FRIENDS OF EDGEWOOD NATURAL PRESERVE
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FRIENDS OF EDGEWOOD NATURAL PRESERVE
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Adapted from an article by Mary Heinricher One plant that you can anticipate encountering on almost any walk at Edgewood is poison oak, one of the most widespread plants in California. Growing west of the Sierra in a wide variety of soil and moisture conditions, it is found at elevations less than 1,200 meters. Its range extends north to British Columbia and south to Baja California. Poison oak may be a low deciduous shrub, but if a means of support is available, it will grow as a vine. The deciduous leaves generally have three leaflets (sometimes more), and an exasperating feature of the plant is the large variability in size and shape of the leaflets, even on a single plant. The leaflets are reddish in color when they first emerge in the spring, and mature to a shiny green. They turn red or yellow in late summer, and fall with water stress. The leaf scars are triangular, a characteristic that can be a useful aid to identification in the winter. The berries are green in the growing season, turning a light tan or off-white at maturity. Poison oak is just one member of a group of plants formerly included in the genus Rhus, but now classified as Toxicodendron. These plants are distributed world-wide, and include the mango, cashew tree, and Japanese lacquer tree. In addition to our own western poison oak (Toxicodendron diversiloba), the United States has poison ivy, poison sumac, and eastern poison oak (Toxicodendron quercus). Poison ivy and poison oak are the most common cause of contact dermatitis in North America. The active principle, which has been isolated in the resinous sap of these plants, is called "urushiol," from the Japanese word for sap. Resin canals containing the sap are found in almost all parts of the plant, including the leaves, stem, roots, flowers, and unripe berries. (The pollen, however, is safe.) The fresh sap is almost clear, but darkens and hardens to form a black lacquer-like substance upon exposure to air. Drops of sap, caused for example by insect damage, are the source of the black spots often seen on poison oak leaves. The characteristic rash results when the skin is exposed to a portion of the plant that has been bruised or damaged in some way so that the resin can contact the skin. (An uninjured plant would thus be innocuous.) This is a true allergic reaction, and it is often said that over 50% of the U.S. population has been sensitized, although this is a very rough figure. Within 5-10 minutes of exposure, the urushiol interacts with proteins in the skin, leading to activation of immune cells. However, the onset of symptoms due to these immune cells is not immediate. Although symptoms usually begin within 2 days of exposure, delays of up to 10 days are not uncommon. The rash (which physicians still call "Rhus dermatitis") begins with itching and redness, and then proceeds to the characteristic miserable watery blisters. Itching is intense. Although poison oak usually goes away by itself within two weeks, serious cases can be treated with corticosteroids. Some people appear to be truly tolerant to poison oak, but the mechanisms for this are not well understood. There are unsubstantiated reports that the Native American population in California was not sensitive to poison oak. Other authorities believe that poison oak leaves were chewed to elicit a form of oral desensitization, i.e. an acquired immunity. This practice was apparently adopted by early settlers, with some deaths resulting. Exposed areas of skin should be washed as soon as possible, that is, within minutes. However, washing is worthwhile even hours later, so that any resin remaining on the skin surface is not spread to adjacent areas. In addition, urushiol remains potent indefinitely under dry conditions. (There is an anecdotal report that someone developed poison oak dermatitis from a specimen that had been stored in a herbarium for almost 100 years.) Thus sap remaining on packs or clothes, including boot laces, can be responsible for many recurrent cases. For the extremely sensitive, there are some barrier creams on the market. However, recent work suggests that those presently available are not very effective. Clearly, the most effective way to prevent poison oak dermatitis is to avoid contact with the plant. |
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