Creeping bentgrass (CBG; Agrostis stolonifera L.) is a problematic weed of cool-season turfgrass. The herbicide mesotrione is often used for selective control, but CBG often recovers from sequential applications. Research evaluated the efficacy of mesotrione-based sequential application regimens for CBG control in kentucky bluegrass (Poa pratensis L.) over a 2-year period. In two separate experiments, identical herbicide regimens were initiated in Oct. 2014 or May 2015 and then reapplied to the same plots in Oct. 2015 or May 2016, respectively. Regimens consisted of various sequential application regimens of mesotrione alone (totaling 560 g·ha–1 annually), three sequential applications of mesotrione (175 g·ha–1) tank-mixed with either triclopyr ester (560 or 1120 g·ha–1) or amicarbazone (50 or 100 g·ha–1), and topramezone (32 or 37 g·ha–1) tank-mixed with triclopyr ester (1120 g·ha–1). At the end of each 2-year experiment, the most effective treatments did not eliminate CBG completely. Among treatment regimens initiated in the fall, the most effective treatments reduced CBG cover 49% to 73% at the conclusion of the experiment in Oct. 2016. At the conclusion of the spring experiment in May 2017, the most effective treatments reduced CBG cover 66% to 94%. Topramezone + triclopyr tank mixtures were less effective than mesotrione-containing treatments on most dates. Mesotrione + amicarbazone tank mixtures reduced CBG more effectively than mesotrione alone, but these tank mixtures also caused severe kentucky bluegrass injury. CBG cover reductions from mesotrione + triclopyr tank mixtures and mesotrione alone were generally similar. Among mesotrione-only regimens, there were no consistent differences in CBG cover reduction. This research indicates that turf managers using a selective herbicide regimen to control CBG in kentucky bluegrass should apply mesotrione at the maximum annual use rate (560 g·ha–1) in two to four sequential applications at 2- to 3-week intervals.
Fumigation of annual bluegrass (Poa annua L.)-infested putting greens before seeding creeping bentgrass (Agrostis stolonifera L.) prevents stand contamination due to annual bluegrass seedling emergence. Dazomet is a soil fumigant labeled for use in putting green renovation; however, limited data are available on efficacy of dazomet controlling annual bluegrass seedling emergence following surface-applications. The objectives of this study were to determine the influence of rate and plastic covering of surface-applied dazomet on annual bluegrass seedling emergence in putting green turf; and safe creeping bentgrass seeding intervals following applications of dazomet to putting green surfaces. Treatments were applied in late summer to the surface of a 20-year-old stand of turf maintained as a putting green and plots were watered immediately after application and throughout each test period. Plastic-covered dazomet treatments had fewer annual bluegrass seedlings than noncovered dazomet treatments. Three plastic-covered dazomet treatments (291, 340, and 388 kg·ha-1) provided complete control of annual bluegrass seedlings during 2000 and 2001. None of the noncovered dazomet treatments provided complete control of annual bluegrass seedling emergence. Results of the seeding interval experiment revealed that creeping bentgrass seedling development was not inhibited in both plastic-covered and noncovered dazomet treatments, when seeded 8, 10, 13, and 16 d after dazomet was applied to the turf surface. Results of this study demonstrate that dazomet, applied at rates ≥291 kg·ha-1 to the surface of a putting green in summer and covered with plastic for 7 d, can control annual bluegrass seedling emergence. Chemical name used: tetrahydro-3,5-dimethyl-2H-1,3,5-thiadiazine-2-thione (dazomet).