Core Ideas: Growing degree day models continue to predict plant growth regulator performance in turfgrass. Data collection should occur until the growth response has dissipated, not based on predefined intervals. Including a dampening coefficient in the sinewave models allows the clipping yield effect to decay with time. Adding a decay coefficient allows sinewave models to fit datasets where a rebound growth phase did not occur. Growing degree day (GDD) models can predict the performance of plant growth regulators (PGRs) applied to creeping bentgrass (Agrostis stolonifera L.). The goal of this letter is to describe experimental design strategies and modeling approaches to create PGR models for different PGRs, application rates, and turf species. Results from testing the models indicate that clipping yield should be measured until the growth response has diminished. This is in contrast to reapplication of a PGR at preselected intervals. During modeling, inclusion of an amplitude-dampening coefficient in the sinewave model allows the PGR effect to dissipate with time.