Phenotypic plasticity in seed dormancy may allow plant species to cope with rapid environmental changes, such as climate warming. In controlled experimental settings, plasticity in dormancy has been found to relate to temperature during seed maturation, but this relationship has not been tested in field conditions. Here we analyse for the first time the variation in dormancy during five successive years in wild populations of the study species Centaurium somedanum, to determine how this variation is related to average temperatures during specific seasons of plant activity. We performed laboratory germination experiments to measure: (1) the degree of dormancy at dispersal; and (2) the sensitivity to dormancy-breaking factors. We calculated average temperatures during four seasons of plant activity (overwintering, vegetative growth, flowering and seed maturation) for each year, and tested the relationship between these temperatures and patterns of dormancy variation, using Generalized Linear Models. Dormancy varied among sites and years, seeds being more dormant in colder years. For the degree of dormancy at dispersal, we found a positive relationship with flowering temperature and a significant effect of collection site. For the sensitivity to dormancy-breaking factors, we found no significant differences among sites, a positive relationship with flowering temperature and a negative relationship with overwintering temperature. Phenotypic plasticity in dormancy in C. somedanum is thus especially marked in the sensitivity to dormancy-breaking factors. Temperatures during overwintering and flowering have differential effects on this plasticity, allowing the plant to detect the gradient of seasonality, a main ecological feature of its distribution. These results highlight the importance of taking into account more than average temperatures when assessing the response of plant phenotypic plasticity to climate change.