<p class="AbstractSummary">Natural ices often fail at stresses much lower than those measured in laboratory settings, complicating our understanding of glacial failure and icy moon crustal fracture. This may be because strength models, which depend on the size of individual ice grains, do not account for the saltwater commonly found in terrestrial and planetary ices. We conducted grain growth experiments, finding that saltwater always modifies grain growth compared to pure ice, and that increasing volume of saltwater introduces a pinning effect limiting this growth. Ice grain size therefore depends directly on liquid fraction, controlled by salinity and temperature. Modeled effects of grain growth on tensile strength following saltwater infiltration find that low-salinity water can reduce ice strength by up to 46 % within 24 hours, narrowing gaps between observations and experiments.