Class I KNOX transcription factors, such as SHOOT MERISTEMLESS (STM) and KNAT1, are known to play a role in the plant shoot apical meristem (SAM), where they are thought to prevent differentiation and hence promote stem cell maintenance. Now, on p. 4311, Urs Fischer and colleagues uncover a role for STM and KNAT1 in another Arabidopsis meristem – the vascular cambium, which is a lateral meristem that gives rise to xylem and phloem cells. They first show that STM and KNAT1 are expressed in undifferentiated cambial cells but also in differentiated phloem and xylem cells. The researchers further demonstrate that xylem fibre formation is reduced in stm and knat1 mutants, suggesting that STM and KNAT1 promote the differentiation of cambial derivatives. In addition, they report that STM and KNAT1 regulate xylem differentiation via transcriptional repression of BLADE-ON-PETIOLE 1 (BOP1) and BOP2, which are not expressed in the SAM. Together, these findings demonstrate that, in contrast to their function in the SAM, STM and KNAT1 promote cell differentiation in the cambium, suggesting that the exact role of these transcription factors in other meristems needs re-examination.