Western poison oak is a member of the group that includes poison ivy (Toxicoden-
dron radicans), poison sumac (T. vernix) and Eastern poison oak (T. quercifolium), all of which are viney shrubs and contain the skin-irritating oil urushiol.  They are not technically poisonous, but rather allergenic, and just as poison ivy is not an ivy, so poison oak is not an oak.  All of these species belong to the sumac or cashew family. Poison oak can be viney or shrubby, or may approximate a small tree with a fairly thick trunk growing to 9' or 10' tall.  The leaves, which is how most people identify it, have three leaflets, the terminal leaflet usually (but not always, see the two small photos below) arising from a separate stem.  They have crenulate or slightly-lobed margins, obtuse tips, are ovate to oblong or sub-orbicular in outline, and the upper surface is glabrous and ± shiny, bright green in summer and becoming dark red in autumn.  The inflorescence is a drooping or erect racemose axillary panicle, and each flower has five sepals, five yellow-green petals and five stamens.  The 1/4" fruits are creamy-white somewhat pulpy spheres sometimes ridged or with black striations.  This species reproduces by creeping rootstocks as well as by seeds.  Western poison oak is one of the most common shrubs in Southern California, ranging widely from Baja to Canada, growing in thickets and wooded places to about 5000' in coastal sage scrub, chaparral and oak woodlands, and blooming from March to May.  All that is apparent of the plant in the wintertime is a tangle of leafless and seemingly lifeless stems, but even these can affect those who brush against them.  Severe blistering and itching may result from direct contact.  I cannot attest from personal experience to the myth that rubbing an affected area with the leaves of mugwort (Artemisia douglasiana) can counteract the harmful and painful effects, but inasmuch as it takes some moments to penetrate the skin, immediate washing with soap and water may successfully remove the oils.  Some of the worst effects of poison oak may be experienced by those who inadvertantly breath in particulates from burning plants, and this is to be avoided at all costs.

Click here for Latin name derivations: 1) Toxicodendron 2) diversilobum.
Pronunciation: tox-i-ko-DEN-dron di-ver-si-LO-bum.
Click here for Botanical Term Meanings.