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Featured Plant

Toxicodendron diversilobum


Scientific Name: Toxicodendron diversilobum

Common Names: Western Poison Oak

Family: Anacardiaceae

Plant Type: Variable: Vine, Shrub, Small Tree

Environment: Grows well in sun or shade.

Bloom: White flowers appear in spring

Uses: Not recommended for planting. Native Americans used the stems to make baskets and the sap to cure ringworm. Chumash Indians used poison-oak sap to remove warts,corns, and calluses; to cauterize sores; and to stop bleeding. They drank a decoction made from poison-oak roots to treat dysentery.

Other: Poison Ivy (T. radicans) and Poison Sumac (T. vernix) contain the same chemical, urushiol, and can cause the same allergic reaction. T. diversilobum is the most widespread shrub in California and has a range from Baja, California north to British Colombia. Poison oak does not cause a reaction in wildlife or livestock, but some pets may react to it.



Toxicodendron diversilobum

Poison Oak is California's most prevalent wild shrub. It will form dense thickets, grow as a climbing vine, shrub or a small tree. Deserts, dense forests and altitudes above 4000-5000 feet are the only restraints to its growth. The common name refers to the striking similarity of the leaves to the genus Quercus (Oak).

The body's response to poison oak, or more specifically to the chemical compound, urushiol, is an allergic reaction. The body's immune system goes haywire and responds to the urushiol as a threat. The body then sets out to contain and destroy it. The resulting itchy, blistery rash can be miserable and in severe cases requires medical attention.

Urushiol is not a poison in the true sense of the word, like arsenic or strychnine. You won't get poison oak by merely being in its presence. You must touch the plant or another object that has been in contact with the plant, such as clothing, tools or the family pet. Urushiol is contained in the leaves, stems, roots and skin of the berries. Consequently, exposure can occur even when it has lost its leaves in the winter. It is particularly seductive in the autumn when the leaves turn a bright, beautiful red. The oil does not dry up and disappear with time. People have been exposed when handling centuries old dried specimens in herbariums. When it is burned, droplets can also be carried on smoke and soot particles, depositing it into the lungs.

About 10-15% of the population are truly not sensitive to urushiol. The other 85% are vulnerable to varying degrees, with 10-15% falling in the category of exquisitely sensitive. The actual degree of any given reaction depends on the amount of urushiol and an individual's sensitivity.

Poison oak has small white flowers and white berries, but the bright fall colors are what give this plant its biggest opportunity for identification. Other times of year, the old rule, "leaves of three let them be" is the best way to steer clear of this toxic plant.


Photos by Docent Joanne Taylor; text by Docent Kathy McNeil; profile by Associate Curator David Kruse-Pickler; Additional photo courtesy of Far Out Flora.


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