The objective of this experiment was to study the yield efficiency, fruit set and flower bud mortality of 'Stevnsbaer' sour cherry (Prunus cerasus L.) on the rootstocks Weiroot 10 and Colt. During three seasons, trees were grown in large containers and irrigated daily with a complete nutrient solution. On Colt, a treatment with additional potassium fertilization in the second year was included. After the second growing season, trees were overwintered in the field (lowest recorded air temperature -12°C) or in a cold storage (4°C). Fruit set and yield efficiency were recorded in the third growing season. In August of the second year, there were significant differences between rootstocks for scion leaf K, P, Ca, Mg, Mn and Zn concentrations. Trees on Colt had very low K concentrations. Applying additional soil K to trees on Colt increased leaf concentrations of K and decreased Ca, Mg and Mn. In the field, 76% (on Weiroot 10) to 89 to 91% (on Colt) of all flowers were killed on extension shoots during winter. Mortality rates were less on spurs, where Weiroot trees had only 29% dead flowers. K-fertilization increased the spur mortality from 62 to 76% on Colt. Trees overwintered in cold storage had only 0 to 8% damaged flowers on spurs and shoots. Fruit set (based on viable flowers) was similar on trees overwintered in the field and cold storage, but fruit set on extension shoots was higher on Weiroot 10 than on Colt, and K-fertilization significantly improved the fruit set on extension shoots on Colt. Yield efficiency in the third season was markedly better on Weiroot 10 compared to Colt. This was due to increased fruit set as well as higher winter survival rates of flowers. Leaf K concentrations on Colt were very low, but additional K-fertilizers only improved yield efficiency when the trees were overwintered without any frosts.