Description: Metrics to track seasonal transitions are needed for a wide variety of ecological and climatological applications. The spring indices have been widely used in earlier studies to model phenological variability and change through time across a wide range of spatial scales. These indices require only daily minimum and maximum temperature observations (e.g., from meteorological records) as input along with latitude, and produce a day of year value corresponding to the simulated average timing of first leaf and first bloom events among three plant cultivars. Core functions to calculate the spring indices require no external dependencies, and data for running several illustrative test cases are included. Instructions and routines for conducing more sophisticated monitoring and modeling studies using the spring indices are also supplied and documented. See Ault et al., 2015 (http://www.sciencedirect.com/science/article/pii/S0098300415001466) for more information.