Toxicodendron diversilobum aka Pacific Poison Oak

Toxicodendron diversilobum is known as western poison oak. Another name is Californian poison oak. It belongs to Anacardiaceae, the cashew family. Its older name is Rhus diversiloba. Its native range spans southwestern Canada to Mexico. Most plants grow west of the Cascades and Sierra Nevada. It grows in coastal scrub and open woods. Inland, it grows on hillsides, in thickets, and beside streams. It also grows in conifer forests and foothill valleys.

This plant sheds its leaves and forms shrubs or climbing vines. Shrubs reach about 3–10 feet (0.9–3.05 metres) tall. Oregon State gives a smaller range for shrubs. That range is 3–7 feet (0.9–2.1 metres). Garden spread reaches about 7 feet (2.13 metres). Climbing stems can reach 50 feet (15 metres). Young bark is smooth and tan. With age, it turns rough and gray-brown. Thin twigs bear short, stiff side shoots and hairy buds.

The leaves grow in an alternate pattern along the stems. They are pinnate, with leaflets arranged along a shared stalk. Most leaves have three leaflets. The end leaflet is larger than the two side leaflets. Their edges may be smooth or toothed. They may also have uneven lobes. New leaves show red mixed with green. They turn green, then red or yellow before falling in autumn. Small flowers form loose, hanging groups from April to June. Sources give their color as white or yellow-green. Separate plants bear male and female flowers. The round fruits are drupes, with a stone inside. Their skins range from white to cream or gray-white. Fruits hang in groups and stay after the leaves fall.

The Missouri Botanical Garden lists USDA hardiness zones 5–9. It favors full sun but accepts some partial shade. Full shade does not suit it. It grows best in moist sandy or medium loam. The soil must drain well. Other soil types suit it if they drain well. It needs moderate water but can cope with drought. Its fall color and fruit display do not make it suitable for gardens. Sources do not recommend planting it there. Reported uses include stems for baskets and dyes. People have also used seed oil for candles. Past use as medicine does not prove it safe. The listed food rating is zero.

All parts contain an oil called urushiol. It can cause severe skin reactions. Signs may start long after contact and last a long time. Animals can carry the oil on their fur. Smoke from burning plants can also expose people to it. Even old, dried plant parts can cause reactions. Wear gloves and other clothes that protect you when handling it. This risk from contact is the main concern in landscapes.