Diffuse knapweed can be a winter annual but is most often biennial starting as a rosette.  One to 3 feet tall with tough tap root.  Usually a single stem will emerge from the rosette and diffusely branch to form a ball 1 to 2 feet tall.  Leaves have no petiole, smaller and less divided higher up the stem.  Plants are a muted sage green and may appear dusty from a distance.  Normally a single flower is borne at the branch tips and are most often white, but can be lavender to light pink or mixed on the same plant.  Seed heads are urn shaped and about 1/2 inch long. Seed head bract tips are stiff and very sharp causing painful itchy irritation when handled.  Reproduces only by seed but high seed volume and tumbleweed characteristic and vigorous germination combine to accelerate spread.

Seedling establishment is the critical period for diffuse knapweed and suppression efforts are most effective during this period.  Cultivation or mechanical removal will destroy plants; however, as conditions favor it will regenerate from the present seed bank.  Most common herbicides (roundup, 2,4-D) are usually effective.  There are growing populations resistant to these chemistries.  Clopyralid (Transline, Redeem, Curtail) is very effective at controlling this weed when used pre-bloom or on fall emerging rosettes.  Aminopyralid (Milestone) has also been shown to be effective rosette to bolt and in the fall.
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Text from Selected Noxious Weeds of Eastern Oregon, photos Richard Old XIDServices Inc.