On Wed, 26 Jun 2013 06:43:21 -0700, Oren wrote:

...purple thistle weed

I think it's a "Bull Thistle" (Cirsium vulgare).

Apparently it only reproduces by seed, but, the seeds must be
removed from the area...
http://www3.picturepush.com/photo/a/...g/13420241.jpg

http://www.cal-ipc.org/ip/management...mber=182.ph p
Plants from Yosemite Valley that were cut at the root crown a few days
after their first flowers appeared and then laid on the ground produced
abundant viable seed (Randall pers. observation). Thus it may be
important to remove cut stems from the area.