Pacific Poison Oak (Toxicodendron diversilobum),
Sumac Family (Anacardiaceae)

Habitat and Range

You’ll see poison oak just about everywhere out at Mt. Pisgah from shady areas with dappled sunlight or in the full sun. It grows in the riparian zone along the river, along the trails in the wooded areas and out in the grasses in the oak-savanna area. You will see it climbing up the sides of trees, growing in the form of a dense shrub or growing as a small plant in the grass.

The pacific poison oak grows in the western region of the United States and British Columbia. It grows west of the Cascades in Washington and Oregon. It grows in California west of the Sierras.

Pacific poison oak can grow from 3 - 10’ tall as a shrub, and it can grow up to a 100’ as a vine.

All parts of Pacific poison-oak (except the pollen) contain an oily substance, urushiol (pronounced: you-ROO-shee-all), that is present throughout the entire year. When these oils come in contact with the skin, you can have an allergic reaction that causes an itchy rash. If burned the oils can also travel in smoke and be inhaled. Many people are immune when they are young, but may become sensitive to it with age.

I recommend that you become very familiar with this plant.

Leaves

The leaves have 3 (sometimes 5) leaflets, hence the saying, “Leaves of three, let it be.” The leaflets can vary in appearance. They can be ovate with a fairly smooth edge to a lobed edge that resembles oak leaves. The upper leaflets usually have a shiny or glossy look. They often turn a beautiful red in mid to late summer or in the fall.

In the fourth photo, you can see how the poison oak leaves resemble the leaves of the pacific white oak. The light green leaves at the top of the photo are poison oak, and the dark green leaves at the bottom of the photo are pacific white oak.

Flowers

The flowers are small, about 1/4 inch in diameter and greenish-white. I often see honeybees on them collecting pollen and nectar.

Fruit

The berries form in a loose cluster. They are white, up to 1/4 inch wide, and lightly striped longitudinally. Many birds eat them in the fall and winter such as chickadees, towhees, bushtits, juncos, and flickers.

Quick Reference:

Height: Shrub: 3 - 10’, vine: 100’
Leaves: 3 leaflets, ovate or lobed
Fruit: White berries
Habitat: Riparian, woodlands or oak-savanna areas

Journal Location:
Mt. Pisgah Arboretum, Eugene, OR
Mt. Pisgah Arboretum Website
Mt. Pisgah Arboretum Plant List