Plumeless thistle (Carduus acanthoides L.)
Plumeless thistle is a biennial forb with purple flowers, deeply lobed spiny leaves, and multi-branched, spiny stems. It reproduces only by seed.
During its first year, the plant forms a basal rosette from a fibrous taproot. In the second year, it produces one to many upright stems that typically grow 3 to 5 feet tall.
Basal leaves may reach up to 7 inches long and are usually deeply lobed, though some may be lance-shaped with toothed margins. Leaves are dark green and arranged alternately along the stem, clasping it at the base and becoming smaller toward the top. Prominent spines occur at leaf tips, along the vertical “wings” of the stems, and sometimes on the underside midrib.
Before flowering, plumeless thistle is often confused with musk thistle; however, plumeless thistle has more heavily divided leaves and more pronounced spiny stem wings, giving it a rougher appearance overall.
Flowering occurs from early summer through fall. Flower heads are smaller than musk thistle, reaching up to 1.5 inches across. Flowers form at the ends of stems and may be solitary or clustered in groups of two to three. Florets range from pink to deep purple and are enclosed by a rounded bulb of spiny bracts.
Seeds are smooth, hairless, oblong achenes (about 5 mm long and 1 mm in diameter).
Quick ID Guide
Life Cycle:
• Biennial
• First-year rosette
• Second-year flowering stems
• Reproduces by seed only
Height:
• 3–5 feet tall
Roots:
• Fibrous taproot
Leaves:
• Dark green
• Deeply lobed
• Very spiny
• Prominent spiny stem wings
• Clasp the stem
Flowers (Early Summer–Fall):
• Pink to deep purple
• Up to 1.5 inches across
• Solitary or in small clusters
• Rounded, spiny bracts
Seeds:
• Smooth, hairless achenes
• Oblong shape
Management Options
Prevention should always be the first line of defense. Maintain healthy, competitive vegetation and minimize soil disturbance to reduce establishment. Use certified weed-free forage, mulch, and gravel, and clean equipment, vehicles, and livestock before moving from infested to uninfested areas.
There are currently no approved biological control agents available for release in Sublette County.
Because plumeless thistle develops a taproot, small infestations can be effectively controlled by hand pulling or digging. Remove as much of the root as possible, cutting the taproot at least 2 inches below the soil surface. Plants should be removed before flowering and seed set. Tilling or hoeing can also provide effective control when the taproot is completely severed.
Targeted grazing by sheep, goats, or cattle may suppress plants before the bolting stage. Grazing becomes less effective once stems elongate. Mowing or chopping will not eliminate established plants, but can reduce seed production if done before flowering. Do not mow once viable seeds are present.
For larger infestations, herbicide treatment may be necessary. Applications are most effective when made to rosettes in spring or fall. Bolting plants can also be treated, though earlier applications generally provide better control. Flowering plants may be cut, and the remaining rosette and stem treated with herbicide to prevent regrowth and seed production. Always read and follow herbicide label instructions, and consult Sublette County Weed & Pest for recommendations on products, timing, and application rates.
