1. Mixed mating and variation in outcrossing rate among populations of the same species are common. Outcrossing can be affected by pollinator activity and floral traits that facilitate or impede autonomous self-fertilization. However, the relative contribution of pollen limitation and evolved differences in the ability to self fertilize to variation in the mating system is poorly understood and can only be disentangled using an experimental approach. 2. We placed arrays of plants from eight Campanula americana populations that varied in autonomous selfing ability (hereafter "autonomy") in sun and shade habitats to create high- and low-pollination environments. Floral visitors were observed and pollen limitation, dichogamy, herkogamy, and pollen persistence within the flower were measured. Outcrossing rate was estimated for a subset of the arrays. 3. Pollen limitation was over three-times higher in the shade than the sun due to differences in pollinator visitation. Populations with high levels of autonomy displayed greater pollen persistence and dichogamy in the more pollen-limited environments than in the high-pollination environments. In these high-autonomy populations, outcrossing rates were tightly associated with pollen limitation, with reduced outcrossing under strong pollen limitation. In contrast, populations with intermediate autonomy levels had less plastic floral traits and their outcrossing rates changed little with pollen limitation.4. Mating system was shaped by both the pollination environment and the magnitude of plasticity in floral traits. The experimental approach used here revealed that plasticity in floral traits provides the ability to adjust mating system in response to limited potential for outcrossing. The lack of plasticity in some populations could explain mismatches between pollinator activity and mating system in natural populations. Finally, flexibility in the mating system may help explain the prevalence of mixed mating.