Growing degree-days as a method to characterize germination, flower pattern, and chemical flower suppression of a mature annual bluegrass [Poa annua var reptans (Hauskins) Timm] fairway in Michigan

Three turfgrass research projects characterized important seasonal events for two mature adjacent fairway populations of annual bluegrass [AB] as they relate to growing degree-days [GDD]. Experiments were conducted between 2001-2006 on a 9 to 15 yr old AB fairway maintained at 1.5 cm at the Hancock Turfgrass Research Center in East Lansing, MI. Environmental data was collected remotely by a Michigan Agricultural Weather Network [MAWN] weather station and verified with on-site measurements. AB seedling emergence [SEM] followed a bimodal pattern with peaks occurring in the spring and fall of each year. A simple growing degree-day [GDD] model accurately predicted mean soil temperature but did not adequately describe rate of SEM. However, logistic regression of mean soil temperature and SEM occurrence verified that SEM in this population fell within published soil temperature optima. Occurrence and rate of flowering of AB were observed for six years. Ordinal dates for onset, peak duration, and completion of flowering were recorded. GDD proved to be a reliable method to predict key flowering events in five of six years. A simple average-calculated GDD model, using a base temperature of -5 C was developed to describe seedhead production (R2=0.65) as compared to previously published model (R2=0.65). Onset, peak period, and completion of flowering were 550, 800 to 1300, and 1550 GDD-5, respectively. Plant growth regulators [PGRs] may be used to suppress AB flowering. Five one-year studies examined the effects of application timing and PGRs, mefluidide [MF], ethephon [EP], or EP plus trinexapac-ethyl [TE] on suppressing AB flowering. AB seedhead suppression, but not the occurrence of turfgrass injury, could be explained by application timing for MF only. Application dates associated with maximum seedhead suppression from MF were used to identify optimum GDD range. Applications occurring between 350-550 GDD-5 resulted in <5% seedhead cover averaged over all years. No injury was associated with EP or EP plus TE. Seedhead suppression was not affected by application timing for EP or EP plus TE when applied between 200-600 GDD-5. EP and EP plus TE applications resulted in <14% seedhead cover averaged over all years. GDD proved to be a reliable method to predict key seedhead production events for AB and PGR application timing for suppression of AB flowering. GDD alone did not explain rate of SEM but did predict conditions conducive for SEM. Regional testing of simple average-derived GDD models will facilitate end-user adoption of web-based decision-making tools related to AB management.