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Musk thistle  (Carduus nutans L.)

Special Announcement Regarding Musk thistle:
Most of our beneficial seed head weevils, an introduced biological control, didn't survive the 2002 drought. This season, we are observing numerous egg cases under the flower heads of the Musk thistles. This appears to be natural recovery response. Look for "pin head size, mud like" deposits (see second picture on the left). These are the egg cases of the Rhinocyllus conicus (seed head weevil), a biological control insect that feeds on the Musk thistle seeds in the larvae growth stage and destroys most (not all) of the seed production. If these egg cases are present in sufficient numbers, then shoveling activities, collecting and disposing of flower heads or spraying adult thistles will interrupt their life cycle and reduce their numbers which ultimately means more seed production. A better management alternative would be to leave the adult thistles and weevils alone during the summer, then manage the new thistle rosettes during the September, October fall window. These fall rosettes will become next years flowering, seeding adults. If by randomly checking you do not find sufficient numbers of egg cases, say minimum of 4 to 6 per flower head, then use the recommended integrated management techniques found on this site.

Musk thistle is a member of the Aster family, Thistle tribe. It is an introduced biennial, winter annual, or rarely annual, which reproduces by seed. The first year’s growth is a large, compact rosette from a large, fleshy, corky taproot. The second year stem is erect, spiny, two to six feet tall and branched at the top. The leaves are alternate, deeply cut or lobed with five points per lobe, very spiny, three to six inches long and extend (clasp) down the stem. The waxy leaves are dark green with a light midrib and mostly white margins. The large and showy flowers are terminal, flat, nodding, 1½ to 2½ inches broad, purple, rarely white, and surrounded by numerous, lance-shaped, spine-tipped bracts. Blooms appear in late May and June and set seed in June or July. Seeds are straw-colored and oblong. Musk thistle is commonly found in pastures, roadsides, and waste places. It prefers moist, bottom land soil, but can be found on drier uplands, also. It is becoming an increasing problem throughout Colorado.

Musk thistle Fact/Info Sheet (PDF) Click Here