The onset of plant dormancy has proven difficult to explain in nearly all ecosystems. Most research has focused on the end-of-season dynamics of deciduous ecosystems, where leaf coloration and leaf fall are the primary phenological responses. More complex are the end-of-season dynamics of grasslands, where the mechanism of dormancy is a gradual response to climatic variables. These complications are magnified in dry grasslands, where the effects of temperature on phenology are modulated by the availability of soil water. Our objectives were to identify the primary drivers influencing the timing of end of season on the shortgrass steppe and determine if the timing of start of season, end of season, or both influences the growing season length of the shortgrass steppe. Our results suggest that temperature and soil water interact to influence the timing of end of season in the shortgrass steppe and that growing season length is strongly related to the date of start of season and less so to the date of end of season. Our findings bear important implications for understanding semiarid ecosystems under climate change. Because future precipitation and temperature tend to diverge, understanding responses in seasonality of greenness as well as productivity in general must take both precipitation and temperature into account.