Asteraceae: Carduus nutans

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Family Name

Asteraceae

Common Name

Musk thistle, nodding thistle

Description

Carduus nutans is a biennial (sometimes annual or short-lived perennial) herb in the Asteraceae family. It develops from a stout, fleshy taproot and does not reproduce asexually via stolons or rhizomes. During its first year, it forms a large basal rosette of spiny, deeply lobed leaves, each up to 40 cm long, with white, woolly undersides and sharp, yellowish spines along the margins. In its second year, it produces a single, stout, erect, and often branched stem, usually 60–200 cm tall, with conspicuous wings (spiny extensions) running down from the leaf bases. Leaves are alternate and deeply lobed with spiny margins; basal and lower leaves can be 10 to 30 centimeters long and 5 to 10 centimeters wide, narrowing upward on the stem. Petioles are winged and spiny-edged. Stems and leaves are covered with cobwebby hairs, especially when young. Flowering occurs from June to September. Inflorescences are solitary or in small clusters at the stem tips; each is a large, pendulous (nodding) flower head, 3–7 cm across. The involucre consists of several series of broad, rigid, spiny-tipped green bracts (phyllaries) about 2 to 3.5 centimeters long and 0.5 to 1 centimeter wide, forming a spiny protective cup around the flower head base. Each flower head contains numerous tubular florets with 15 to 25 bright purple to pink ligulate corollas, each ligule approximately 1.5 to 2.5 centimeters long and 3 to 5 millimeters wide. Florets have five lobes; stamens number five with anthers fused around the ovary. The pistil consists of a single ovary with a two-branched style. The fruit is a small, ribbed achene about 4 to 5 millimeters long topped with a pappus of fine hairs facilitating wind dispersal. Fruits mature late summer into fall. Musk thistle is an introduced species in South Dakota, most common in disturbed sites, roadsides, pastures, fields, and rangelands, and is found statewide but especially abundant in the east and central regions.

Additional Notes

Musk thistle is not native to North America and is classified as a noxious weed in South Dakota and many other states due to its invasive growth, prolific seed production, and negative impact on native plants and grazing lands. The species has little value for wildlife or humans and is not used in horticulture. While some thistles are edible (young stems or roots), musk thistle is rarely used due to its spines.