Surprise Field Office

Noxious Weeds

Scotch Thistle (Onopordum acanthium)
Sunflower Family (Asteraceae)

Photo of Scotch Thistle

Description:  Biennial or winter annual up to 10 feet, with large, fleshy taproot. Stems erect, branching with sharp, spiny leaf wings extending down onto stem. Leaves are large, coarsely lobed, finely, densely hairy on both sides, with very sharp yellow spines on margins. Basal leaves may be over 2 feet long and 1 foot wide. Flowers solitary, numerous, 1 to 2 inches in diameter, violet to reddish. Phyllaries spine-tipped. Seeds deep brown to black, distincly wrinkled, and 3/16 inch long borne in a honeycomb-shaped receptical.

Habitat:  Native to Europe and Mediterranean. Invades most habitats: waste areas, roadsides, dry meadows, rangelands, pastures, and sometimes waterways. Reproduces by seed which are viable for 30+ years. Cut or chopped plants may still flower and set seed. Forms dense patches which are impenetrable to livestock, wildlife, and humans. Extremely aggressive.

Distribution:  Scotch thistle is widespread in northeastern California and northwestern Nevada (Washoe County). The largest infestations are in New Pine Creek and Surprise Valley areas (Modoc County), and Big Valley (Lassen County and Modoc County).

Flowering Period: Mid-June to September.

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