Centaurea solstitialis L.
Yellow Starthistle is an erect, rigid, branching annual. Leaves are deeply lobed at the base but not lobed further up. Leaves are small and pointed. Stems have winged structures, are covered with a cottony fiber and are up to 30" tall.
Flowers are yellow and terminal. Flower bracts are tipped with a straw-colored 3/4 inch thorn.
The root is taproot in form. Seeds are smooth, light-colored, often mottled, 1/8" long and notched just above the base.
Photos and information adapted from "Weeds of the West" by Tom Whitson