Acroptilon repens L.
Russian Knapweed is a greyish perennial up to 3' tall and typically in dense clumps.  Leaves are alternate and of several types.  Upper are small, narrow, smooth edged; stem are larger with small toothed margins; basal are deeply notched.  Stems branch profusely with terminal flowers.  Flowers are thistle-like with scaly seed head and lavender color.  Roots are very dark and heavily scaled.  Seeds are flattened, ivory in color and held in cup shaped seed heads.
Photos and information adapted from "Weeds of the West" by Tom Whitson