SuMacS: WeSteRn PoiSon oak: toxicodendron

EUdICOTS: AnAcArdiAceAe • 45

western poison oak
Toxicodendron diversilobum (Torr. & A. Grey) E. GreeneA lovely plant with variable forms. It can grow as a low weak-stemmed herb-like shrub, as a woody shrub 0.5-4 meterstall, or as a vine up to 4 meters long that trails up and over other plants. In wetter parts of the state, its stems reach up to25 meters long; occasionally those individuals are treelike, growing alongside larger plants such as true oak trees
and toyon. Leaves shiny, hairless above, divided into 3 leaflets (rarely 5) that may be small or large, smooth-edged or lobed.
Terminal leaflet 1-13 cm long; its petiolule is long and slender, not winged with leaf blade tissue. Leaves are typically bronze
when first out of bud, turn green with maturity, and change to red, orange, or yellow before they drop off in fall.
Flowers unisexual, generally appear with or after the leaves break bud. Stamens 5 in male flowers, vestigial in female flowers(anthers, if present, are empty). Pistil 1 in female flowers, vestigial in male flowers. Styles 3 in female flowers, fusedat base, with 3 terminal branches. Petals 5(4), creamy-yellow to greenish. Fruit a spherical white berry, 1.5-6 mm wide.
In flower (mostly) Mar-Apr.
One of the most hazardous plants in California, according to Dieter H. Wilken in The Jepson Manual (Baldwin etal. 2012), western poison oak grows abundantly throughout our area along the coast, coastal canyons, waterways,foothills, woodlands, and mountains. It occurs in all of our wilderness parks and along most forest trails.
The toxin within its sap, urushiol (pronounced oo-RU-sheall), is present year-round within and on the surface of theplant. When you contact the leaves or stems, it oozes onto
your skin and can cause allergic contact dermatitis (ACD), an
intense itching sensation accompanied by fluid-filled blisters.
Scratching the rash only makes it worse by tearing open
the blisters and may lead to secondary bacterial infection.
It generally takes 1-2 (to 10) days after contact for ACD toappear, and it worsens and persists for 2-3 weeks. According
to Crosby (2004), “Although the liquid in the blisters isharmless, contact with the affected area can spread surface
allergen to other parts of the body.” It can also be transferredby contaminated clothing, shoes, tools, and pet fur. Local
biologists and naturalists report that Tecnu® products by
Tec Labs and Zanfel® by Zanfel Laboratories can be used towash affected skin and contaminated clothing.
The extremely shiny lacquer that coats Japanese-madepottery and furniture is made from the sap of a related
plant, the Japanese lacquer tree or urushi (Toxicodendron
vernicifluum (Stokes) F. Barkley). The toxin urushiol was
first isolated from the sap of this tree and named after it(Japanese, urushi = lacquer). Urushi lacquer can cause ACD
when in liquid form but not after it dries and cures. Legend
has it that people of Asian or Native American descent areless sensitive to urushiol than others. We do not know if thisis true. The most prudent course of action, regardless of yourancestry, is to avoid touching this plant.
The genus was originally named by J.P. de Tournefort forits toxicity (Greek, toxicos = poison; dendron = tree) andformally published in 1735 by C. Linnaeus. The epithet wasgiven in 1838 by J. Torrey and A. Gray for its lobed leaves(Latin, diversus = separated or turned; lobos = lobe).
similar. Skunkbrush (Rhus aromatica): in open sun among
chaparral; a shrub with arching branches; flowers appear before
the leaves; leaves divided into three or more wavy- or squareedged leaflets. Usually grows in dry areas near coast live oak(Quercus agrifolia, Fagaceae). California blackberry (Rubusursinus, Rosaceae): in moist soil near shady watercourses;
always a vine, never a shrub; sharp prickles along stems, petioles,and leaf undersides; leaves 3-lobed or pinnately divided into
3-5 lobes, most always pointed and toothed; large white
petals, 5-25 mm long; fruit an egg-shaped aggregate of tiny
fleshy berries, each dark red to black. Especially common
near watercourses. Virgin’s-bowers (Clematis spp.,
Ranunculaceae): in moist or dry soil; always a vine; leaves
opposite, 1-2 pinnate, leaflets often lobed and toothed; petioles grasp
and twine around other plants and objects; flowers muchlarger and obvious; fruit a cluster of dry seeds, each with afuzzy tail.
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western poison oak
Leaflets three, let it be!

its attractive clusters ofcreamy yellow flowers
are best watchedthrough binoculars
or a camera lensfemale flowers

Lookalikes

Western Poison Oak

Skunkbrush

California Blackberry

Virgin’s-bowers

4/23/13 2:09 PM