Radial growth increases plant girth, and is driven by proliferating cells in the cambium that give rise to vascular tissues (xylem and phloem). However, the identity and behaviour of cambial stem cells has remained incompletely understood, more than 150 years after their existence was first postulated. Now, Dongbo Shi, Thomas Greb and colleagues use the Arabidopsis hypocotyl to trace stem cell contributions to radial growth. EdU pulse labelling reveals that proliferative cells are restricted to a single domain within the cambium, and that their descendants contribute to both xylem and phloem. The proliferating cambium can be further refined into three functionally distinct domains – proximal, central and distal – which are defined by gene expression. Using a cell lineage tracing technique based on these expression domains, the authors reveal that stem cells in the central cambial domain are capable of contributing to both xylem and phloem. Fine-tuning this technique to generate single cell clones, the relationship between cambial domain and determination is then clarified: the proximal domain exclusively generates xylem, the distal domain exclusively generates phloem, and the central domain harbours ‘bifacial’ stem cells that can give rise to both fates. This work thus generates a fate map for radial growth and reveals the identity, location and potential of the long sought-after cambial stem cells.