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St. Augustine grass [(Stenotaphrum secundatum (Walt.) Kuntz.] is the preferred warm-season turfgrass for Florida's commercial and residential landscapes with an estimated 0.7 million hectare under growth and management. Limited published information is available on St. Augustinegrass response to plant growth retardants (PGRs). A 2-yr study was implemented to monitor St. Augustinegrass turf quality, lateral stolon growth, percent cover, mowing frequency, cumulative turfgrass clippings, and seedhead suppression following PGR application. Treatments were applied on 23 June 1995 and 22 June 1996 as a single application (SIA) at label use rate (LUR) or as twin split applications (TSA) at half LUR each: the sequential application was only used when mowing interval equaled the untreated. The PGRs and rates were flurprimidol [α-(1-methylethyl)-α-[4-(trifluoro-methoxy) phenyl]-5-pyrimidine-methanol] and paclobutrazol [(+/-)-(R*,R*)β-[(4-chloro-phenyl)methyl]-α-(1,1-dimethylethyl)-1H-1,2,4-triazole1-ethanol] at 1.12 kg ha-1 for SIA and 0.56 kg ha-1 for TSA, trinexapac-ethyl [4-(cyclopropyl-α-hydroxymethylene)-3,5-dioxocydohexane carboxylic acid ethylester] and mefluidide [N-[2,4-dimethyl-5-[[trifluoromethyl)sulfonyl]amino]phenyl]acetamide] at 0.28 kg ha-1 for SIA and 0.14 kg ha-1 for TSA, and imazapic [(±)-2-[4,5-dihydrol-4-methyl4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-methyl-3-pyridinecarboxylic acid] at 0.028 kg ha-1 for SIA and 0.014 kg ha-1 for TSA. Responses were observed for a 12-wk period following initial application, and turf quality was acceptable (>7) for all PGRs. Turf quality for imazapic was generally better than the untreated for Weeks 6 to 10. Greatest control of lateral stolon growth 10 wk after initial application was achieved with TSA of imazapic (68%) and mefluidide (61%). Percent cover 12 wk after initial application was lowest for SIA and TSA imazapic (66 and 53%, respectively). Greatest reduction in mowing frequency was provided by trinexapac-ethyl (50%), while flurprimidol and mefluidide reduced mowing frequency by 26 and 20%, respectively. The only PGR that reduced cumulative turfgrass clippings (CTC) was trinexapac-ethyl (63%). Greatest seedhead inhibition during peak production (about 35%) was provided by imazapic and mefluidide. The two most effective PGRs were trinexapac-ethyl (reduced mowing frequency and CTC) and imazapic (controlled lateral stolon growth and seedhead production), while mefluidide demonstrated some potential. Combinations of these products could be examined in future studies. [ABSTRACT FROM AUTHOR]