Poison oak is not an oak but member of the sumac family. It grows as a shrub or occasionally as a vine, with shiny leaves composed of three coarsely lobed leaflets whose shape is somewhat similar to that of leaves of white oaks. The leaf shape is reflected in the scientific name of poison oak, Toxicodendron diversiloba, which means "diversely lobed poison tree".

Poison oak has small, greeenish-white flowers that bloom in the spring, with male and female flowers usually occurring on separate plants. The female flowers develop into clusters of white berries in the late summer and fall. To recognize poison oak, it is useful to remember the old rhyme: "leaflets three, let them be; berries white, poisonous sight." In the fall poison oak leaves turn a beautiful crimson, making them easy to pick out among the other vegetation.

Poison oak is closely related to poison ivy, poison sumac, and the Asian lacquer tree (the source of natural lacquer). These plants have abundant resin canals just beneath their bark, in their leaves and under the skin of their fruits. When the sap or resin of these plants contacts the skin of sensitive individuals, it can cause a painful allergic reaction characterized by a rash, severe itching, and blistering.

The substance responsible for this reaction is called urushiol, which is actually a mixture of phenolic compounds known as catechols. Urushiol takes its name from the Japanese word urushi, meaning lacquer. Some hypersensitive individuals will react to as little as 2 mg of urushiol. An estimated 80-90% of Americans will react to 50 mg of urushiol, about the same weight as a grain of salt. Urushiol is easily transferred from clothing or pet fur to skin, but is difficult to wash off skin. It readily penetrates the epidermis and becomes bound to cell membranes, where it may activate an allergic immune response. White blood cells may attack and kill skin cells bound with urushiol, leading to a blistering rash. Allergic reaction typically appears in 1-3 days but may take several days or weeks.

Urushiol does not appear to affect wildlife. The leaves and fruits of poison oak are eaten by deer and birds. Some humans also appear to be insensitive to urushiol. There are numerous cases, however, of previously insenitive individuals becoming sensitive upon repeated or heavy exposure to urushiol.

Although reliable data are in short supply, urushiol sensitivity appears to have been much less common or severe among Native American populations prior to European contact than it presently is among European-Americans. Several cultures in central California used poison oak twigs in basketry. The Karok and other northern California cultures wrapped food in poison oak leaves for baking. The Pomo used poison oak lacquer to treat warts, ringworm and rattlesnake bites, and also used it as a black dye, not unlike the Chinese and Japanese use of the lacquer tree. It has been suggested that Asians are also less likely to be sensitive to urushiol. The reasons for insensitivity in some individuals or ethnic groups are not clear. Insensitivity may reflect a genetic difference in the production of receptors for urushiol on immune system cells, or possibly it may reflect a difference in immune system development in response to early exposure to urushiol.

Persons exposed to poison oak should wash their exposed skin as well as their clothing and any pets that may have urushiol on their fur. Rubbing alcohol may be more effective in removing urushiol than soap and water, although once urushiol has penetrated the skin there is little chance of washing it off. Creams containing oxidizing enzymes can destroy urushiol on the skin surface, but will have little effect once it has penetrated the epidermis. The most accepted treatment for poison oak rash is hydrocortisone creams, lotions, or sprays. Calamine lotion may help with mild cases. Severe cases should be treated by a doctor.

Poison oak is widespread in California and Oregon, but is uncommon north of the Columbia River. Poison oak reaches the northern limit of its distribution on the Sunshine Coast of British Columbia, and is found only sporadically north of the southern Puget Sound praries. In Seattle, a small patch of poison oak is known in Lincoln Park and a large one occupies the south slope of Pinoy Hill in Seward Park.

Why does Seward Park have a large patch of a plant that is rare in our region? Poison oak prefers a drier, better-drained habitat than is commonly found in western Washington. It is usually found on dry rocky soils. Because Pinoy Hill in Seward Park is composed primarily of bedrock rather than of glacial till like most other Seattle hills, the soil is relatively thin and well-drained, and the south-facing slope receives plenty of sun to help keep it dry. Poison oak is found in a few other sunny locations in Seward Park, but most of it occurs on the south hillside just east of the Nature Center, playground and picnic shelter #1.

Poison oak often associates with other plants that like sunny locations and well-drained soils. Another locally uncommon plant found high on the south side of Pinoy Hill among the poison oaks is snowbrush or sticky-laurel (Ceanothus velutinus). This evergreen shrub has large oval, somewhat sticky leaves that are distincitive in having three prominent veins. The name snowbrush refers to the dense clusters of small white flowers.

The seeds of snowbrush are heat-resistant and are activated by fire, so that new seedlings germinate when other vegetation has burned away, leaving an open area in which snowbrush can thrive. The roots of snowbrush host nitrogen-fixing bacteria, giving it an additional advantage in colonizing open areas.

Look for snowbrush near the top of the hill below the Douglas-firs. (Binoculars are helpful for seeing the distinctively veined leaves). The few plants found here in Seward Park may be the only ones in Seattle. Other shrubs share the hillside with poison oak and snowbrush. Some, like hazelnut, saskatoon and snowberry, are found in the forest to the north as well. Others, like hairy honeysuckle and tall Oregon grape, are largely replaced in the forest by more shade-tolerant relatives (orange honeysuckle and Cascade Oregon grape, respectively).

Prominent among the trees associated with poison oak is our native Garry oak (Quercus garryana). A small grove of these elegant and drought-tolerant trees is found east of the brushy hillside dominated by poison oak, just beyond the turn-around loop of the south parking lot. These slow-growing trees live to be hundreds of years old. They are becoming less common in the Puget Sound region both because of development and because without fires (which they can survive), they eventually become shaded out by conifers.

A frequent associate of Garry oak is madrona (Arbutus menziesii), perhaps our most strikingly beautiful native tree, with its leathery evergreen leaves and peeling bark. The south side of Seward Park has the largest grove of madronas in the city, with trees scattered through both the oak meadow and the brush community. Madronas are also dispersed through the old growth douglas-fir forest on the northern part of the peninsula.