St. Augustine Growth Responses to Various Plant Growth Retardents

Jan Weinbrecht and Grady Miller
University of Florida, Gainesville
Institute of Food and Agricultural Sciences

St. Augustinegrass is the preferred warm season turfgrass for Florida's commercial and residential landscapes with an estimated 1.5 million acres under growth and management. However, limited research information is available regarding St. Augustinegrass growth response to currently labeled or potentially registered plant growth retardants (PGRs). Previous PGR research has demonstrated excellent responses in fine warm season turfgrasses such as bermudagrass and centipedegrass. A two year study was conducted at the University of Florida Turfgrass Envirotron to evaluate 'Floralawn' St. Augustinegrass growth responses following PGR application.

St. Augustinegrass growth responses were monitored twice-weekly for a six week period following Cutless 50 WP and TGR Turf Enhancer 50 WP at 1.0 lb product/A, Primo at 1.0 qt/A, Embark at 1.0 pt/A , Plateau at 1.6 fl. oz/A, and an untreated control. Cutless, TGR, and Primo are classified as Type II PGRs. Application of Type II PGRs results in a suppression of gibberellic acid, the naturally occurring hormone responsible for cell elongation. Embark and Plateau are classified as Type I PGRs and inhibit cell division and amino acid synthesis, respectively. Interference with these physiological processes temporarily stops plant growth. Any growth suppression or inhibition following a PGR application is temporary, with normal plant growth resuming within three to eight weeks following application.

Observed growth responses included turf quality on a 1-10 basis, where 10=best turf quality and 5=minimal commercial acceptability, turfgrass clippings reduction, and lateral stoloniferous growth reduction. Of particular interest was root growth response in terms of root length density, or total root length per volume of soil.

Embark was one of the first PGRs to be introduced to fine turfgrass management. It provides good vegetative growth inhibition, and an excellent job of suppressing seedhead formation. Turf quality following Embark (>=8.3) was similar to the untreated at any time during evaluation. Root length density was similarly not affected following Embark at any of the bi-weekly tracings, or as a cumulative value at the end of the six week evaluation period. A 25% clippings reduction was evident for the first two weeks, with shoot growth after that equal to the untreated. Lateral stoloniferous growth did not appear to be inhibited following Embark during this study. However, based on the information in a related field study, Embark at 1.0 pt/A reduced lateral growth ~30% three to four weeks following application.

Cutless and TGR 50 have been registered for use in fine turfgrass situations since the mid 1980's. They have traditionally provided a longer duration of growth suppression, but lack an ability to inhibit seedhead formation. In this trial, Cutless and TGR 50 actually enhanced turf quality during the latter part of the study (weeks four through six). This has been a common observation with these materials in previous PGR/fine turfgrass research. St. Augustinegrass root length density was unaffected at any biweekly rating, or as a cumulative value at the end of the trial period. Cutless and TGR 50 provided ~48% reduction in turfgrass clippings during week two, while Cutless averaged a 65% clippings reduction during week four. Neither Cutless nor TGR 50 limited lateral stoloniferous growth.

Primo was registered for use in fine turfgrass situations in the early 1990's. Like Cutless and TGR 50, it provides an extended period of growth suppression, with no seedhead control activity. Like Cutless and TGR 50, Primo enhanced St. Augustinegrass turf quality (~9.0) four to six weeks after treatment. No reductions in St. Augustinegrass root length density following Primo were evident during these trials. A 75% reduction in turfgrass clippings was evident at week four. Primo reduced lateral stoloniferous growth (55% four weeks after application.

Plateau is a herbicide Type I PGR registered for roadside turf in the mid 1990's, and applied in "sub-lethal" doses, provides excellent vegetative growth inhibition and seedhead control. Plateau provided the lowest turf quality (~7.2) at two weeks following application. Recovery was complete by week four where it equaled the untreated for the remainder of the study. St. Augustinegrass root length density was inhibited ~68% four weeks following application. However, root length density values for Plateau at week six was >50% higher than values for the untreated control at week six. This surge in root growth resulted in cumulative six week root length density value for Plateau equaling root length density values for all treatments. Turfgrass clippings were reduced 89% through week two only. Lateral stoloniferous growth was inhibited an average 63% for four weeks following application.In summary, four to six weeks following a single PGR application is a likely period for warm season turfgrasses to display above ground injury and recovery. The ability to simultaneously view above and below ground 'Floralawn' St. Augustinegrass growth responses following PGR application within this time frame demonstrated that all PGRs evaluated provided fair to good turf quality and clippings reduction, and with the exception of Plateau, root length density was unaffected by PGR application. The initially lower root length density values associated with Plateau were viewed as temporary inhibition and not a long-term reduction, as cumulative root length density at six weeks were equal across all treatments. Additionally, lateral stoloniferous growth was significantly limited by Plateau, which suggests potential as a landscape chemical edging material.