Carduus nutans L.
Musk thistle is a biennial or winter annual growing up to 7' tall.  A rosette is formed the first year with a flowering, elongating stem the 2nd year.

Leaves are dark green with a light midrib, hairless on both sides with long sharp spines. The stems are hairless. 

Flowers are solitary, terminal, nodding heads, 11/2" to 3" across.  Color is ranges from deep rose to purple.  It has a fleshy taproot, hollow near the surface. 

Reproduction is by seed only, 20,000+ per plant.  Viability is at least 10 years.
Photos and information adapted from "Weeds of the West" by Tom Whitson