This image is from the Rockies, so it is western wallflower. A little to the east it might be prairie rocket. Really all the same plant, more or less. The boundaries of this species seem to be a little elastic, so its technical name may depend on which botanist's territory you're in. It may also have orange or even purple petals, depending again on just where your limits are. Western wallflower grows mostly in open areas in AK, CO, IA, ID, IL, KS, MN, MO, MT, ND, NE, NM, OK, SD, TX, WY, AB, BC, MB, ON, QC, and SK. Clear Creek Co CO, 6/16/13. Mustard family, Brassicaceae.