Lactuca ludoviciana (Nutt.) Riddell
Family: Asteraceae
prairie lettuce, prairie wild lettuce, western wild lettuce
[Lactuca campestris Greene,  more...]
Lactuca ludoviciana image
from USDA Plants website  
Etymology: Lactuca: name for "milk"
Plants: erect, biennial/perennial, 1'-5' tall forb with milky juice; stems leafy, smooth
Leaves: bluish, with spiny edges and midrib
Flowers: head 1" wide with 20-56 bluish-white to yellow rays; inflorescence of a few, relatively large heads in loosely-branched clusters; blooms July-Sept.
Fruits: blackish, flat, dry seed with only one obvious line on each side
Habitat: sun; prairies, meadows
Conservation Status: Native