A common problem for decision makers in selecting frost control options is uncertainty about the level of injury that can be caused by low temperatures. During the past few years, the concept of lethal temperature (LT) at which 10% of the bud population dies (LT10) has been used as an index for evaluating the vulnerability of flower buds to low temperature conditions. This concept has shown to be a useful tool for frost control decision-making. However, the current methods used to obtain LT values assume no spatial or temporal variability, which results in a high level of uncertainty. The goal of this study was to develop an approach that decreases the uncertainty based on the known effects of temperature on bud vulnerability. A growth chamber experiment was conducted to determine flower bud vulnerability to low temperature as a function of temperature. The results from this study showed that thermal time expressed in degree days could explain changes in floral bud development and vulnerability to frost injury. According to our findings, LT10 is a fully acceptable index for determining flower bud vulnerability to low temperatures in orchard crops. Based on this information, we found that among the five apple and cherry cultivars analyzed, ‘Gala’ is the least vulnerable to low temperature because it starts at the beginning of spring with a high level of hardiness and increases its vulnerability at a low rate. The approach described in this article may enhance decision-making certainty associated with the timing and methods to increase air temperature in orchards during low-temperature events to avoid frost damage.