Poison Oak

Toxicodendron diversilobum (syn. Rhus diversiloba), commonly named Pacific poison oak or western poison oak, and known by it scientific names Toxicodendron diversilobum or Rhus diversiloba), is a woody shrub-like vine in the Anacardiaceae or sumac family inhabiting conifer and mixed broadleaf forests, woodlands, grasslands, and chaparral biomes. It is widely distributed in California. Indeed, the native inhabitants of California before the arrival of the Spanish in the 1700s referred to what is today Los Angeles as Yangna or Iyaanga, which means poison oak place.
As with other members of the Toxicodendron genus, T. diversilobum causes itching and allergic rashes in roughly four-fifths of humans who touch or brush up against it. It can have even more serious consequences in those who inhale the smoke from it when it is burned.
Toxicodendron diversilobum leaves and twigs have a surface oil, urushiol, which causes an allergic reaction in the form of contact dermatitis – an immune-mediated skin inflammation involving inflammation, colorless bumps, severe itching, and blistering. Repeated exposure ups sensitization to urushiol, which also exists in the peel of mangos.
Toxicodendron diversilobum is common in various habitats, from riversides with adequate water to areas of the foothills near the desert where rainfall is far less or barely extant. It often occurs in chaparral and woodlands, coastal sage scrub, grasslands, and oak woodlands; and Douglas-fir (Pseudotsuga menzesii), hemlock–Sitka spruce, Sequoia sempervirens (coast redwood), Pinus ponderosa (Ponderosa pine), and mixed evergreen forests.
It thrives in shady and dappled light through full and direct sunlight conditions, at elevations below 5,000 feet. The vining form can climb up large shrub and tree trunks into their canopies. Sometimes it kills the support plant by smothering or breaking it.
Poison oak is variable in both appearance growth patterns. It will grow into a foot-and-a-half to 13-foot high shrub in open sunlight, a treelike vine of up to 30 feet, and grow to more than 100 feet long with a three to eight inch trunk, or as dense thickets in shaded areas. It breeds by spreading rhizomes and by seeds.
The plant is winter deciduous, with its stems leafless after cold weather sets in, though sometimes bearing berry clusters. The leafless stems occasionally sport black dried sap marks.
The plant has three leaflets that are one-and-a-half to 4 inches long, with scalloped, toothed, or lobed edges, in some degree resembling the lobed leaves of a true oak, though more glossy. Leaves are bronze upon unfolding in February to March, bright green in the spring, yellow-green to reddish in the summer, and bright red or pink from late July to October. White flowers form in the spring. When the plant is well nourished, the flowers become greenish-white or tan berries.
Black-tailed deer, mule deer, California ground squirrels, western gray squirrels, and other indigenous fauna feed on the phosphorus, calcium, and sulfur-rich leaves. Some bird species will shelter in the plant and feed upon its berries. Domesticated animals do not appear to be sensitized to urushiol.
The plant has medicinal properties. Indigenous California Indians used the plant’s sap to cure ringworm, and applied a poultice of fresh leaves as a palliative to rattlesnake bites. An infusion of dried roots, or buds eaten in the spring, were taken by some native peoples for an immunity from the plant’s poisons. Chumash Indians used T. diversilobum sap to remove warts, corns, and calluses; to cauterize sores; and to stop bleeding. A decoction made from the roots was utilized to treat dysentery.