Background and Aims Cleistogamy is considered to be an adaptive strategy resulting in plasticity in CH and CL flower production depending on environmental conditions and plant size. The aim of this study was to investigate whether CH and CL flower production in Portulaca oleracea is genetically differentiated among populations in association with climatic conditions. Methods First, we conducted growth experiments with P. oleracea seedlings from 16 populations under two temperature conditions. Second, we sowed seeds originating from the parents in the first experiment and grew the resulting plants to investigate whether flower production is heritable and plants in the same population show the same pattern of flower production. Key Results Two types of plants that produced only CH or CL flowers (referred to as CH and CL plants, respectively) were mainly observed, and the growing temperature conditions did not affect flower production. The frequency of CL plants increased with a decrease in the mean temperature in the original population. CL plants tended to begin reproduction earlier than CH plants, and the probability that a CH plant would flower decreased under the low-growing temperature conditions. Thus, CL plants may have some advantages in unfavourable environments in which early reproduction is necessary due to a short growing season and/or CH flowers cannot open due to low temperatures. The progeny originating from CH and CL plants also produced only CH and CL flowers, respectively, suggesting that there is a genetic basis for the dimorphism in flower production in P. oleracea, represented by CH and CL plants. Conclusions In contrast to the previous hypothesis that the production of both CH and CL flowers would be plastic, the genotypes producing either CH or CL flowers occurred at different frequencies under varying climatic conditions.