Poison oak (toxicodendron diversilobum, Pacific poison oak, western poison oak) is a shrub and vine native to the sumac (anacardiaceae) family, which includes poison ivy, and mango and cashew producing plants. Poison oak gets its name from the irritant that is produced, known to cause serious rashes even if leaves are absent from the shrub. It’s found on the entirety of the North American coast, from Los Angeles to British Columbia, and east to Nevada, and is related to eastern poison oak. It is common in many different habitats, from chaparral (charred by infrequent fires) to riparian zones, and thrives in conditions with direct sunlight. The form with vines can climb up large trees, and if left to its own devices can kill the plant supporting it.
Its twigs are characterized by short (about one inch) branches coming from the main stem, and are poisonous even if leaves are absent in winter. The dense shrub can reach four meters in height, and can create dense thickets. Its leaves have three leaflets (“leaves in three, let it be” is a popular saying to look out for poison oak), and resemble oak trees with rounded, lobed margins. The leaves are pinnately venated and the veins reach all the way to the margins. It is usually green but can also have a red tinge around the edge of the leaf, and is usually shiny but absence of a sheen does not mean there are no poisonous properties. The skin irritant is urushiol, or 3-heptadecylcatechol. It also produces berries, edible to birds, which use the stems for shelter. No other fauna (domesticated canines, livestock, horses, California ground squirrels, western gray squirrels, black-tailed or mule deer) have demonstrated a reaction to urushiol, and many native groups would eat the leaves to build immunity to it.