Plant growth regulator and mowing height effects on seasonal root growth of penncross creeping bentgrass

Summer decline of creeping bentgrass [Agrostis palustris Huds. (=Agrostis stolonifera var. palustris (Huds.) Farw.] root growth is an annual concern in many regions of the USA. A 2-yr field study was conducted to investigate the effects of mowing height and plant growth regulator (PGR) treatments on root biomass (RB) patterns in 'Penncross' creeping bentgrass, grown on a Wakulla sand (sandy, siliceous, Thermic Psammentic Hapludults) with 94% sand, 4% silt, 2% clay, 24 mg g(-1) organic matter, and a pH of 6.1. Plots were maintained at 3.2, 4.0, or 4.8 mm throughout the experiment. Plant growth regulator treatments were trinexapac-ethyl, [4-(cyclopropyl-alpha-hydroxymethylene)-3,5-dioxo-cyclohexane carboxylic acid ethyl ester] applied at 0.05 kg a.i. ha(-1) and paclobutrazol, {(+/-)-(R*,R*)-beta-[(4-chloro-phenyl) methyl]-alpha-(1,1-dimethylethyl)-1H-1,2,4-triazole-1-ethanol} applied at 0.14, 0.28, or 0.56 kg a.i. ha(-1). Plant growth regulator treatments initially were applied in April 1997 and were reapplied monthly for 24 mo. Annual variation in air and soil temperature accounted for variable RB across years. Root biomass declined 76% from a May, maximum to a minimum in September; however, RB increased between September and January. Turf mowed to a height of 3.2 mm consistently had lower RB than turf maintained at 4.0 or 4.8 mm, and had slower early autumn RB recovery. When averaged across mowing heights, only paclobutrazol (0.56 kg a.i. ha(-1)) reduced RB and no PGR increased rooting when compared with nontreated turf. From September to January of each year, a PGR by mowing height interaction showed that paclobutrazol applied at 0.56 kg a.i. ha(-1) delayed RB recovery in turf mowed to a height of 4.8 mm, while no PGR delayed RB recovery in turf maintained at the 3.2 mm mowing height. Slower RB recovery in turf mowed to 3.2 mm following the September minimum RB was attributed to reductions in turf quality and density, rather than the effects of PGRs. Paclobutrazol only reduced Penncross RB when applied at twice the labeled rate and with greater application frequency than is typical for most PGR use patterns. These results showed that labeled rates of paclobutrazol and trinexapac-ethyl did not adversely affect root growth under putting green conditions.