Infestation of Musk Thistle Sierra Valley

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Infestation of Musk Thistle

Sierra Valley

Nodding Thistle

Sierra Valley Resource Conservation District

Po Box 3562

Quincy, CA 95971

P: 530-283-0455

Carduus nutans

Mature Plant of Musk Thistle


Erect thistles with prickly winged stems and leaves. Plants exist as basal rosettes until flowering shoots develop at maturity. The thistle head weevil (Rhinocyllus conicus), an introduced biocontrol agent, attacks Carduus species and several other thistles, including some native thistles (Cirsium spp.). Control of Carduus thistle infestations by the weevil varies by species and regionally from excellent to poor. The largest known location of musk thistle is found at Mt. Shasta (Siskiyou County). Also found scattered through Dog Valley (Sierra County) and southern Long Valley (Lassen County and Plumas County); and along Highway 395, Interstate 80, Reno, and Verdi along Truckee River (Washoe County, Nevada).


First recorded in Northern America in Pennsylvania in 1852, musk thistle was introduced accidentally to the east coast in ship ballast water. This invasive weed spread to the Midwest in the early 20th century, and was established in early 1940s.


  • Habitat: Pastures, forest, roadsides, rangelands, waste areas, ditches

  • Life Cycle: Biennial

  • Flowering Time: June to Mid- Fall

  • Spread: Wind carries thousands of seeds

  • Height: 1-6 feet, commonly 3-4 feet

  • Noxious Weed List: California A-rated

Musk Thistle

Nodding Thistle


  • Flower: Single, flat, very large nodding flower at the end of bent, naked stem

  • Flower Color: Deep pink to purple

  • Stem: Naked and bent towards flower, winged spines toward base

Carduus nutans

Musk Thistle

Why is it a threat to Sierra Valley?

  • Long taproot contributes to soil erosion

  • Seeds remain viable in the soil for over ten years

  • Large infestations limit access for animals

Think you may have this invasive weed or seen one?

  • Please contact your local Resource Conservation District.

Sierra Valley RCD

Po Box 3562

Quincy, CA 95971


  • A member of the staff will come identify the plant and record the weed’s location.

  • District staff will work with you to determine the best method of control that fits your property and the invasive weed’s present. With your permission, the District will monitor the site after control measures are completed to ensure control efforts succeeded.

noxious weed control depends on you.”

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