The adaptive value of cleistogamous (closed and obligately selfing) flowers has been explored in theoretical and empirical studies, yet the effects of cleistogamy on reproductive isolation between diverging taxa has previously received little consideration. Two subspecies of the annual plant Triodanis perfoliata have diverged in their relative allocation to cleistogamous (CL) versus open or chasmogamous (CH) flowers. The subspecies have widely overlapping geographic distributions and produce viable hybrid offspring. Here, we use morphological diagnostic traits of T. perfoliata subsp. biflora and subsp. perfoliata to characterize their spatial distribution and test for evidence of hybridization at a site in Pitt County, NC. Sampling along a transect in the zone of contact revealed the presence of two distinct morphological types that were spatially mixed. In a separate study, offspring of CL and CH flowers from both subspecies were raised in a growth room, and diagnostic morphological traits were confirmed to differ significantly between the subspecies. In subsp. biflora, we found a significant difference between offspring of CH and CL flowers for both diagnostic traits analyzed, providing evidence for hybridization; for each trait tested, the mean for CH offspring in subsp. biflora deviated in the direction of the mean for subsp. perfoliata. In contrast, in subsp. perfoliata, we found no significant differences in morphological traits between offspring of CH and CL flowers. We suggest that differences in the number of CH flowers produced by each subspecies might explain the observed asymmetry in hybridization.