Meristems play an essential role in plant growth and development. Shoot and root apical meristems are responsible for primary elongation growth in shoots and roots, respectively. Secondary or radial growth that follows is dependent on the vascular cambium, a circular meristem that produces secondary xylem (wood) and secondary phloem (inner bark). Little is known about the vascular cambium, despite its importance to wood formation in trees. Class I KNOX homeodomain transcription factors are important regulators of meristem maintenance in plants. Members of this class, including BREVIPEDICELLUS (BP) maintain the shoot apical meristem in part by preserving boundaries that keep stem cells separate from differentiating organs. The role of boundary genes in the vascular cambium is mostly unknown. Here, I provide evidence that spatial regulation of boundary genes by Class I KNOX genes is important for different reasons in the vascular cambiums of stem and root-hypocotyl in Arabidopsis thaliana, a model