Vernalization, the cold-induced acceleration of flowering, involves the epigenetic silencing of the floral repressor gene FLOWERING LOCUS C (FLC). We investigated the molecular basis for variation in vernalization in Arabidopsis natural accessions adapted to different climates. A major variable was the degree to which different periods of cold caused stable FLC silencing. In accessions requiring long vernalization, FLC expression was reactivated following nonsaturating vernalization, but this reactivation was progressively attenuated with increasing cold exposure. This response was correlated with the rate of accumulation of FLC histone H3 Lys 27 trimethylation (H3K27me3). Thus, variation in epigenetic silencing of FLC appears to have contributed to Arabidopsis adaptation.