Water availability is a potent regulator of development in plants and acts as a positional cue to induce root branching through a process termed hydropatterning. The mechanism by which roots perceive the spatial distribution of water to position lateral branches is unknown. Here we reveal that a root's developmental competence for hydropatterning is limited to the root tip, where tissue growth occurs. Mathematical modeling suggests that water uptake during growth creates spatial biases in tissue water potential, and we show that these gradients predict the position of future lateral branches. By altering growth dynamics with exogenous chemical and environmental treatments, we demonstrate that growth is necessary to allow roots to distinguish environments with relatively high or low water availability and pattern branching accordingly. Furthermore, we show that these cues regulate a number of other physiologically important pathways. Our work supports a sense-by-growth mechanism governing lateral root hydropatterning, in which water availability cues are rendered interpretable through growth-sustained water movement.