In sea beet, Beta vulgaris ssp. maritima, seed dormancy largely controls germination timing, is highly heritable and varies geographically; climate might select for this trait. Wagmann et al. assess geographical variation for seed dormancy among 85 sea beet populations across the whole of France. Under controlled conditions, they find a highly variable proportion of seeds to be dormant, and this follows a geographical pattern ranging from lower dormancy at high latitudes to higher dormancy at low latitudes. Regression points to summer temperature as a plausible selective factor. Whilst variation in dormancy seems to stem in part from genetic differentiation, maternal phenotypic plasticity may also play a role.