Timing of flowering is an adaptive trait regulated by environmental cues and has been intensively studied in annual plants, but in perennials it is currently not well characterized. Quilot-Turion et al. apply cold treatments to two locally adapted populations of the perennial Arabidopsis lyrata from central Europe and Scandinavia and study responses to vernalization. They find that the population from central Europe flowers more rapidly and invests more in inflorescence growth than the Scandinavian population, which has longer leaves and higher leaf rosettes. QTL mapping in the F2 population reveals genomic regions governing these traits and some QTL × vernalization interactions. The results indicate that the two populations have diverged in their plastic responses to vernalization in traits closely connected to fitness through changes in many genomic areas.