Bermudagrass is a troublesome weed of zoysiagrass golf-course fairways. Field research was conducted in 2009 and 2010 evaluating bermudagrass suppression with applications of fluazifop plus triclopyr at various timings. Three rates of fluazifop (0.10, 0.21, and 0.32 kg ai ha⁻¹) were applied with triclopyr (1.12 kg ae ha⁻¹) once six thresholds of growing-degree-day accumulation (GDD_10C) had been reached: 200, 450, 825, 1,275, 1,775, and 2,250 GDD_10C. Yearly accumulated GDD_10C values were calculated with a base temperature of 10 C beginning on 1 January. Applications at 200 and 2,250 GDD_10C suppressed bermudagrass ≥ 90% at 5 WAT each year. Increased rates of fluazifop did not provide additional bermudagrass suppression at these timings. Cooling accumulation models may be needed to time fall applications, as the 1,775 GDD_10C timing in 2009 provided similar bermudagrass suppression to the 2,250 GDD_10C timing in 2010. Late-spring and midsummer applications at 450 GDD_10C, 825 GDD_10C, and 1,275 GDD_10C only suppressed bermudagrass 4 to 16% at 6 wk after treatment (WAT) in 2009 and 0 to 57% at 6 WAT in 2010. Zoysiagrass injury measured < 25% for all timings and decreased to 0 to 7% by 5 WAT each year. Future studies should evaluate bermudagrass suppression with other herbicides with the use of growing-degree-day and cooling accumulation models.

Nomenclature: Fluazifop; triclopyr; bermudagrass (Cynodon dactylon L. Pers.) ‘Riviera’; zoysiagrass (Zoysia japonica Steud.) ‘Zenith’.