Pacific Poison Oak

Poison oak is a deciduous shrub, which like POISON IVY IS in the SUMAC FAMILY, native to North America.

Its leaves contain a compound that causes a rash on human skin.

Poison Oak can Specifically Refer to:

Toxicodendron pubescens, which grows in Eastern North America

Toxicodendron diversilobum, which grows in Western Coast of North America

Poison oak (Toxicodendron diversilobum) and its EASTERN COUNTERPART poison ivy (T. radicans) are two of the North American plants most painful to humans.

 Note: These species were formerly placed in the genus Rhus. Poison oak and a related, look-alike shrub, Rhus trilobata belong to the SUMAC family (Anacardiaceae).

Poison oak is widespread throughout the mountains and valleys of the western USA. It thrives in shady canyons and riparian habitats. It commonly grows as a climbing vine with aerial (adventitious) roots that adhere to the trunks of oaks and sycamores.

Rocky Mountain poison oak (Toxicodendron rydbergii) occurs in canyons throughout western Canada. Because the two species of western poison oak look like a vine as they develop, some authors list poison oak as a subspecies of eastern poison ivy.

Poison oak often grows like a climbing vine.

The cautionary rhyme "leaves of three, let it be" applies to poison oak, as well as to poison ivy (Toxicodendron radicans). Toxicodendron diversilobum, commonly named Pacific poison oak or western poison oak (syn. Rhus diversiloba), is in the Anacardiaceae family (the sumac family).

The woody vine or shrub is widely distributed in western North America, inhabiting conifer and mixed broadleaf forests, woodlands, grasslands, and chaparral biomes. It is known for causing itching and allergic rashes in many humans, after contact by touch or smoke inhalation.

The Pacific poison-oak Toxicodendron species is found in California, Nevada, Oregon, Washington, and British Columbia. The closely related Eastern poison oak (Toxicodendron pubescens) is native to the Southeastern United States. Pacific poison-oak and Western poison-ivy (Toxicodendron rydbergii) hybridize in the Columbia River Gorge area.

Toxicodendron diversilobum is common in various habitats. It thrives in shady and dappled light through full and direct sunlight conditions, at elevations below 5,000 feet. The vine form can climb up large shrub and tree trunks into their canopies. Sometimes it kills the support plant by smothering or breaking it.

The plant often occurs in chaparral and woodlands, coastal sage scrub, grasslands, and oak woodlands; and Douglas-fir (Pseudotsuga menzesii), Hemlock—Sitka spruce, Coast redwood (Sequoia sempervirens), Yellow Pine (Pinus ponderosa), and mixed evergreen forests.

Description

Toxicodendron diversilobum, Pacific or western poison oak, is extremely variable in growth habit and leaf appearance. It grows as a dense one and a half to thirteen foot, tall shrub in open sunlight; also as a tree like vine ten to thirty feet high and may be more than more than one hundred feet long with a three to eight inch trunk. It reproduces by spreading rhizomes and by seeds.

Pacific Poison-oak foliage.

The plant is deciduous, so that after cold weather sets in, the stems are leafless and bear only the occasional cluster of berries. Without leaves, poison oak stems may sometimes be identified by occasional black marks where its milky sap may have oozed and dried.

The leaves are divided into three (rarely five,seven or nine) leaflets, one and one a half to four inches long, with scalloped, toothed, or lobed edges. They generally resemble the lobed leaves of a true oak, though the Pacific poison oak leaves will tend to be glossier. Leaves are typically bronze when first 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, from March to June. If they are fertilized, they develop into greenish-white or tan berries.

Botanist John Howell observed the toxicity of Toxicodendron diversilobum obscures its merits:

"In spring, the ivory flowers bloom on the sunny hill or in sheltered glade, in summer its fine green leaves contrast refreshingly with dried and tawny grassland, in autumn its colours flame more brilliantly than in any other native, but one great fault, its poisonous juice, nullifies its every other virtue and renders this beautiful shrub the most disparaged of all within our region”

Pacific poison oak leaves and twigs have a surface oil, urushiol, which causes an allergic reaction. It causes contact dermatitis- an immune-mediated skin inflammation- IN 4/5 OF HUMANS However, most, if not all, will become sensitized over time with repeated or more concentrated exposure to urushiol.

Pacific Poison-Oak grows on drier rocky slopes at lower elevations on south-eastern Vancouver Island and nearby Gulf Island.

Reactions:

Urushiol-induced contact dermatitis from poison oak.

Pacific poison oak skin contact first causes itching; then evolves into dermatitis with inflammation, colourless bumps, severe itching, and blistering. In the dormant deciduous seasons the plant can be difficult to recognize, however leafless branches and twigs contact also causes allergic reactions.

Contrary to what has been written by other authors and posted on other web-sites Urushiol volatilizes when burned; SO IT IS, INDEED, CARRIED THROUGH THE AIR to unsuspecting victims.

 And, human exposure to Toxicodendron diversilobum smoke is extremely hazardous, from wildfires, controlled burns, or disposal fires. The smoke can poison people who thought they were immune. The resin can persist on pets and clothing for months and is also ejected in fine droplets into the air when the plants are pulled.

Branches used to toast food over campfires can cause reactions internally and externally, although, or so it is claimed, the Karok peoples traditionally used them as a cooking tool.

Ecology: Black-tailed deer, Mule deer, ground squirrels, Western grey squirrels, and other indigenous fauna feed on the leaves of the plant. It is rich in phosphorus, calcium, and sulphur. Bird species use the berries for food, and utilize the plant structure for shelter. Native animals, horses, livestock, canine pets, DO NOT demonstrate reactions to urushiol.

Due to human allergic reactions, Pacific poison oak plants are usually eradicated in gardens and public landscaped areas. It can be a weed in agricultural fields, orchards, and vineyards. It is usually removed by pruning, herbicides, digging out, or a combination of these.

Uses:

Native North Americans used the plant's stems and shoots to make baskets, the sap to cure ringworm, and as a poultice of fresh leaves applied to rattlesnake bites. The juice or soot was used as a black dye for sedge basket elements, tattoos, and skin darkening.

An infusion of dried roots, or buds, eaten in the spring, were taken by some native peoples for an immunity from the plant poisons.

Chumash peoples used Pacific poison-oak sap to remove warts, corns, and calluses; to cauterize sores; and to stop bleeding. They also drank a decoction made from Pacific poison-oak roots to treat dysentery.
© Al (Alex-Alexander) D Girvan> All rights reserved.