Description:

MUSK THISTLE — Carduus nutans L.

Asteraceae — (Sunflower family)

Musk thistle is biennial or sometimes a winter annual, with a rosette that forms the first year and a flowering stem that elongates up to six feet the second year. Leaves are dark green with light green midrib, deeply lobed, hairless on both sides and have long, sharp spines. Leaves extend onto the stem giving a winged appearance. Flower heads are terminal, solitary, one and a half to three inches in diameter, and usually bent over. Flowers appear in midsummer. Flowers are deep rose, violet or purple, occasionally white; they are subtended by broad, spine-tipped bracts, which are a very distinctive identification feature. Fruits are 3/16 inch long, shiny, yellowish-brown with a plume of white hair like fibers.

Musk thistle was introduced to the United States in the early part of the century and is now widespread throughout the United States and Canada. It is native to Southern Europe and Western Asia. It invades pasture, range and forest lands along with roadsides, waste areas, ditch banks, stream banks and grain fields and can grow under a wide range of environmental conditions. It spreads rapidly by seeds only, forming nearly impenetrable stands which crowd out desirable forages. Each plant may produce up to 120,000 seeds with each seed remaining viable (able to germinate) after ten years in the soil. Chemical control is effective. An introduced biological control agent, the musk thistle weevil, feeds on the seeds and can limit the spread of this plant.

Growth Habit: Biennial, or winter annual, erect up to 7` tall. Freely branching. Rosette formed 1st year, flowering stem elongates 2nd year.

Leaves: Dark green with light midrib, hairless on both sides, long sharp spines.

Stems: Hairless.

Flower: Solitary, terminal, nodding heads, 1½" to 3" in diameter, deep rose to violet to purple.

Roots: Fleshy taproot, hollow near ground surface.

Seeds: Can be in excess of 20,000 per plant with 90% viable. 90% may germinate in first two years.

Seeds may germinate after 10 years in the soil.

Other: Reproduce by seed only.

(Courtesy of Weeds of the West)

Control Strategies:

Early control of Musk Thistle is important. While chemical control is effective, digging up the rosettes each year until the growth ceases can also control the plant. Mature plants can be dug up so long as a good portion of the taproot is removed. Be sure to burn or destroy all flower heads that are close to maturity. Biological control is working very well. If you have Musk thistle that must be controlled, treat the rosettes spring and fall until a heavy grass stand can be established. Most herbicides work on the rosettes, but none work on bolting plants.

To learn about biological control agents for Musk Thistle visit this site.