Flora of Scotts Creek Watershed
Toxicodendron diversilobum - Pacific Poison Oak
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Family Name: Anacardiaceae
Common Names: Pacific Poison Oak, Western Poison Oak
Origin: Native to California
Habitat: Widespread. Forests, woodlands, chaparrals, riparian zones, canyons, disturbed sites. Below 1700 m.
Life Form and Duration: Perennial Shrub (0.5-4 m, occasionally may be tree-like) or vine (less than 25 m)
Comments on overall growth: Form and appearance is very variable depending on conditions and location. In riparian habitats and shady areas commonly grows as a climbing vine through the trees and up rocks. In open areas such as chaparral and coastal scrub can grow as a multistemmed shrub in dense thickets.
Bark and Stem Description: Gray to red-brown, hairless or hairy.
Foliage Type: Deciduous
Foliage Description: Leaves are alternate and pinnately compound with 3-5 leaflets ("leaves of three, let it be"), round to oblong in shape, margins are entire to wavy or slightly lobed, upper surface is shiny and hairless. Size and color vary, based on environment and season. The terminal leaflet is the largest, ranging in size from 1-13 cm long and 1-8 cm wide, lateral leaflets are 1-7 cm long and 1-6 cm wide. Plants growing under cover generally have larger smoother leaves while those out in the open are smaller, more wrinkled, leathery, and redder. Leaves are generally shiny red in early spring, become green, then red again in the fall before they drop.
Infloresence Description: Flowers are in loose panicles (branched) located in leaf axils, drooping.
Flower Description: Flowers yellow-green, small about 3 mm across, 5-merous, radial.
Flowering Season: April-July
Fruit Type: Drupe
Fruit Description: Spherical, grayish white to grayish brown, 6 mm in diameter, becoming leathery, glabrous to finely bristly.
Common Associates: Understory of oak-madrone woods, redwood-Douglas fir forests, redwood stands
Other Comments: "Urishol oil on leaves, stems and fruits causes varying degrees of rashes in most people. The oil can stay potent on any surface for up to five years. Over 90% of people are allergic to urishol oil, the more you are exposed to the oil the more likely you are to break out in a rash. If the urishol oil is washed off the skin within 15 minutes of contact a reaction may be prevented. For those reacting after their first exposure, it generally takes longer for symptoms to develop, around 7-10 days. For the rest a blistery, itching rash begins to show in 12-36 hours. Reactions usually last 12-15 days. The rash can not be spread by scratching, as long as all the urishol oil has been removed from the skin by washing with soap and water. Spots and blisters appearing after the first signs are not due to spreading by scratching, but from less severe contact with the oil or on less sensitive areas. See: <http://poisonivy.aesir.com/> for additional information."