New owner of 12 yr old yard.
By mid-July 2012 these were covering 10'x24' area on east, morning sun side of our shed.
I treated them as weeds.
Conyza canadensis? If so, it is a weed.
It does appear that vngarden is correct.
I think it is horseweed too: http://davesgarden.com/guides/pf/go/54589/