Pacific Poison Oak
- Sumac / Cashew Family (Anacardiaceae)
- Eudicots are a major lineage of flowering plants; see family for general characteristics
- Woody shrub or vine
- Drought deciduous (drops leaves during the dry season or droughts); leaves turn red by late summer
- Leaflets (leaf-like structures of compound leaf) of three
- Middle leaflet has a longer petiole (leaf stalk) than the other two
- Highly variable in appearance
- Inflorescence (flower arrangement) a raceme (unbranched stem with stalked flowers opening from the bottom up) or panicle (branching stem with flowers opening from the bottom up)
- Small, cream-colored, fragrant flowers
- Usually unisexual, with male and female flowers on separate plants (dioecious)
- Ovary superior (above the attachment of other flower parts)
- Fruit is a pale green to light tan berry-like drupe (a fleshy fruit with usually 1 seed in a hard inner shell–a stone fruit)
- Vine can grow to 82 ft.
- Native to California
- Grows in a variety of habitats, including chaparral, coastal scrub, conifer and mixed broadleaf forests, woodlands, and grasslands
- Most widespread shrub in California
- See Calflora for statewide observations of this plant
- Outside California, grows from British Colombia south into Washington, Oregon, Nevada, and into northern Baja California, Mexico
- Grows at elevations to 5,410 ft.
Uses (Picking or removing any natural material from public land is illegal)
- Bees frequently visit the flowers
- Honey made from poison oak nectar is not allergenic (okay to eat)
- Berries eaten by at least 50 bird species, including wrentits, woodpeckers, quail, thrashers, towhees, robins, and western bluebirds (Sialia mexicana)
- Leaves, stems, and berries eaten by a variety of mammals, including bear, deer, squirrels, and the dusky-footed woodrat (Neotoma fuscipes annectens)
- Thickets provide shelter for many small animals
- Native people had many uses for poison oak
- Stems were used in basketry
- Leaves were used to wrap bulbs or acorn meal before baking in earthen ovens
- Pomo people of Northern California used the sap to dye baskets and create tattoos
- “A design was drawn on the face with poison oak juice, then soot was pricked in with a sharp pointed needle from the nutmeg tree. This gave a blue-green color, unfading” (Murphy 1959)
- Sap was also used medicinally to cure ringworm, warts, corns, and calluses, as well as to cauterize wounds
- CAUTION – Sap produced in resin canals of the stems, roots, leaves, flowers, and berries (but not pollen) can cause a severe contact dermatitis
- Can transfer to human skin by direct contact or via dog fur, clothing, and tools
- Around 80-85% of humans are susceptible (DiTomaso and Lanini 2009)
- Sensitivity varies–for some people, a molecular trace can cause the allergic reaction (Armstrong 2011)
- Rash usually appears several days after contact and may last several weeks
- Smoke inhalation is extremely dangerous
- 100-year-old herbarium specimens protected from oxidation have caused rashes
- Toxicodendron (tox-i-ko-DEN-dron) – from the Greek toxikos, “toxic/poison,” and dendron, “tree”
- diversilobum (di-ver-si-LO-bum) – from the Latin for “diversely lobed,” referring to the leaves
- Poison oak – a misnomer as the plant’s resin is an allergen, not a poison, and despite the similar shape of the leaves, poison oak is not an oak
- Humans are the only animals known to get a rash from poison oak
- If measured by the number of lost working hours, the dermatitis caused by poison oak makes it the most hazardous plant in California (DiTomaso and Lanini 2009)
- Resinous sap is called by a Japanese word, urushiol
- Contained in resin canals and found only on damaged plant surfaces
- Sap darkens and hardens to a lacquer when exposed to air
- Sap of the related Japanese lacquer tree (Toxicodendron vernicifluum) is used in the creation of traditional lacquerware (urushi)
- Shininess of young poison oak leaves is caused by a protective wax, not urushiol
- How were Native people able to use this plant for so many purposes without suffering ill effects?
- Some may have acquired an immunity from early or repeated exposure, although the many documented native remedies for the rash suggest that some were not immune
- They may have handled the plant very carefully!
- Male and female flowers are difficult to distinguish as both have stamens and pistils: stamens are sterile in female flowers; pistil is atrophied in male flowers
- Seed dispersal is facilitated by berry-eating birds and mammals
- Passage through the gut helps break the hard seed coat (DiTomaso and Lanini 2009)
- Seed dispersal via ingestion by vertebrates is called endozoochory
- Like coyote brush (Baccharis pilularis), poison oak quickly resprouts after disturbance and is a nurse plant for other species, providing shade and protection from herbivores
- New and aging leaves often turn brilliant shades of red; poison oak is one of the few California native plants with vibrant fall colors
- Associated with the fungus-like microorganism Phytophthora ramorum, which causes Sudden Oak Death (SOD)
- At Edgewood, the 2 species known to be highly susceptible to SOD are coast live oak (Quercus agrifolia var. agrifolia) and Pacific madrone saplings (Arbutus menziesii)
- For a complete list of known hosts and host associates see USDA Risk Analysis for Phytophthora ramorum, pp. 6-9
- May be confused with California blackberry (Rubus ursinus) and Himalayan blackberry (R. armeniacus)
- Blackberries also have leaflets of three, but the leaves are hairy and the stems have prickles
- Poison oak leaflets are shiny, and the stems lack prickles
- Mnemonic – Leaves of three, let it be. If it’s shiny, watch your hiney. If it’s hairy, it’s a berry. Berries white, a poisonous sight.
- Found in a wide range of habitats, including riparian, woodland, and dry chaparral
- See iNaturalist for observations of this plant
- Flowers March – May
Armstrong, W.P. 2011, Jan. 16. Poison Oak: More Than Just Scratching The Surface. Wayne’s Word: An Online Textbook of Natural History.
DiTomaso, J.M. and Lanini, W.T. 2009. Poison Oak Management Guidelines. UC ANR Publication 7431. UC Statewide IPM [Integrated pest Management] Program, University of California, Davis, California.
Howard, J.L. 1994. Toxicodendron diversilobum. Fire Effects Information System. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory.
Marionchild, K. 2014. Secrets of the Oak Woodlands: Plants and Animals among California’s Oaks. Heyday, Berkeley, California.
Murphey, E. 1959. Indian Uses of Native Plants. Mendocino County Historical Society. Fort Bragg, California. p. 56.