The Use of Gibberellic Acid Inhibitors for the Control of Annual Bluegrass (Poa annua L.) in Creeping Bentgrass (Agrostis palustris Huds.) Putting Greens

Annual Bluegrass (Poa annua L.) has become a major problem for temperate zone golf courses, particularly on putting greens.  Because P. annua is so invasive and difficult to control, it often becomes a substantial percentage of the stand, resulting in the need for increased amounts of water and pesticides.  In this study, three plant growth regulators (PGRs), specifically the gibberellic acid inhibitors flurprimidol, paclobutrazol, and trinexapac-ethyl, were used in a variety of combinations and rates to find an effective P. annua control that also minimized turf injury.  Creeping bentgrass (Agrostis palustris Huds.) plugs containing an average of 20.6% established P. annua were removed from a putting green and potted in 2.6 L plastic containers.  The turf was maintained in the greenhouse under a 16 hr photoperiod and mowed daily to a height of 0.5 cm.  Eleven treatments were applied at 14 day intervals over a 15 week period, with measurements of percent P. annua and visual ratings of turf injury taken preceding each application.  The combination of paclobutrazol and trinexapac-ethyl at the rates of 45.4 and 10.6 g active ingredient (a.i.) per acre, respectively, provided the most effective control. This treatment reduced P. annua by an average of 71.9% and also resulted in the lowest turf injury of any PGR treatment.  Three other treatments, paclobutrazol and flurprimidol at 45.4 and 56.7 g a.i. per acre, trinexapac-ethyl and flurprimidol at 10.6 and 56.7 g a.i. per acre, and paclobutrazol, trinexapac-ethyl and flurprimidol at 79.8, 15.9 and 70.9 g a.i. per acre, produced statistically similar reductions in P. annua, with only the last of these three treatments showing significantly more turf injury.  The highest rates of turf injury were observed in treatments containing all three PGRs at the highest application rates.  These results indicate that the combination of paclobutrazol and trinexapac-ethyl may provide effective control of P. annua on golf putting greens.