Lactuca serriola L.  

An annual or biennial. Stems erect, up to 150 cm tall, usually spiny along the lower half. Leaves oblong, stiff, glaucous, from entire to pinnatifid, with spiny bristles along the margins and on the midrib beneath, and sometimes along the lateral veins. Upper leaves sagittate, blades held vertically pointing roughly in a north-south direction. Flower heads pale yellow, about 13 mm across, on peduncles up to 3 cm long, in a lax, much-branched, pyramidal panicle. Only ray florets present. Involucre glaucous, 8 to 13 mm long, cylindrical, with many rows of bracts. Achenes flattened, 3 to 4  mm long, with a beak up to 5 mm long. Pappus of rough hairs up to 5 mm long.