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

Try pulling one up... you may find they're attached to an underground
root and stolon system, in which case you may be dealing with
Canada thistle, Cirsium arvense, a noxious weed*

Hmmm... I don't know what a "root and stolon" system looks like.

Most looked like taproots - like this:
http://www3.picturepush.com/photo/a/...g/13419976.jpg

Or this:
http://www4.picturepush.com/photo/a/...g/13419977.jpg

And this:
http://www5.picturepush.com/photo/a/...g/13419978.jpg

However, some came out as 'clump' roots - like this:
http://www2.picturepush.com/photo/a/...g/13419980.jpg

And this:
http://www3.picturepush.com/photo/a/...g/13419981.jpg

PS: It was only half way through the eradication task that I
belatedly realized shoving a garden hose nozzle into the center
of the plant and blasting the roots loose was the way to go!
http://www4.picturepush.com/photo/a/...g/13419992.jpg