Western poison oak (T. Diversilobum)

Also known as Pacific poison oak;
Toxicodendron diversilobum

This toxicodendron is located along the Pacific coast. It comes in the form of a climbing vine or shrub. It is named “oak” because its leaf is reminiscent of white oaks, with some curves on the edge of the leaf. The western poison oak dermatitis looks much like poison ivy. The first European to discover western poison oak was David Douglas between 1825 and 1830 in the Columbia River valley.

Western poison oak growing as a climbing vine.

Western poison oak growing as a climbing vine.

Western poison oak growing as a brush.

Western poison oak growing as a brush.


Leaf

As , its leaf is reminiscent of white oak and it is easy to confuse. Its alternate leaves, in groups of three leaflets or rarely 5, 7, or 9, while other toxicodendrons always have only three leaflets. Each leaflet can measure up to 4 in (10 cm) long and has a glossy texture. The appearance of the leaves can vary greatly from one plant to another: scalloped, toothed, or lobed edges. In the fall, the leaves turn to orange or red, like other toxicodendrons.

Western poison oak shrub at fall.

Western poison oak shrub at fall.


Trunk and stems

The color of the stems and trunk is a grayish brown. Under good conditions in open sunlight, it grows as a dense shrub with a trunk up to 8 in (20 cm) diameter. Often, it can grow as a 3 to 4 feet (1.25 m) high bush. In shaded areas it grows as a climbing vine. With its climber variety, it generates fine aerial roots that allow it to cling to the bark and climb on trees.

Western Poison Oak (T. diversilobum) as a climbing vine.

Western Poison Oak (T. diversilobum) as a climbing vine.

Western poison oak (T. diversilobum) shrub. Source: Bri Weldon on Flicker

Western poison oak (T. diversilobum) shrub. Source: Bri Weldon on Flicker


Flowers and fruits

The flowers in clusters, at the base of the stems, are pale, yellow green. These flowers are very small, often only half an inch (1 cm) in diameter and appear all throughout spring (April – May). The fruit is small, round, striped (like a pumpkin), and yellowish or greenish. The fruits come in drupes near the flowers, at the base of the stems, and appear in late summer.

Western poison oak flowering. Source: Eeekster

Western poison oak flowering. Source: Eeekster

Western poison oak (T. diversilobum) fruits. Source: Noah Elhardt

Western poison oak (T. diversilobum) fruits. Source: Noah Elhardt


Propagation

This vigorous plant is propagated by its seeds, located within fruit. The fruit can drop near the plant or birds can eat the fruit and expel the seeds later in their droppings. Another plant can then push into this place, if the germination is done correctly. Also, the plant spreads using its horizontal rootstocks (rhizomes), which are stem tissue. A root system can cover several feet in diameter.


Habitat and distribution

Western poison oak is adapted to a great range of environments like moist evergreen forest, woodlands, dry chaparral and by rivers and creeks. It grows in many different soils but seems to prefer sunny places, clear and not too dry. This toxicodendron is located along the Pacific coast from Vancouver, including southern Vancouver Island in a few remote areas to the Baja California peninsula. Note that where western poison oak grows, in western British Columbia and the northwestern United States, is absent.

Western poison oak habitat area (Toxicodendron diversilobum).

Western poison oak habitat area (Toxicodendron diversilobum).


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