Toxicodendron diversilobum
Toxicodendron diversilobum (syn. Rhus diversiloba; Western Poison-oak or Pacific Poison-oak) is found only on the Pacific Coast of the United States and of Canada. It is extremely common in that region, where it is the predominant species of the genus; the closely related Atlantic Poison-oak (T. pubescens) occurs on the Atlantic Coast.

# Appearance
Western Poison-oak is extremely variable in growth habit and leaf appearance. It grows as a dense shrub in open sunlight, or as a climbing vine in shaded areas. Like Poison ivy, it reproduces by creeping rootstocks or by seeds. The leaves are divided into 3 leaflets, 312 to 10 centimeters long, with scalloped, toothed, or lobed edges- generally resembling the leaves of a true oak, though the Western Poison-oak leaves will tend to be more glossy. Leaves are generally bright green in the spring (or bronze when first unfolding), yellow-green to reddish in the summer, and bright red or pink in the fall. White flowers form in the spring and, if fertilized, develop into greenish- white or tan berries. Toxicodendron diversilobum is winter deciduous, so that after cold weather sets in the stems are leafless and bear only the occasional cluster of berries.  Without leaves, poison oak stems may sometimes be identified by occasional black marks where sap may have oozed and dried.
- Green phase
Green phase
- Red phase
Red phase
- Berries
Berries

# Habitat
Western Poison-oak is found only on the Pacific Coast, where it is common, and ranges from Southern Canada to the Baja California peninsula. It is one of California's most prevalent woody shrubs but also climbs, vine-like, up the sides of trees.  The plant is often found in oak woodlands and Douglas-fir (Pseudotsuga) forests.  Along the Avenue of the Giants in northern California, the vine form may be seen climbing many feet up the trunks of Coast Redwoods (Sequoia sempervirens).  It can also be found in damp, shady areas near running water and out of direct sunlight.  Any trail leading to a waterfall on California's coast will most likely be home to western poison-oak.

# Toxin
Western Poison-oak leaves and twigs have a surface oil, urushiol, which causes an allergic reaction. Around 15% to 30% of people have no allergic response, but most if not all will become sensitized over time with repeated or more concentrated exposure to urushiol.
For those who are affected by urushiol, it causes a very irritating rash. In extreme cases, corticosteroids are needed to treat rashes and severe itching.  The first symptom of contact is a severe itching of the skin that develops into reddish colored inflammation or non-colored bumps, and then blistering of the skin occurs. In severe cases, clear fluids ooze from open blistered sores. Once the urushiol poison has had contact with the skin, it is quickly bound to the skin. The poison can be washed off within a short time after contact, but once bound to the skin, it cannot be washed away.
After the urushiol is removed, the rash cannot usually be spread by contact with an affected area or by scratching. The oozing fluids released by itching blisters do not spread the poison. However, scratching can open the skin especially in cases with significant blistering, making it possible for the skin to become infected by opportunistic bacteria (known as a secondary infection). The appearance of a spreading rash indicates that some areas received more of the poison and reacted sooner than other areas. The blisters and oozing result from blood vessels that develop gaps and leak fluid through the skin; if the skin is cooled, the vessels constrict and leak less. If poison oak, ivy, etc., is burned and the smoke then inhaled, this rash will appear on the lining of the lungs, causing extreme pain and possibly fatal respiratory difficulty. If eaten, the digestive tract, airway, kidneys or other organs can be damaged.
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.

# Treatment
For a discussion of prevention and treatment options, see the

# Quotation
"In spring the ivory flowers bloom on the sunny hill or in sheltered glade, in summer its fine green leaves contrast refreshingly with dried and tawny grassland, in autumn its colors flame more brilliantly than in any other native: but one great fault, its poisonous juice, nullifies its every other virtue and renders this beautiful shrub the most disparaged of all within our region."

# Note on name hyphenation
The hyphenated form "Poison-oak" is used, rather than "Poison Oak" to clearly indicate that it is not a variety of oak, just as "Poison-ivy" is not a variety of ivy.