Centaurea diffusa Lam.
Diffuse Knapweed is a annual or biennial up to 2' tall.  A rosette forms the 1st year with a flowering, elongating stalk the 2nd year.  Leaves are greyish green, alternate, basal leaves whorled, upper leaves reduced and covered with fine hair.  A single, erect, hairy stem grows from a taproot and is branched near the base. Flowers are solitary mostly white with a few pink or lavender.  Seedhead bracts end as sharp rigid spines. Seeds are oblong, dark brown or gray with longitudinal lines.
Photos and information adapted from "Weeds of the West" by Tom Whitson