HERMAPHRODITISM comprises outcrossing whenever the proximity of male and female organs allows self-fertilization1 and interference between sexual functions2. Many floral traits of animal-pollinated angiosperms encourage cross-fertilization3, as recognized by Darwin4-6; however, these characteristics may also allow pollination between flowers on the same individual (geitonogamous self-pollination) 7,8. Simultaneous display of many flowers exemplifies this conflict. Although large floral displays promote outcrossing through enhanced pollinator attraction9, they could be costly in terms of lost mating opportunities10,11 if geitonogamy decreased outcrossed siring success by reducing pollen transfer between plants (pollen discounting12). We report here that, after manipulating the flower number of bee-pollinated Eichhornia paniculata plants, we observed the predicted higher selfing and lower outcrossed siring success for larger inflorescences. Given the reduced fitness resulting when pollen receipt by one flower interferes with pollen export by another, we propose broadening traditional interpretations of floral design and display to recognize their roles in reducing geitonogamous pollen discounting.