Carduus nutans L.

Musk Thistle

© SRTurner

Family - Asteraceae/Cardueae

Habit - Coarse biennial or annual forb, from stout taproot.

Stems - Erect, to 3 m, glabrous to finely hairy, often branching, winged (from extended leaf bases). Wings with stiff, straw-colored spines along margins.

	Overwintering basal rosette.
© SRTurner

	Mid-upper stem and leaves.
© SRTurner

	Upper stem and leaf base.
© SRTurner

	Lower stem.
© DETenaglia

Leaves - Basal leaves in a rosette, to 60 cm long, oblong, elliptic, or lanceolate, variously lobed, spiny-margined, both surfaces glabrous or pubescent. Stem leaves alternate, to 30 cm, lanceolate to broadly elliptic, variously lobed, the lobes mostly triangular to ovate, spiny-margined, sessile with bases decurrent on stem and forming wings, glabrous or pubescent on both surfaces, sometimes glaucous.

	Lower leaf.
© SRTurner

	Lower leaf lobe, adaxial surface.
© SRTurner

	Lower leaf lobe, abaxial surface.
© SRTurner

Inflorescence - Solitary terminal heads, usually nodding, 2-7 cm broad, the stalk long and usually relatively naked, usually densely felty-hairy.

	Inflorescence.
© SRTurner

Heads - Involucral bracts 15-50 mm long (including the spiny tip), 2-8 mm wide, lanceolate to narrowly ovate (often slightly constricted in the basal 1/2), the outer and median ones spreading to reflexed above the midpoint, greenish purple, with prominent midvein, gradually or more commonly abruptly tapered to a hard, straw-colored or occasionally purple, spiny tip, the surfaces glabrous or felty- to cobwebby-hairy. Inner bracts with spines soft or missing, erect to spreading, purple, with prominent midvein.

	Flowering heads.
© SRTurner

	Flowering head.
© SRTurner

	Involucre.
© SRTurner

	Laterally sectioned head.
© SRTurner

Flowers - Ray flowers absent. Disk flowers 18-30 mm long, the corollas 5-lobed, dark pink. Pappus of capillary bristles 13-18 mm long, white. Stamens 5, mostly included. Style exserted well beyond corolla, pinkish to lilac at apex.

	Closeup of anther tubes, stigmas, and pollen.
© SRTurner

	Individual florets.
© SRTurner

Fruits - Achenes glabrous, appearing basally attached, 2.5-4.0 mm long, oblong or slightly narrower at the symmetrical base, somewhat flattened and sometimes slightly 4-angled in cross-section, the tip usually with a slightly raised rim, the surface somewhat shiny, light brown to grayish brown, with numerous longitudinal, darker brown stripes.

Flowering - June - October.

Habitat - Pastures, prairies, fields, fencerows, disturbed sites, roadsides, railroads.

Origin - Native to Eurasia.

Lookalikes - Other thistles (Cirsium spp.), though this one is readily distinguished by its general appearance.

Other info. - This is an aggressive species which is considered a noxious weed in Missouri as well as throughout most of the continental U.S. In Steyermark's time (1963), it had been recorded from only four counties in the state. Since then it has spread to probably every county, and is a common sight along many highways during the summer.

Large numbers of achenes are produced by each plant, and these can travel great distances with the wind by virtue of the large, silky pappus. In addition to mowing and spraying with foliar herbicide, control attempts have included biological approaches. Beginning in 1979, controlled releases of two insect predators of the plant were carried out in Missouri, and these showed some degree of success in controlling the plant's spread. Unfortunately, one of the weevils employed in this approach also attacks native thistles, resulting in adverse impacts to populations of an imperiled species, Cirsium muticum. The decline of this species could, in turn, threaten populations of the endangered swamp metalmark butterfly (Calephelis muticum), which relies upon the plant for food during its larval stage. This cascade of effects illustrates both the interconnectedness of ecological systems, and the peril of attempting to manipulate them, even with the best of intentions.

Three subspecies of Carduus nutans are currently recognized in Missouri. The plant is not totally lacking in redeeming qualities, as the flower heads are extremely popular among insects, serving as nectar sources for bumblebees and several species of butterfly. The nodding pose of the mature flower heads gives rise to the species name.

Photographs taken at Flemming Park, Jackson County, MO., 5-13-00 (DETenaglia); also at Marais Temps Clair Conservation Area, St. Charles County, MO, 9-30-2006, Weldon Spring Conservation Area, St. Charles County, MO, 6-9-2007, and near Labadie, Franklin County, MO, 6-7-2022 (SRTurner).