Leaves ternate or 3 foliate (3 leaflets make up the leaf), leaflets ovate (egg-shaped), round, or oblong (longer than wide with margins parallel between base and tip); crenulate (margins cut with small, rounded teeth or lobes) toward the tip; 1-13 cm long, 1-8 cm wide (terminal leaflet), 1-7 cm long and 1-6 cm wide (lateral leaflets); short petioles (≤1 cm long). Leaflets are thin but somewhat sclerophyllous, shiny bronze chartreuse green in the spring, dark green in the summer, and bright red/splotchy red/green in the fall. The plant is winter deciduous. The leaflets are glabrous (smooth-surfaced, in this case sometimes shiny) or very faintly pubescent (hairy/downy), darker on top and paler underneath. Flowers are small (petals ~0.2-0.4 cm long), whitish to yellow-greenish, with 5 each petals, sepals, and stamens, and carried on panicles that come out of the leaf axils (where the leaf meets the stem). Blooms April to May. Fruit is a white, creamy, tan, or black- striped round berry about 0.4-0.7 cm wide, yielding a flattened rough small seed about 0.3-0.6 cm across. Growth form is variable: It can appear as an erect shrub or even small tree in full sun anywhere from 0.5 to 4 m tall or, in shadier substory situations, as a climbing liana or vine that can extend along the ground or up another tree some 25 m. Twigs and stems are greyish or reddish-brown. All parts of the plant secrete or contain urushiol, to which 90% of the human population is severely allergic. Some people are not allergic, but some of these are merely enjoying "beginner's luck" -- urushiol allergy can develop over the course of several exposures to it. Even a billionth of a gram can induce hives and violent itching. Direct contact is the most common way to get it, but it can also be spread during a fire when the plant burns. "Leaflets three, leave it be!"
Toxicodendron diversilobum aka Rhus diversiloba (Anacardiaceae): poison oak or western poison oak
The development of this key was partially funded through the Geoscience Diversity Enhancement Program (Award #0703798) and through a course of re-assigned time provided by the CSULB Scholarly and Creative Activities Committee. Thanks also to the students in sections of biogeography, introductory physical geography, GDEP, and LSAMP for "test-driving" various editions of this key.