Pattern formation is typically controlled through the interaction between molecular signals within a given tissue. During early embryonic development, roots of the model plant Arabidopsis thaliana have a radially symmetric pattern, but a heterogeneous input of the hormone auxin from the two cotyledons forces the vascular cylinder to develop a diarch pattern with two xylem poles. Molecular analyses and mathematical approaches have uncovered the regulatory circuit that propagates this initial auxin signal into a stable cellular pattern. The diarch pattern seen in Arabidopsis is relatively uncommon amongst flowering plants, with most species having between three and eight xylem poles. Here, we use multiscale mathematical modelling to demonstrate that this regulatory module does not require a heterogeneous auxin input to specify vascular pattern. Instead pattern can emerge dynamically, with its final form dependent upon spatial constraints and growth. The predictions of our simulations compare with experimental observations of xylem pole number across a range of species, as well as in transgenic systems in Arabidopsis in which we manipulate the size of the vascular cylinder. Through considering the spatial constraints, our model is able to explain much of the diversity seen in different flowering plant species.