Growing degree-days optimize trinexapac-ethyl reapplications on ultradwarf bermudagrass putting greens: I. Predicting the maximum suppression point

Applying trinexapac-ethyl (TE) is a common practice for growth suppression and quality improvement of ultradwarf bermudagrass [Cynodon dactylon (L.) Pers. x C. transvaalensis Burtt-Davy] putting greens. Previous research suggested that growing degree-days (GDD) effectively predict the suppression of creeping bentgrass (Agrostis stolonifera L.) putting greens following a TE application, and additional research showed similar results on ultradwarf bermudagrass putting greens. Whereas temperature may be the main factor influencing turfgrass suppression following a TE application, it is likely that other environmental factors affect suppression as well. The objective of this research was to identify the optimal variable unit for predicting suppression, particularly the maximum suppression point (MSP), after a TE application on a ‘MiniVerde’ ultradwarf bermudagrass putting green. Tested variable units included calendar days (DAT), GDD (base temperatures of 0–12 °C), soil temperature (2.5 cm), global horizontal irradiance (GHI), and photosynthetically active radiation (PAR). The GDD with a base temperature of 0 °C (GDD0) model predicted suppression better than the GDD with a base temperature of 10 °C (GDD10) model, GHI model, and PAR model. According to the GDD0 model (pseudo-R2,.564; SE: 0.195), the MSP occurred at 262 GDD0 with a suppression magnitude of 61%. The models did not indicate a rebound period following the single TE application.