Poison Oak

Toxicodendron diversilobum

young rosy leaves
The rosy growth of spring | East Basin, south side | Feb. 2011

Poison oak (Toxicodendron diversilobum) is not an oak; nor is it technically poisonous. It has attractive leaves that are bronze when young in the spring, shiny green in the summer and gold and red in the autumn before they fall.  It may be the most feared plant in California.

 “Leaves of three, let it be”

The plant produces a substance, urushiol, that most people are allergic to.  It produces an unbearable, itching, blistery, oozing skin rash that is “nothing more than the immune system gone haywire, fighting some harmless substance, like Don Quixote charging at windmills.” 56 Only humans are bothered. Animals graze on the foliage with impunity, and many birds find the seeds a delicacy.

Other Common Names:

Western poison oak, Pacific poison oak

Description 4,5,26,59,514

Poison oak has variable growth form, largely dependent on its immediate habitat. In open areas it can form a low, weak-stemmed shrub or a dense woody thicket up to 16 feet tall (4 m). In forested or partly shaded areas it is often a thick-stemmed vine that clings to trees with aerial roots and may reach 75 feet (25 m) or more in height. Poison oak has an extensive near-surface rhizome system which allows vegetative propagation. Stem, leaves and fruit have numerous resin ducts, which coat the plant with an oily resin that contains urushiol that cause severe dermatitis.

Leaves are bronze in early spring turning bright green, with smooth, shiny surfaces. They are 4¾ inches (12 cm) or less in length, usually with three ovate, elliptic or obovate leaflets (occasionally five). The margins are variously lobed or toothed and may resemble typical oak leaves. The terminal leaflet is often slightly longer and has a long, slender stem (petiolule); lateral leaflets have very short petiolules,  if any. Leaves are deciduous, turning golden and red in the autumn before falling.

Flowers are usually dioecious, forming loose clusters from leaf axils. The sexes can be difficult to distinguish without close inspection (closer than one usually wants to do in the field). They are small, less than ¼ inch (0.8 cm) with usually five (occasionally six or seven) cream colored petals. A nectary disk surrounds the base of the pistil, producing copious nectar. In a mature male flower, petals are flared outward and downward. There are five, sometimes six, stamens with yellow anthers that extend beyond the throat of the flower. The pistil is rudimentary.  The female flower is smaller and petals are less flared, having more of a disk shape; it lacks the bright yellow anthers. The single pistil has three styles that are fused at the base and forming three unequal lobes. Poison oak flowers March – April, following the appearance of new leaves.

Fruits are small, rounded and pale. Technically, the fruit is a “drupe”, like a plum or apricot. There is a thin, papery outer layer (the fuzzy skin of an apricot), a pale fleshy middle layer striated with black resin ducts (the flesh of an apricot) and a hard third layer surrounding a seed (the pit of an apricot).

three-parted leaves

Leaves usually trifoliate | East Basin, south side | April 2019

male flowers

Male flowers | East Basin, south side | April 2020

Cluster of female flowers | Photo credit: Margaret Fillius | Mission Trails Regional Park | March 2016

Distribution 5,7,11,27,89

Poison oak is native to the coastal states of North America, from southern British Columbia into northern Baja California

It may be the most widespread shrub in California,5 where it is common below  5000 ft (1500 m) especially west of the Sierras and the Mohave desert.  It prefers cool, somewhat shaded areas with a bit of moisture, such as canyons, river edges and and north facing slopes, but can be found almost anywhere.

Fortunately, in the Reserve, there is only one patch of poison oak adjacent to public trails. This is a sizeable thicket of one or more male plants on the main trail to La Orilla,  east of the Santa Helena trailhead, near where the trail crosses the drainage channel that is lined with young cottonwoods. Recently, the Rangers have marked it with warning signs.

 

Classification 2,11,34,44

Poison oak is a dicot angiosperm in the sumac family, Anacardiaceae. Members of this family have small, five-petaled flowers, each of which produces one seed with a hard seed coat that is surrounded by fleshy tissue or some form of gelatinous coating (such a fruit is called a drupe).

Many members of the Anacardiaceae, such as cashews and mangos, are familiar food crops, others, such as poison oak, poison ivy and poison sumac, contain urushiol, which produces severe contact dermatitis. Other family members include the closely related lemonade berry (Rhus integrifolia) and laurel sumac (Malosma laurina).

Toxicodendron is a genus of approximately 14 species that were formerly placed in the genus Rhus. All members of the genus produce the oil urushiol. Poison oak is the only member of the Toxicodendron genus to occur in California. Western poison ivy (Toxicodendron rydbergii) is a different species and is not found in California.

Alternate Scientific Names:

Rhus diversilobum

Jepson eFlora Taxon Page
warning sign before poison oak plant

East Basin, south side | April 2020

poison oak thicket with shiny green leaves

Thicket in summer | East Basin, south side | April 2020

tangle of leafless branches in winter

Thicket on left in winter | Feb. 2018

Ecology 5,514

The evolutionary pressures that produced urushiol in plant sap are uncertain as are the present-day adaptive values of this substance. It does not seem to deter large grazers since many mammals browse on poison oak and in some parts of California it is the most important forage for black-tail deer.5 Many birds eat the seeds. It is possible that urushiol protects the plant from bacterial or fungal infection following injury. However, that is speculation.

poison oak thicket with shiny green leaves

East Basin, south side | April 2019

close-up of stem with oval pores

Close-up of stem. Do not touch. | East Basin, south side | April 2020

Shiny green leaves turning red in fall

Thicket developing autumn color | East Basin, south side | Aug. 2010

Human Uses

Did the early California Indians have immunity to poison oak and if so, was it acquired or inherited?514 This remains a controversial question.

Reports of the uses of poison oak by Native Californians are confusing. The Chumash15 are said to have used powdered poison oak to heal wounds and the juice of the plant to stanch blood flow. They also harvested leaves in the spring when the sap was flowing well, and held leaves to a wart or persistent sore, letting the “tears” of the plant drip into it. The juice turned black on the skin. When the juice dried and fell off, the sore was healed. The root of poison oak was boiled and the cooled decoction used to treat dysentery. The Kumeyaay used a similar decoction to improve vision. In northern California, the Pomo used poison oak stems in baskets and poison oak ashes or sap in tattoos.5,15,282  This usage implies immunity to urushiol.

Some Chumash believed that spitting on the plant would confer immunity, as would occasionally eating a poison oak leaf.

By the 1950’s, the Chumash had ceased using poison oak for medicinal purposes and herbal remedies for the skin rash were being developed. A decoction of mugwort leaves (Artemisia douglasiana) was used,15 also a decoction from the gum of gumplant (Grindelia spp.).514

 

poison oak thicket with shiny green leaves

East Basin, south side | July 2010

three-parted leaves and dangling flower cluster

The middle leaflet has a longer stem | East Basin, south side |

male flowers

Male flowers | East Basin, south side | April 2020

Interesting Facts 41,514

Another species in the sumac genus is the Japanese lacquer tree (or Chinese lacquer tree, T. vernicifluum). The sap of this tree is the basis for the exquisite finish that characterizes Japanese lacquerware. Its use dates back thousands of years probably originating in China. Because the lacquer contains urushiol, it is prepared only by skilled artisans. Our word urushiol comes from the Japanese word for that lacquer, “urushi”.

Thicket from which many leaves have fallen

East Basin, south side | Aug. 2018

young bronze leaves

Earliest spring leaves are bronze | East Basin, south side | March 2020

dangling flower cluster

Male flowers | East Basin, south side | April 2019

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