Plants have to cope with constantly changing conditions and need to respond to environmental stresses and seasonal changes in temperature and photoperiod. Alignment of their development with particular seasons requires memory mechanisms and an ability to integrate noisy temperature signals over long time scales. An increasingly well understood example of how seasonal changes influence development is vernalization, the acceleration of flowering by prolonged cold. Vernalization has been dissected in Arabidopsis thaliana and shown to involve a Polycomb-based epigenetic memory system. This minireview summarizes our current understanding of this mechanism and its modulation through adaptation. A key concept that has emerged is that cell-autonomous switching between epigenetic states can provide the basis for quantitative accumulation of environmental memory.