On Wed, 26 Jun 2013 15:42:03 +0000, Kay Lancaster wrote:

*Noxious weed is a legal definition, meaning the plant is a peril to
agriculture.

At first, I thought lovely thorny plant was a "Purple Starthistle",
(Centaurea calcitrapa), which is an invasive weed in the San Francisco
Bay Area:
http://www.cal-ipc.org/ip/management...mber=182.ph p

But, now I think it's a Bull Thistle (Cirsium vulgare), which is also
prominant in the bay area, based on the fact that this looks like it:
http://www.cal-ipc.org/ip/management...um_vulgare.php

This site says it has a taproot and that "cut flowerheads can still develop
viable seed", but I would have no idea how:
http://www.cdfa.ca.gov/plant/ipc/weedinfo/cirsium.htm