Flowering time is thought to be a critical trait for adaptation to climate, and the developmental signaling pathways controlling flowering have been well characterized at the molecular level in model species such as Arabidopsis thaliana.  To understand how natural variation in candidate flowering time genes affects phenotype and fitness in different climates and seasonal environments, we have conducted common garden experiments in Finland, England, western and eastern Germany, and Spain with over 300 inbred “ecotypes” originating across the species’ European climatic range. Plantings were timed to coincide with natural germination phenology in each site, including fall, spring and summer plantings in sites with multiple generations per year. Natural variation in candidate flowering time genes is associated with life history variation and with relative fitness, but these effects differ among sites and seasonal cohorts.  In particular, natural null alleles of the major flowering time gene FRIGIDA result in a shift from a winter annual life history to an autumn flowering life history allowing multiple generations per year, but only in sites with mild oceanic climates.