Mowing height and post-application irrigation influence annual bluegrass control with methiozolin

Methiozolin was registered for preemergence and postemergence annual bluegrass control in 2019 and offers good cool-season turfgrass safety. The methiozolin label indicates that a higher rate is required to effectively control annual bluegrass (Poa annua L.) at mowing heights greater than that of typical golf course putting greens. Additionally, the herbicide label recommends post-application irrigation for effective annual bluegrass control that is difficult to administer timely due to demands of golf play. The objective of this research was to determine the effect of mowing height and post-application irrigation on annual bluegrass control with methiozolin. Three field studies were initiated in Blacksburg, VA, on creeping bentgrass (Agrostis stolonifera L.) and Kentucky bluegrass (Poa pratensis L.) research fairways as a split-plot design, with three mowing heights (0.15, 0.30, and 0.6 inches) as main plots. Subplots were arranged as a four by two factorial with four levels of methiozolin rate (0.45–3.6 lb ai acre⁻¹) and two levels of post-application irrigation (0 and 0.2 inches). At all locations, as mowing height increased, more methiozolin was required to control annual bluegrass 80% (C₈₀). At two of the three locations, post application irrigation increased annual bluegrass control at 0.3 inches or higher heights of cut. When turf was maintained at 0.3 inches, annual bluegrass C₈₀ was reduced by approximately 50% with post application irrigation compared to no irrigation in two of the three trials. Results from these studies support product label recommendations regarding increased use rates with higher heights of cut but cannot confirm the need for post-application irrigation in putting green height turf.