How to Manage Pests

Identification: Weed Photo Gallery

Pacific poison-oak

Scientific name: Toxicodendron diversilobum (Cashew Family: Anacardiaceae)

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Pacific poison-oak is a native perennial broadleaf vine or shrub that is sometimes treelike in form. Poison-oak is found throughout California, except the Great Basin and southwestern edge of the Mojave Desert, up to 5400 feet (1650 m). It inhabits oak woodlands, chaparral, conifer and mixed conifer forests. It is particularly troublesome in the dry sandy soils of the foothills and dry-farmed mountain orchards.

Poison-oak is considered one of the most hazardous plants in the western United States. Its milky, poisonous oil, containing a compound called urushiol, can cause a severe skin rash. Urushiol is found throughout all plant parts except for pollen. Direct contact with broken plant tissue or contact with tools, clothing, or pets that have touched the tissue can lead to contact dermatitis in individuals with an allergic response. Severe respiratory irritation can be induced by breathing the smoke from burning plant material. Repeated exposure often results in increased sensitivity.

Seedling

Cotyledons (seed leaves) are egg shaped to oblong, sit atop short stalks and often are slightly glossy. First leaves consist of three leaflets with a lance-shaped terminal leaflet that is much longer than the football-shaped lateral leaflets. Leaves are opposite to one another along the stem.

Mature plant

The diversity in leaf size and shape accounts for the Latin species name “diversilobum”. When Pacific poison-oak grows as a shrub, it can reach up to 13 feet (4 m) tall. When it grows in a vinelike or treelike form, stems can reach up to 82 feet (25 m) long. Twigs are hairless to sparsely hairy and gray to reddish brown. Leaves consist of three, and sometimes up to five leaflets but three leaflet leaves are most commonly found. The terminal leaflet has a rounded or tapered base that ends in a short stalk. Leaflet edges are smooth, wavy, or have slightly rounded lobes. The upper leaf surface is hairless, or nearly so, and usually slightly glossy. The lower surface usually has sparse, short hairs. Leaves turn bright red in the autumn. Roots and underground stems are extensive and woody.

Flower

Flowering takes place from April through May. Male and female flowers develop on different plants. Small, five-petaled, yellowish-green flowers with flower stalks are produced along loose, drooping branches.

Fruit

Fruit consist of tiny, round berries that are cream colored or creamy white with dark lines and turn brown as they age. They range from 1/17 to 1/4 of an inch (1.5–6 mm) in diameter, are hairless or have sparse stiff hairs, and contain one seed each.

Reproduction

Reproduces from seed and by underground stems.

Related species/Similar looking plants

Skunkbrush

More information

• Broadleaf ID illustration
• Calflora's distribution map
• See UC IPM's Pest Management Guidelines for agricultural information
• See UC IPM's Poison Oak Pest Note for landscape information