Maximizing seedhead suppression of Poa annua L. in Michigan through use of growing degree-days to predict optimum application timing for ethephon and ethephon plus trinexapac-ethyl

Research was conducted from 2002 to 2006 to determine optimum application timings for the chemical seedhead suppression of annual bluegrass (AB) [Poa annua L. var. reptans (Hauskins) Timm.] by single applications of the plant growth regulators ethephon (EP) (2-chloroethylphosphonic acid) or EP plus trinexapac-ethyl (TE) [4-(cyclopropyl-alpha-hydroxymethylene)-3, 5-dioxo-cyclohexanecarboxylic acid ethyl ester]. The objectives were to evaluate AB seedhead suppression from single applications of EP and EP plus TE, and to evaluate the simple average-calculated growing degree-days (GDD) and Baskerville–Emin GDD models for their reliability to predict best application timing dates from year to year. Plots were established on a 10- to 15-yr-old AB fairway maintained at 1.5 cm and receiving 0.5 to 0.6 cm daily automatic irrigation throughout the growing season and 120 kg N ha^–1 yr^–1 from 2002 to 2006. The soil type is a Marlette sandy loam (fine-loamy, mixed, semiactive, mesic Oxyaquic Glossudalf). Plant growth regulators were applied at 3.74 and 0.08 kg a.i. ha^–1 for EP and TE, respectively, between 24 March and 13 May. Ethephon interacted with application timing in 2002 and 2003. Ethephon plus TE interacted with application timing in 2004, but not 2005 or 2006. Ethephon and EP plus TE did not provide less than 10% seedhead cover in 2003 or 2006, respectively. For EP and EP plus TE, application timing affected seedhead control in 3 of 5 yr.