Poison Oak (Toxicodendron diversilobum) is a deciduous shrub or vine that is covered in a highly allergenic oil known as urushiol. It is extremely variable in appearance. It can grow as a dense shrub in full sun, anywhere between 2 and 13 feet tall, or with more shade, it may grow as a vine or as a groundcover, forming stems anywhere from 10 to 100 feet long. It reproduces via spreading roots in addition to by seed.
Poison Oak is native to all of Woodland and to all of Yolo County except for some small rain shadows just east of the foothills and just east of the Capay Hills.
Poison Oak is commonly identified by its three-part compound leaves, which inspired the rhyme, "Leaves of three? Let them be." However, it loses its leaves each fall and winter, which can make it especially hard to identify. The bare branches still remain covered with urushiol all winter and are fully capable of causing allergic dermatitis.
Poison Oak leaves re-emerge around February and are typically bronze-colored through March, then bright green through May, yellow-green in June and early July, then red or pink from late July until they fall off around October.
Poison Oak bears white flowers from March through June, followed by greenish-white berries.
Clematis, Blackberries, and Sumac are sometimes mistaken for poison oak. Sumac is actually closely related to Poison Oak and bears trace amounts of urushiol. Blackberries are not closely related and are often armed with thorns but never armed with urushiol. Clematis is even less related and is extremely unlikely to do you any harm.
