Growing Degree Day Models of Plant Growth Regulator Efficacy on Creeping Bentgrass Putting Greens.

Golf course superintendents in the Lubbock, TX area struggle with excessive shoot growth of creeping bentgrass (Agrostis stolonifera L.) putting greens during summer months that can result in scalping and poor playability.  Plant growth regulators (PGRs) are used extensively on golf course putting greens to reduce shoot elongation.  There are three commercially available chemical PGRs; trinexapac-ethyl  (Primo Maxx), flurprimidol (Cutless MEC), and paclobutrazol (Trimmit 2SC).  Newer, commercially available PGRs (Legacy and Musketeer) have combined two or more of these chemistries.  Legacy contains trinexapac-ethyl and flurprimidol, whereas Musketeer combines all three chemistries.  Previous research has demonstrated reduced effectiveness with trinexapac-ethyl as temperatures increase; however, similar studies have not been conducted on other PGRs in the transition zone.  The objective of this study was to construct growing degree day (GDD) models for commercially available PGRs.  Treatments were applied from May to August six to eight weeks apart in 2014 and 2015 to demonstrate their full progression at two locations in Lubbock, TX.  Clippings were collected from each plot two times per week, oven dried, and weighed to determine relative growth to untreated controls.  Maximum suppression was achieved from 180 to 300 GDD following application.  Products containing paclobutrazol provided greater suppression for a longer period of time.  Flurprimidol, trinexapac-ethyl, and flurprimidol + trinexapac-ethyl lost suppression at 400 GDD; however, the three-way combination with all PGR’s suppressed growth for 550 GDD and paclobutrazol alone effectively reduced shoot growth for 650 GDD.  Many turf managers from this region have suggested that PGRs other than trinexapac-ethyl are chosen due to their lack of a rebound effect.  In contrast, our data demonstrated accelerated shoot growth following suppression.  The results from this research have been communicated with regional golf course superintendents to assist with PGR application scheduling to maintain adequate suppression.