Musk thistle (Carduus nutans L.)
Musk thistle is a biennial forb that reproduces only by seed. In its first year, it forms a basal rosette. During the second year, it produces one or more flowering stems.
Plants grow from a fibrous taproot. Second-year stems can reach up to 7 feet tall and may branch. Leaves are alternate and clasp the stem, decreasing in size upward. Basal leaves may reach 6 inches long and are typically deeply lobed, though some may be lance-shaped with toothed margins. Leaves are green but often have a frosted or silvery appearance, sometimes with pinkish or silver margins. Sharp white spines occur on leaf tips, along stem “wings,” and occasionally on the underside midrib.
Flowering occurs from early summer through fall. Flower heads are large—up to 3 inches across—and form at the ends of stems. Florets range from pink to deep purple and form a dense central disk. The large, spiny bracts beneath the flower head are a distinguishing feature. Mature flower heads droop or nod, giving the plant its characteristic appearance.
Seeds are oblong achenes (about 5 mm long) with dark longitudinal markings and a fine pappus that aids in dispersal.
Quick ID Guide
Life Cycle:
• Biennial
• First-year rosette
• Second-year flowering stems
• Reproduces by seed only
Height:
• Up to 7 feet tall
Roots:
• Fibrous taproot
Leaves:
• Deeply lobed
• Frosted or silvery appearance
• White spines
• Clasp stem
• Spiny stem wings
Flowers (Early Summer–Fall):
• Pink to deep purple
• Up to 3 inches across
• Large, spiny bracts
• Drooping (“nodding”) heads
Seeds:
• Oblong achenes
• Dark longitudinal spots
• Fine pappus
Management Options
Prevention and sound cultural practices should be emphasized whenever possible to limit the spread and establishment of musk thistle.
There are currently no biological control agents approved for release in Sublette County.
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Because musk thistle produces a taproot, small patches can be controlled by hand pulling or digging, provided the entire taproot is removed. Mechanical methods, such as tilling or hoeing, are also effective if the root is severed at least 2 inches below the soil surface. Targeted grazing by sheep, goats, or cattle may help suppress plants before bolting. While mowing or chopping will not eradicate established infestations, it can reduce seed production if done before seeds mature. Avoid mowing once viable seeds are present.
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Extensive infestations may require herbicide applications. Treatments are most effective when applied to rosettes in the spring or fall, although bolting plants can also be treated successfully. Flowering plants may be cut, and the remaining rosette and stem treated with herbicide to prevent regrowth and seed development. Always read and follow label instructions before applying any herbicide. Contact Sublette County Weed & Pest for guidance on product selection, application rates, and timing.
