Significance RNA-directed DNA methylation (RdDM) provides a system for targeting DNA methylation to asymmetric CHH (H = A, C, or T) sites. This RdDM activity is often considered a mechanism for transcriptional silencing of transposons. However, many of the RdDM targets in the maize genome are located near genes or regulatory elements. We find that the regions of elevated CHH methylation, termed mCHH islands, are the boundaries between highly methylated (CG, CHG), silenced chromatin and more active chromatin. Analysis of RdDM mutants suggests that the function of the boundary is to promote and reinforce silencing of the transposable elements located near genes rather than to protect the euchromatic state of the genes.