Plant hormones are chemicals that change plant growth or development. Growers use natural hormones or synthetic chemicals that behave like hormones to control weeds, roots, fruit, seeds and ripening.

Selective herbicides contain synthetic auxins. They cause susceptible broad-leaved weeds to grow abnormally and die. Narrow-leaved cereal grasses are less sensitive to these chemicals or can metabolize them differently, so they survive at the applied dose. Differences in leaf orientation and spray interception may contribute to selectivity, but they are not the main explanation.