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|Western poison oak (larger leaves;
small leaves are another plant)
at base of oak tree
(Torr. & A.Gray) Greene
Toxicodendron diversilobum, western poison oak or Pacific poison oak (syn. Rhus diversiloba) is in the Anacardiaceae family (the sumac family) and is a plant best known for its ability to cause allergic rashes and itching after contact. Western poison oak is found only on the Pacific Coast of the United States and of Canada.
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Western poison oak occurs only on the Pacific Coast of North America, 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, and can be found growing as single stems in grassland, often as part of early stage succession where woodland has been removed, and serving as a nurse plant for other species.
It is found in damp, semishady areas near running water and also thrives in direct sunlight, requiring water only in early spring. Any trail leading to a waterfall on California's coast may likely be home to western poison oak; it can also be found in some inland mountain ranges, such as the Cascades.
The plant toxin produced by members of the genus Toxicodendron, called urushiol, is known for causing an uncomfortable, and sometimes painful, skin reaction. Urushiol is the main component of the oily resin that is found on the stems and leaves of poison ivy and several other related species. It causes contact dermatitis — an immune-mediated skin inflammation (Kalish et al., 1994) — in four-fifths of humans.
The active components of urushiol have been determined by Billets (1975) to be unsaturated congeners of 3-heptadecylcatechol with up to three double bonds in an unbranched C17 side chain. In poison ivy, these components are unique in that they contain a -CH2CH2- group in an unbranched alkyl side chain.
Deer species and other animals feed on the leaves of the plant — which are rich in phosphorus, calcium and sulfur, while bird species utilize the plant structure for shelter. These animals do not seem to demonstrate any sort of reaction to urushiol. Poison oak is widely distributed in western North America, inhabiting both forests and chaparral biomes.
Western Poison oak leaves and twigs have a surface oil, urushiol, which causes an allergic reaction. Around 15% to 30% of people have no allergic response, but most, if not all, will become sensitized over time with repeated or more concentrated exposure to urushiol.
Due to this adverse effect, poison oak plants are routinely eradicated upon discovery. For example, the Sunset Western Garden Book states if poison oak is discovered on the gardener's property, "destroy it with chemical brush killer."
Effects of poison oak are similar to those of poison ivy. It first causes severe itching, evolves into inflammation, colorless bumps, and then blistering when scratched. In late fall or winter, there are no leaves on the plants, so they can be difficult to recognize. People have occasionally used its branches to toast marshmallows or hot dogs over a campfire with results that sent them to the hospital.
Western poison oak is extremely variable in growth habit and leaf appearance. It grows as a dense shrub in open sunlight, a tree with an 8–20 cm (3.1–7.9 in) trunk under conditions with ample sunlight, very wet winter/spring and dry summer, or as a climbing vine in shaded areas. Like poison-ivy, it reproduces by creeping rootstocks or by seeds. The leaves are divided into three (rarely 5, 7, or 9) leaflets, 3.5 to 10 centimetres (1.4 to 3.9 in) 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 typically bronze when first unfolding, bright green in the spring, 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. T. 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 its milky sap may have oozed and dried.
Botanist John Howell observed Toxicodendron diversilobum's toxicity 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 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."
See also 
- C. Michael Hogan (2008) "Western poison-oak: Toxicodendron diversilobum", GlobalTwitcher, ed. Nicklas Strömberg
- Billets, S., et al. "New GLC analysis of urushiol congeners in different plant parts of poison ivy, Toxicodendron radicans." (1978). JPS. Volume 67, issue 4, pgs 483–485.
- Howstuffworks "How Poison Ivy Works"
- Contact-Poisonous Plants of the World
- Sunset Western Garden Book [5th edition], (Menlo Park: Sunset Publishing, 1988), p. 506
- Howell, John Thomas; Frank Almeda, Wilma Follette, Catherine Best (2007). Marin Flora. California Academy of Sciences; California Native Plant Society. p. 264.
- American Academy of Dermatology — Poison Oak information
- Western Poison-Oak Photo Gallery
- Poison Oak/Poison Ivy Information Center
- Billets, S., Corbett, M. D. "Characterization of poison oak urushiol." (1975). Journal of Pharmaceutical Sciences. Volume 64, issue 10, pgs 1715–1718.
- Hogan, C. M. "Toxicodendron diversilobum: Western poison-oak." (2008). Global Twitcher.
- Kalish, S., et al. "Processing of urushiol (poison ivy) hapten by both endogenous and exogenous pathways for presentation to T cells in vitro." (1994). JCI. Volume 93, no 5, pgs 2039–2047.