Poison Oak - A Plant to Avoid

Prepared by Robert Smith
Mountain News, January 1999

This uncherished native plant of California, the botanical name of which is Toxicodendron diversilobum (an older botanical designation being Rhus diversiloba), is a close relative of poison ivy, which is widespread in the eastern and central United States. The sap oil (urushiol) produces an allergic reaction in most persons who touch it. Regarding Califomia's poison oak, Dr. Glenn Keator writes in his Plants of the East Bay Parks (published by MDIA in cooperation with Roberts Rinehart Publishers, 1994):

"Poison Oak has earned for itself an indelible reputation; approach it always with care. Even those who have immunity may later lose it; better to be safe than sorry. Should you touch the twigs (even the bare twigs in winter) or leaves, rinse your hands immediately and wash with mild soap; this should remove oils. Remember to wash your dog if it has accompanied you, and wash your clothes as well."

"Despite the consequences of its toxicity, poison oak is an interesting -- even attractive -- deciduous shrub. Thriving on disturbance, it has ventured into a wide range of different plant communities, behaving in each one according to circumstances: near the coast it lies prostrate next to the ground; in bright light it forms a dense shrub; in shade it climbs toward the sun, often ascending thirty to forty feet up a tree."

"New growth is signaled in early spring by a flush of glossy reddish new leaves. By mid-spring, dangling chains of whitish-green flowers perfume the air, attracting bees for pollination. This perfume is harmless to humans and actually enhances many a spring outing. By summer, there are whitish berries the birds consume, and in fall the foliage turns brilliant red before falling. When leaves color up in summer, it's a sign that the dry period has stressed the shrubs, telling them it's time to lose leaves before they lose too much water. Out of leaf, poison oak may be identified by the long upright main branches with short, stubby side branches."

Since allergic reactions to poison oak are unpredictable, one should avoid contact with the plant, both leaves and branches. Anyone with known sensitivity who is hiking in infested areas may with to apply the preparation Ivy Block (which prevents the oil from being absorbed by the skin) before starting a hike, but usually long-sleeve shirts and trousers-plus due wariness on the trail are sufficient precautions. Urushiol is absorbed into the skin within minutes after contact, hence the recommended washing of affected skin after contact apparently has limited benefit. Redness and blisters appear after about 24 hours, and can linger for two weeks. Calamine lotion applied to itching, inflamed skin can have a soothing effect.

For more information, here are books (available in paperback) related to Poison Oak/Ivy:

Nature's Revenge: The Secrets of Poison Ivy, Poison Oak, Poison Sumac, and Their Remedies, by Susan C. Hauser (Lyons Press, 1996) Gives a very reader-friendly presentation of relevant topics.

Poison Ivy, Poison Oak, Poison Sumac, and Their Relatives, by Edward Frankel (Boxwood Press, 1991). Frankel discusses where the plant grows, its characteristics, and remedies for persons afflicted by it.

The Poison Ivy, Oak and Sumac Book: A Short Natural History and Cautionary Account, by Thomas E. Anderson (Acton Circle Publish Co., 1995). Lots of scholarly research with sometimes surprising information about the plants and human allergic reactions.

Spring Poison Oak (Toxicodendron diversilobum)

Mike Woodring

Summer Poison Oak (Toxicodendron diversilobum)

Yulan Tong

Fall Poison Oak (Toxicodendron diversilobum)

Mike Woodring

Winter Poison Oak (Toxicodendron diversilobum)

Mike Woodring

© 2019 by Mount Diablo Interpretive Association