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Poison Oak is part of the Sumac (Anacardiaceae)family, Toxicodendron diversilobum or Rhus diversiloba is the binomial name for Poison Oak in the Western United States and south to Mexico. Tocicodendron pubescen or Rhus pubescen is the name for the Atlantic variety. This information refers to the Western variety.
Habit: Deciduous, small to medium sized, erect shrubs 3' to 10' (3 m) foot in height, or tree-climbing vines, with shiny, dark green leaves which turn varying shades of red and yellow in the fall. Stems often hairy.
Leaves: Alternate, pinnately compound, with 3 (rarely 5) ovate to ovate-rhombic or obovate leaflets, the terminal leaflet is larger than the lateral leaflets and has a distinct petiole, the lateral leaflets are subsessile: margins wavy to shallowly lobed, some times deeply lobed but rarely smooth; surfaces smooth and shiny, initially reddish-green in the early spring, but soon turning green; red or yellow in the fall.
Flowers: Small, long-stemmed, inconspicuous, yellowish-green; borne in loose, pendulous (hanging) clusters (panicles)from leaf axils; fragrant. April–July
Fruit: Round to subglobose, grayish white, fleshy fruit with grooves, striated drupes which persist after the leaves have fallen; by late fall only the stems or penducles remain.
Stems: New twigs light brown to tan colored, pubescent; when a climbing vine the stems are equipped with brown tendrils; short stiff lateral branchlets numerous; buds naked.
Habitat and Range: Common in western Cascades and Sierra Nevada, south from Puget Sound to Mexico in open forested sites and in valleys. Occurs on moist to dry, well-drained sites in the sun or shade; from southern British Columbia south to southern California on the west of the Cascades and Sierra Nevada Mountains. Most common in the valleys along fence rows and in pastures, also in the woods in the surrounding foothills. In the mountains it is found on the drier ridges and south and west slopes. Where summer precipitation is sufficient to encourage the growth of competing vegetation, poison oak disappears.
Indicator Value: Dry, hot sites with well drained soils.
Fire sensitivity: Sprouts easily from wide-spreading root systems.
Remarks: Poison Oak is toxic to most individuals. Immunity is a relative thing, for individuals who have not been bothered by poison Oak for some years may find themselves, eventually affected. Fumes from the burning plant are especially toxic and immediate medical help is suggested. The Oil (Urushiol) causes allergic reaction in many people, resulting in a burning or itching rash. Rash is only spread by the oil of the plant and is not contagious. Usually lasts from 5 to 12 days although in extreme cases can last up to 30 days or longer. Bees are attracted to the flowers in the spring, but none of the toxicity of the plant is transmitted through the nectar. Horses, cattle, and goats can browse the species with immunity, but household pets that have wandered through the plant are carriers of the plants oils and should be bathed as soon as possible
The cautionary rhyme "leaves of three, let it be" applies to poison oak, as well as to poison ivy (Toxicodendron radicans).
- Manual of Oregon Trees and Shrubs, W. R. Randall 1958, R. F. Keniston 1968, D. N. Bever 1974,1981, E. C. Jensen 1981,1988,1990
- Key Species for Plant Associations on the Rogue River, Siskiyou and Umpqua National Forests, Compiled by Anita Seda in association with: Thomas Atzet and David Wheeler, 1989 USDA Forest Service Pacific Northwest Region
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