Najas marina is a summer annual of shallow lakes that reaches its distributional limit in the Norfolk Broads of eastern England. Seeds are shed in autumn and overwinter in lake sediments. We investigated the influence of growing-season length on seed maturity and germinability and the cumulative effects of winter temperatures on patterns of germinability. Seeds were stratified for up to 1 year and tested for germinability at 4-week intervals. Data-loggers recorded temperatures simultaneously at the field site and in the cold room. Cumulative temperature below a threshold (thermal time) was used to relate behaviour in cold storage to phenology. Most seeds were dormant at collection. Cold storage of imbibed seed resulted in loss of dormancy. The proportion of non-dormant seeds peaked after a cold treatment equivalent to one winter, i.e. coinciding with favourable temperatures for germination in the field. Continued cold stratification resulted in a cycle of secondary dormancy and its subsequent relief; the thermal time characterizing the second, lower, peak in germinability coincided approximately with that attained in the field in the second spring after collection. Shortening the growing season of Najas marina, by collecting material before natural senescence was complete, reduced the proportion of mature seeds, lowered average germinability and increased germination times. The thermal capacity of water bodies causes a lag in spring warming, whereas declining light becomes limiting in autumn. Najas marina is crucially dependent on the length of growing season to produce sufficient viable seed. A low-temperature requirement to break seed dormancy prevents inappropriate autumn germination. In a prolonged winter (necessarily followed by a shorter growing season), a significant proportion of seed would go into secondary dormancy and remain in the seed bank until the second spring. In annual plants, such requirements for warm summers and cold winters could contribute to a continental-climate distribution in northern Europe. In the case of Najas marina this might explain its limit at the eastern extremity of England and thus its rarity.