Root pressure can contribute to upward xylem water flow but it is difficult to measure directly. De Swaef et al. make continuous measurements of sap flow and stem diameter variations and combine them with a mechanistic flow and storage model in tomato, Solanum lycopersicum. They find that transpiration-driven sap flow rates are typically inversely related to stem diameter changes; however, this inverse relationship is no longer valid under conditions of low transpiration. This decoupling between sap flow rates and stem diameter variations is mathematically related to root pressure, thus allowing it to be estimated in a non-destructive, repeatable manner.