Poison Oak, Pacific Poison-oak: Toxicodendron diversilobum (Synonyms: Rhus diversiloba, Toxicodendron radicans, Toxicodendron radicans ssp. diversilobum)

Newly leafed out greenery of poison oak as seen at left on USFS lands just east of Major Creek, Columbia River Gorge......April 3, 2018. The photo at right shows ripening fruits of poison oak as seen in clear cuts along the Pacific Crest Trail between Bonneville Dam and Gillette Lake, Columbia River Gorge National Scenic Area.......September 9, 2018.

The photo at right shows Rhus diversiloba in flower from Catherine Creek, Columbia River Gorge..........May 13, 2000.

Poison oak is a problematic shrub or climbing vine from 1-15 meters high. The herbage ranges from glabrous to pubescent, and all parts of the plant contain a volatile oil which causes a severe dermatitis (rash) on many humans. The leaves are trilobate, the individual leaves ovate to obovate in shape with wavy to fairly deeply lobed margins. They range from 3-7 cm long and have rounded to shortly acute tips. Leaf surfaces are commonly a shiny green, while new foliage is commonly a deep burgundy red.

The inflorescence consists of loose, axillary panicles. The whitish flowers are either male or female. They range from 1-2 mm long and have 5 petals and 5 sepals (which are about half the length of the petals and pubescent to glabrous). The glabrous fruits are globose, whitish to greenish-white and about 5 mm long.

As described above, all parts of this plant contain dermatitis-causing oils. Exposure to any part of the plant is thus to be discouraged. No part of the plant should be eaten, and one should avoid its smoke when the plant is burned. Poison oak is a major annoyance to hikers, fishermen and hunters, beach-goers, land owners and fighters of forest fires.

Along the coast, poison oak may be found in the open or in scrub areas, or as part of the forest understory in areas behind the beaches. In drier inland areas, it is common at low elevation in valley or canyon bottoms.

Toxicodendron diversilobum is found primarily west of the Cascade and Sierra Nevada crests from southwestern British Columbia south through the Puget Trough to northern Baja California. It extends eastward through the Columbia River Gorge to Klickitat County in Washington, and may also be found in the Virginia Mts. to the west of Pyramid Lake in Nevada. In western Oregon, it is largely coastal, although it extends in land to the Cascades in the Siskiyou Mts. of southwestern Oregon.

In the Columbia River Gorge it may be found between the elevations of 100'-1800' from the mouth of the Sandy River east towards The Dalles, OR. Poison oak may on a casual glance be confused with Oregon white oak, which is often found in the same locality. The latter tree has simple, pinnately lobed leaves.

Poison oak beginning to bloom at left along the Memaloose Trail, Columbia River Gorge......May 26, 2012. The photo at right shows new foliage as seen at Catherine Creek.......April 19, 2017.

New leaves and some flower buds of poison oak as seen atop the first bench along the Lyle-Cherry Orchard Trail, Columbia River Gorge.......April 2, 2015.

Poison oak leaves under oaks on slopes at left above the Klickitat River north of Klickitat, WA......April 29, 2021. The photo at right shows poison oak leaves and flower buds along the old haul road about one-half mile north of Klickitat, WA.....April 29, 2022.

Poison oak as photographed at left at Cathernine Creek, Columbia River Gorge......May 13, 2000. The photo at right shows the bright fall foliage and white fruits of poison oak as seen at Catherine Creek, Columbia River Gorge........September 22, 2017.