Transcription in eukaryotic genomes generates an extensive array of non-protein-coding RNA, the functional significance of which is mostly unknown. We are investigating the link between non-coding RNA and chromatin regulation through analysis of FLC — a regulator of flowering time in Arabidopsis and a target of several chromatin pathways. Here we use an unbiased strategy to characterize non-coding transcripts of FLC and show that sense/antisense transcript levels correlate in a range of mutants and treatments, but change independently in cold-treated plants. Prolonged cold epigenetically silences FLC in a Polycomb-mediated process called vernalization. Our data indicate that upregulation of long non-coding antisense transcripts covering the entire FLC locus may be part of the cold-sensing mechanism. Induction of these antisense transcripts occurs earlier than, and is independent of, other vernalization markers and coincides with a reduction in sense transcription. We show that addition of the FLC antisense promoter sequences to a reporter gene is sufficient to confer cold-induced silencing of the reporter. Our data indicate that cold-induced FLC antisense transcripts have an early role in the epigenetic silencing of FLC, acting to silence FLC transcription transiently. Recruitment of the Polycomb machinery then confers the epigenetic memory. Antisense transcription events originating from 3′ ends of genes might be a general mechanism to regulate the corresponding sense transcription in a condition/stage-dependent manner.