Musk thistle Carduus nutans L.
Family: Asteraceae (Composite family)
Life cycle: Biennial
Native status: Introduced as an ornamental for its showy flowers
Habitat: Pastures, roadsides,
General description: First year plants form basal rosette up to 2 ft wide; leaves deeply lobed with spine tipped margins. In second year flower stalk elongates up to 5 ft tall; numerous pink to purple flower heads up to 2 in wide, largest of Iowa’s thistles; head surrounded by sharp bracts. Base of flower heads is flatter than bull or Canada thistle.
Key ID traits: Margins of leaves often silverish, giving them a frosted appearance. On flowering plants, base of leaf extends down stem forming spiny wings. Few hairs on leaves
Similar species: Bull thistle has deeply lobed leaves and prominent spines on the lobes. Canada thistle is much smaller and leaves are not as deeply lobed. Field thistle is a native plant with few hairs on upper leaf surface, but lower surface is densely covered with white hairs. Field thistle flowers much later in season than musk and other thistles.
Miscellaneous: A small percentage of musk thistleplants act as annuals, these plants flower later in the summer and are much shorter than two-year old plants. Musk thistle was introduced to North America as a garden flower by European colonists. The musk thistle weevil was introduced as a biological control agent in the 1980’s and is common across the state. Its larvae feed on developing seed within the flower head.