Potassium (K) is an important nutrient for winter hardiness in alfalfa (Medicago sativa L.). This study determined the effect of K rate and application timing, fall dormancy, and harvest stress on forage yield, nutritive value, root reserves, and winter survival in soils with different clay mineralogy. The experiments were conducted in Lisbon and Milnor, ND, in 2019 and 2020. Potassium treatments of 0, 168, and 336 kg K2O ha-1 at single- and split-application were applied to three cultivars. Half of the experimental units were stressed by harvesting mid-September, while the other half was non-stressed by harvesting in October. Soil K was higher with a split-application, compared with a single-application of K at the same rate. Total seasonal forage yield was significantly lower when no K was applied. Stressed alfalfa had lower root protein in both years and starch was lower in Milnor 2019 and Lisbon 2020 compared with non-stressed.