| Poison oak |
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 Larger, more lobed poison oak leaves (larger, many with red) at base of oak tree (smaller, multiple-lobed bright green leaves) |
| Scientific classification |
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| | Binomial name |
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| Toxicodendron diversilobum |
Western poison oak (Toxicodendron diversilobum or, previously, 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.
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. Western Poison Oak is found only on the Pacific Coast, where it is common, and ranges from Southern Canada to Baja California. It is California's most prevalent woody shrub and is often found in oak woodlands. In comparison to true oaks, the leaves will tend to be rather more glossy.
Toxin
Poison oak leaves and twigs have a surface oil that is an irritant. Depending upon individual suceptability (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 strong soap and cold water. (Warm water will tend to open pores on the skin and will lead to deeper irritation.)
 Small leaf variation - ground shrub After the oil has been removed, the poison oak cannot 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 opportunisitic bacteria (known as a secondary infection).
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. Inhalation of smoke from burning poison oak can result in life-threatening lung damage.
(There is an extensive section concerning treatment within the poison ivy article.)
Quotation
"In spring its ivory flowers perfume the sunny hill or sheltered glade, in summer its fine green leaves contrast refreshingly with dried and tawny garassland, 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
External links
Treatment discussion (http://www.aad.org/Kids/plants.html)
History, description, and treatment (http://www.surviveoutdoors.com/reference/poisonoak.asp)
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