Western Poison-oak
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 Western Poison-oak (larger leaves;
small leaves are another plant) at base of oak tree |
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| Toxicodendron diversilobum (Torr. & A.Gray) Greene |
Western Poison-oak or Pacific Poison-oak (Toxicodendron diversilobum or formerly Rhus diversiloba) is found only on the Pacific Coast of the United States and of Canada. It is extremely common in that region, where it is the predominant species of the genus; the closely related Atlantic Poison-oak (T. pubescens) occurs on the Atlantic Coast.
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[edit] Appearance
Western Poison-oak is extremely variable in growth habit and leaf appearance. It grows as a dense shrub in open sunlight, or as a climbing vine in shaded areas. Like Poison ivy, it reproduces by creeping rootstocks or by seeds. The leaves are divided into 3 leaflets, 3½ to 10 centimeters long, with scalloped, toothed, or lobed edges- generally resembling the leaves of a true oak, though the Western Poison-oak leaves will tend to be more glossy. Leaves are generally bright green in the spring (or bronze when first unfolding), yellow-green to reddish in the summer, and bright red or pink in the fall. White flowers form in the spring and, if fertilized, develop into greenish- white or tan berries. Toxicodendron diversilobum is winter 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 sap may have oozed and dried.
 Green phase |
 Red phase |
 Berries |
[edit] Habitat
Western Poison-oak is found only on the Pacific Coast, where it is common, and ranges from Southern Canada to the Baja California peninsula. It is one of California's most prevalent woody shrubs but also climbs, vine-like, up the sides of trees. The plant is often found in oak woodlands and Douglas-fir (Pseudotsuga) forests. Along the Avenue of the Giants in northern California, the vine form may be seen climbing many feet up the trunks of Coast Redwoods (Sequoia sempervirens). It can also be found in damp, shady areas near running water and out of direct sunlight. Any trail leading to a waterfall on California's coast will most likely be home to western poison-oak.
[edit] Toxin
Western Poison-oak leaves and twigs have a surface oil, urushiol, that is an allergenic irritant. Depending upon individual susceptibility (85% of humans will develop allergic reactions) and exposure, the symptoms can range from trivial to life threatening. The common effect is an irritating, itchy rash, often accompanied with blistering of the skin.
Before the oil is removed, it can be transferred from one part of the body to another, from one person to another by contact or by transfer through an object (such as an automobile seat belt), from a pet to a person, or from clothing to skin when clothes are prepared for washing. If exposed, the recommended immediate treatment is to wash the exposed area with Tecnu (a strong soap available at many pharmacies in the western United States) and cold water. Warm water will tend to open pores on the skin and will lead to deeper irritation.
After the oil is removed, the poison-oak cannot usually be spread by contact with an affected area or by scratching. However, scratching can open the skin especially in cases with significant blistering, making it possible for the skin to become infected by opportunistic bacteria (known as a secondary infection).
[edit] Treatment
Mild cases of poison-oak rash are treated with calamine lotion and other drug store aids while severe cases may require medical treatment and even hospitalization. Topical diphenhydramine (Benadryl) may provide relief in localized areas, but it should not be used on large areas of the body due to the potential for serious or life-threatening systemic side effects. Inhalation of smoke from burning poison-oak can result in life-threatening lung damage.
For more information on poison-oak rashes, see Urushiol-induced contact dermatitis.
[edit] Quotation
"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 colors 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."
- John Thomas Howell: Marin Flora
[edit] Note on name hyphenation
The hyphenated form "Poison-oak" is used, rather than "Poison Oak" to clearly indicate that it is not a variety of oak, just as "Poison-ivy" is not a variety of ivy.
