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The comprehension of plant biology and the response to the environment is fundamental to achieve the optimal skills to manage and conserve the fine equilibrium between biotic and abiotic parameters regulating natural biodiversity in salt marshes. The behaviour of annuals living in these stressful conditions is poorly understood and constitutes a good model for a better understanding of this relationship. We thus identified the determinant environmental factors involved in population survival of Halopeplis amplexicaulis, a threatened annual species inhabiting salt marshes. To achieve this objective, maternal climatic parameters were analyzed in seeds collected in different years, and correlated to seed dimensions (length, width and length/width ratio) and mass, and also to germination responses of the next generation. Our results proved a direct correlation between seed factors and the maternal environment determining the hydroperiod length, particularly autumn and winter temperatures, responsible of life cycle extent. Population survival is ensured by a narrow adaptation to climatic conditions through a natural halopriming of seeds. The combination of the two complementary germination strategies verified (opportunistic vs. bet-hedging) depending on the climatic conditions of the ripening year, is revealed as a density regulation mechanism controlling seed bank dynamics.