Cornflower (Cyanus segetum), an iconic weed species, was used to investigate the implications of climate change on the behavior of long day dependent species while earlier cereal harvest may prematurely end the period of flowering and seed production in weeds. Variability in flowering, with or without vernalization, of European C. segetum populations situated along a 45-55 degrees North latitudinal gradient was studied. Long day requirement was confirmed. Vernalization requirements are not stringent in C. segetum the proportion of plants that were not sensitive or that responded strictly or weakly to vernalization varied across populations. The mean sum of growing degree days (GOD) for first flowering under a 14h day length at 15 degrees C was 1344 and 2072, with and without vernalization, respectively, whatever the population. A growth temperature at 25 degrees C had no effect on GDD except for a population from mountains. Advancing the date of harvest by ten days in a field experiment resulted in one-third fewer seeds produced, which could accelerate its decline, but polymorphism for vernalization and temperature responses could allow adaption.