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MUSK THISTLE — Carduus nutans L.

Asteraceae — (Sunflower family)

Biennial, Musk thistle seeds sprout and become a rosette the first year.  The 6 to 8 inch leaves are dark green with a light green vein, the leaf is deeply scalloped with white spiny edges.
The second year, each rosette sends up several very branching flower stems that grow up to 7 ft tall.  The spiny leaves grow directly from the stem.  The base of the leaf continues to grow downward onto the stem of the plant, causing the stem to look ruffly.  It is not grazed by livestock or wildlife. 
By mid summer each stem is topped by a single flower head measuring up to 3 inches across which usually hangs downward or is at least bent to one side.  Each purple, rose or violet (occasionally white) flower is supported by rows of wide spiny scale like bracts.  The plant grows from seed only and produces up to 120,000 seeds per plant which are able to float on the wind for miles and are able to germinate after 10 years. 
Musk thistle has a stout fleshy  taproot.  Untreated this weed can produce an almost impenetrable infestation which crowds out other desirable plants. Since no chemical works well on the mature plant, it is important to treat the rosette stage or dig it up.  Digging the mature plant is successful if you can dig up most of the root AND burn the flower heads.  Biological control is also effective.  It is far cheaper to keep this weed out than to eradicate it.

The following is courtesy of Weeds of the West:

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)