Crystallisation is influenced by various non-equilibrium factors such as thermal history, mechanical perturbations, and flow, however, the role of imposed mass fluxes on when crystallisation first becomes macroscopically observable remains underexplored. Here, the authors show experimentally that thermodiffusive and isothermal diffusive mass fluxes can cause aqueous potassium chloride to crystallise at lower local supersaturation ratios compared to spatially isothermal reference systems, highlighting the importance of spatially varying temperature and concentration fields for precise crystallisation control.