Checking several of my wildflower reference books, Spotted knapweed
(Centaurea maculosa) is indeed what we know here in Michigan as Star thistle.
While the western (yellow) variety of knapweed may well be a pest weed, I
agree with Ellen Anglin that our light purple flowered plant is a wonderful
nectar source here in Michigan.  It is our major honey flow, at least in
southeastern Michigan. I have never seen it to invade hay or grain fields
here - it always keeps to roadsides and unused lands, and seems easily to be
replaced by lawns and gardens when fallow farms go up for sale and turn into
urbanized subdivisions.

One of the nicest features of star thistle honey is that it granlates very
slowly.  I had a couple barrels of the 1999 harvest which were still
perfectly liquid (but very stiff) at the end of April 2000.

Ted Fischer
Dexter, Michigan USA