Sunflower (Helianthus annuus L.) seed industry needs nondormant seeds (i.e., seeds with the capacity to germinate in a wide range of environmental conditions) for processing and packaging. Thus, the presence of persistent seed dormancy is an undesirable trait. Dormancy level depends on the genotype and on the maternal environment. The aim of the present work was to study, in a wide range of sunflower genotypes, the effect of variations in sowing date on achene dormancy level and to investigate if these changes are related to variations in (a) embryo or coat‐imposed dormancy, (b) dormancy expression at high or low incubation temperatures, and (c) embryo sensitivity to abscisic acid (ABA). Two experiments were performed. In Experiment I, 20 sunflower genotypes were sown in two contrasting planting dates during three consecutive years, whereas in Experiment II, 1579 genotype was sown in five dates along a 5‐mo period. At harvest and during postharvest storage, fruit structures were incubated at different incubation temperatures in water and ABA. Descriptive multivariate analyses showed that late sowings provoked an enhancement of achene germination at harvest and a faster dormancy release during postharvest storage in all genotypes. However, the magnitude of the response, the temperature at which dormancy was expressed, and the tissue responsible for achene dormancy varied among genotypes. The mechanism that partially explains this dormancy alleviation appears to be the sensitivity of embryo germination to ABA inhibition. Result suggest that sowing date can be managed to obtain less‐dormant sunflower seed lots without significant yield loss.