fur) can cause the rash if it comes into contact with the skin.
Severe respiratory irritation can be induced by breathing the smoke from burning plant material. Repeated exposure often results in increased sensitivity.