Poison Oak

Pacific Poison Oak
Toxicodendron diversilobum
NATIVE

Description (Jepson, PlantID.net)

• Eudicotyledon
  • Eudicots are a major lineage of flowering plants; see family for general characteristics
• Sumac Family (Anacardiaceae)
• Woody shrub or vine
  • Drought deciduous (drops leaves during the dry season or droughts)
• Leaves
  • 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
  • New and aging leaves can be red
• Flowers
  • 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.

Distribution

• 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 (San Mateo County Parks prohibits removal of any natural material)

• 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 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” (Murphey 1959)
  • Sap was also used medicinally to cure ringworm, warts, corns, and calluses, as well as to cauterize wounds
  • Coast Miwok, Kashaya Pomo, and Maidu used the ashes to create tattoos. (Bay Nature 2013)
• CAUTION – Sap produced in resin canals of the stems, roots, leaves, flowers, and berries (but not pollen) can cause a severe contact dermatitis (Armstrong 2011)
  • Transfers to human skin by direct contact or via dog fur, clothing, tools, etc.
  • Rash appears one to several days after contact and may last several weeks
  • Inhalation of smoke is extremely dangerous

Name Derivation

• 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
  • Spanish Californios called it yiedra maligna, “evil ivy” (Bay Nature 2013)

Adaptations

• Climbs trees using adventitious roots and/or stems that wedge in crevices
  • Tends to climb straight up its support, in contrast to vines such as hairy honeysuckle (Lonicera hispidula) that twine around their supports
• 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
  • Acts as a nurse plant for other species, providing shade and protection from herbivores
• Deciduous leaves drop during the dry season or droughts
  • Aging leaves often turn brilliant shades of red
  • One of the few California native plants with vibrant fall colors

Notes

• Resin in the sap that causes poison-oak dermatitis is called by the Japanese word urushiol
  • Contained in resin canals and on damaged plant surfaces
  • Mixture of compounds known as alkyl catechols, also found in poison ivy and poison sumac
  • Darkens and hardens to a lacquer when exposed to air
    • Resin 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
• Humans and a few other primates are the only animals known to get a rash naturally from poison oak (Bay Nature 2013)
  • Urushiol oil sensitivity has been induced in laboratory guinea pigs (Ginsberg 1937)
  • 80-90% of human adults will develop a rash if exposed to 50 micrograms of urushiol, less than the weight of a single grain of table salt (CDC 2018)
    • If measured by the number of lost working hours, it’s the most hazardous plant in California (DiTomaso and Lanini 2009)
    • 100-year-old herbarium specimens protected from oxidation have caused rashes
  • 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 (Bay Nature 2013)
• 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

ID Tips

• 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

At Edgewood

• Found in a wide range of habitats, including riparian, woodland, and dry chaparral
  • See iNaturalist for observations of this plant
• Flowers March – May

Specific References

Armstrong, W.P. 2011, Jan. 16. Poison oak: More than just scratching the surface. Wayne’s Word: An Online Textbook of Natural History.

Bay Nature. 2013, Jun. 30. Leaves of three: The rash effects of poison oak. 

Center for Disease Control (CDC). 2018. Poisonous plants: Types of exposure. Poisonous Plants. National Institute for Occupational Safety and Health.

DiTomaso, J.M., and W.T. Lanini. 2009. Poison Oak Management Guidelines. UC ANR Publication 7431. Statewide Integrated Pest Management Program, University of California, Davis, California.

Ginsberg, J.E., F.T. Becker, S.W. Becker. 1937, December. Sensitization of guinea-pigs to poison ivy. Abstract. Arch Derm Syphilol. 1937:1165–1170. JAMA Network.

Howard, J.L. 1994. Toxicodendron diversilobum. Fire Effects Information System. United States 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. Pg. 56.

General References

Calflora Database. 2014. Berkeley, California.

Calscape. 2018. California Native Plant Society.

Charters, M.L. 2015. California Plant Names: Latin and Greek Meanings and Derivations.

Charters, M.L. 2017. Southern California Wildflowers: Guide to the Pronunciation of Specific, Generic and Family Names.

Corelli, T. 2004. Flowering Plants of Edgewood Natural Preserve (2nd. ed.). Monocot Press, Half Moon Bay, California.

Dave’s Garden.

Elpel, T.J. 2013. Botany in a Day: The Patterns Method of Plant Identification. HOPS Press, Pony, Montana.

Flora of North America. efloras.org.

Harris, J.G., and M.W. Harris. 2013. Plant Identification Terminology: An Illustrated Glossary. Spring Lake Publishing, Spring Lake, Utah.

Keator, G. 2009. California Plant Families. University of California Press, Berkeley and Los Angeles, California.

Native American Ethnobotany DB.

Regents of the University of California. Jepson eFlora. Jepson Herbarium. University of California, Berkeley.