Climate can influence plant demographic processes and life stages in different ways, but such details are often ignored in analyses that focus on adult life stages and annual climate averages. In particular, the effects of climate on seeds may be hugely important under climate change. Climate is known to influence seed survival and germination, which in turn can strongly affect population persistence and community dynamics. We investigated climate and other environmental effects on seed viability and germination probabilities of six winter annual plant species persisting in small, isolated habitat fragments in the Mediterranean-climate region of southwestern Australia. Seeds were collected from southern (cool) and northern (warm) bushland remnants and factorially placed into each location to assess the effects of natural dormancy alleviation via after-ripening. Seeds were then exposed to cool and warm germination treatments (representing average germination conditions in the two remnants). For five of the six species, seeds from warm maternal populations had higher germination probabilities (or germinated more seeds sooner). Regardless of maternal population, germination probabilities were higher (or germination was more rapid) for seeds that were after-ripened in the warm remnant for almost all species. For all species, germination was higher (or more rapid) under the warmer germination temperatures. We also found strong microsite effects on seed viability for some species. In the absence of adaptation in dormancy regulation and germination physiology, our results indicate that most of the winter annual species studied will germinate higher fractions of seeds under future climate conditions due to the cumulative effects of warmer maternal, after-ripening, and germination environments. The fate of isolated populations under climate change may therefore depend strongly on postgermination survival and reproduction to prevent seed bank depletion.