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TURFGRASS SCIENCE

St. Augustinegrass Response to Plant Growth Retardants

^a Dep. Horticulture, D-136 Poole Ag. Center, Clemson University, Clemson, SC 29634-0375
^b Dep. Applied Economics and Statistics, Clemson University, Clemson, SC 29634-0375
^c Dep. Env. Horticulture, Univ. of Florida, Gainesville, FL 32611-0670

^* Corresponding author (bmccrty{at}clemson.edu).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
St. Augustinegrass [(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 [-(1-methylethyl)--[4-(trifluoro-methoxy)phenyl]-5-pyrimidine-methanol] and paclobutrazol [(+/–)-(R*,R*)-ß-[(4-chloro-phenyl)methyl]--(1,1-dimethylethyl)-1H-1,2,4-triazole-1-ethanol] at 1.12 kg ha^–1 for SIA and 0.56 kg ha^–1 for TSA, trinexapac-ethyl [4-(cyclopropyl--hydroxymethylene)-3,5-dioxocyclohexane 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-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-methyl-3-pyridine-carboxylic 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.

Abbreviations: CTC, cumulative turfgrass clippings • LUR, label use rate • PGR, plant growth retardant • SIA, single initial application • TQ, turfgrass quality • TSA, twin split applications

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
ST. AUGUSTINEGRASS is the primary warm-season turfgrass in Florida, accounting for 0.7 million hectare and contributing $2 billion in added value to Florida's economy (Hodges et al., 1994). Uses include residential and commercial landscape, golf courses, and roadsides. St. Augustinegrass is well adapted to various soils and climatic regions, establishes rapidly, has a high degree of salt tolerance, and demonstrates a cultivar-dependent range of shade tolerance (McCarty, 2003). The major disadvantages of St. Augustinegrass include susceptibility to the southern chinch-bug (Blissus insularis Barber) and high energy inputs of supplemental irrigation and mowing practices to maintain an attractive turfgrass sward (McCarty, 2003).

To achieve acceptable St. Augustinegrass turf quality during the active growing season, a weekly minimum of 2.3 cm water is required (Peacock and Dudeck, 1984). If irrigation or rainfall becomes excessive, however, St. Augustinegrass will experience rapid vertical growth requiring more frequent mowing and a profusion of turfgrass clippings. Furthermore, 40% of energy requirements for turfgrass maintenance is attributed to fossil fuels (Busey and Parker, 1992). As fossil fuel demands increase, fuel efficiency for lawn maintenance will become increasingly important.

One approach for addressing these problems, aside from turfgrass breeding programs and improved equipment technology, would be the use of plant growth retardants (PGRs) for turfgrass growth suppression and reduced mowing frequency. PGRs suppress growth, reduce mowing frequency, and limit turfgrass clippings on golf course ‘Tifway’ bermudagrass [Cynodon transvaalensis Burtt-Davy x C. dactylon (L.) Pers] (Johnson, 1994) and Kentucky bluegrass (Poa pratensis L.) (Ervin and Koski, 2001). Because of current solid waste management policies that ban, or penalize with impact fees, the contribution of yard debris to landfills, reduction in turfgrass clippings would be an important benefit to lawn care professionals.

Limited published information exists on St. Augustinegrass growth response to commercially available PGRs. The objectives of this study were to compare (i) turfgrass quality, (ii) lateral stolon growth, (iii) seasonal mowing frequency and accumulative turfgrass clippings, and (iv) seedhead growth inhibition–suppression of St. Augustinegrass following application of PGRs.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Site Description and Preparation
Field studies were initiated 23 June 1995 and 22 June 1996 at the University of Florida G.C. Horn Memorial Turfgrass Field Laboratory in Gainesville, FL (29°42' N, 82°24' W) to evaluate above ground vegetative growth response of ‘Floralawn’ St. Augustinegrass to various plant growth retardants. The soil was a loamy, siliceous, hyperthermic Grossarenic Paleudult Arredondo fine sand with 15 g kg^–1 OM and 7.5 pH.

Spring fertilization was applied on 6 May each year at 43.5 kg nitrogen ha^–1 in a 16-4-8 (N:P₂O₅:K₂O) fertilizer. Turfgrass was mowed when 10.0-cm height was reached, and a mowing height of 7.5 cm was maintained throughout the trials. During the second year, test plots were treated on 18 May with atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine) and asulam (methyl [4-aminophenyl) sulfonyl]carbamate] at 2.2 kg ha^–1 for control of white clover (Trifolium repens L.) and southern crabgrass [Digitaria ciliaris Retz (Koel.)] and selective removal of encroached centipedegrass [(Eremochloa ophiuroides (Munro) Hack].

Plant Growth Retardant Treatments and Application
Five PGRs were applied as a single initial application (SIA) at label use rate (LUR) or twin split applications (TSA) at half LUR each. Initial applications were made 23 June 1995 and 22 June 1996. The treatments were flurprimidol and paclobutrazol at 1.12 kg ha^–1 for the SIA and 0.56 kg ha^–1 for TSA, trinexapac-ethyl and mefluidide at 0.28 kg ha^–1 for the SIA and 0.14 kg ha^–1 for TSA, imazapic at 0.028 kg ha^–1 for the SIA and 0.014 kg ha^–1 for TSA, and untreated control (Table 1). Imazapic was an experimental material at trial initiation. For TSA, the second application was applied only when mowing interval (days between successive mowings) of the PGR and untreated plots were equal and was limited to one per 12-wk season. Treatments were applied from 3.0-L bottles with a CO₂ powered backpack system calibrated to deliver 378.5 L ha^–1. Surfactants were not included in any applications.

Characteristics Measured
Turfgrass quality (TQ) was a composite visual rating of phytotoxicity and overall appearance. Evaluation was based on a scale of 1 to 10, where 1 equaled completely dead and brown turfgrass and 10 equaled healthy green turfgrass. TQ ratings were performed weekly for 12 wk. Several measures of vegetative growth were obtained: lateral stolon growth, percent cover, seasonal mowing frequency, cumulative turfgrass clippings, and seedhead production.

Three weeks before treatment initiation, a 60 cm wide by 2 cm deep sod cut was removed through the center of each 3.1-m² plot to facilitate monitoring of lateral stolon growth. Lateral growth measurements were obtained to assess the practical use of PGRs as chemical edgers in landscape maintenance operations and as a mowing reduction management tool in St. Augustinegrass sod farms. Hand weeding was employed weekly to eliminate weed competition with St. Augustinegrass stolons growing in the sod cut area. Five stolons were randomly selected in each plot and monitored for lateral growth by placing a 0.6- by 7.5-cm wooden peg adjacent to each stolon tip. Beginning at Week 2, and biweekly thereafter, the distance from the peg position to the stolon tip was measured. Pegs were then relocated to the new stolon tip position. The five measurements for each plot were averaged each sampling date.

Percent cover was examined at 10 and 12 wk after initial treatment application in the regrowth area to evaluate long term PGR impact on regrowth and recuperative capacity. A 0.6- by 1.6-m grid was constructed with 2.5-cm² galvanized wire mesh attached to a PVC frame fitted to the original sod cut area. The grid was placed over the stolon regrowth area, and each 2.5-cm² cell was examined and counted if any St. Augustinegrass leaf or stolon tissue was observed. The number of counted cells was divided by the total number of grid cells (1560) and multiplied by 100 to obtain percent turfgrass cover. The grid was precisely placed at the same location on each observation date.

Mowing frequency denotes the number of mowings per treatment over a 12-wk growing season. Each treatment was mowed to 7.5 cm when average turfgrass height, measured from the soil surface to mature leaf blade tips, reached 10.0 cm. The dry weight of cumulative turfgrass clippings (CTC) was determined for each treatment. Five 1.55-m² sections of each plot were harvested with a commercial walk-behind rotary mower with a 0.5-m swath, custom designed chute, and clear plastic catcher. Harvested samples were oven dried for 72 h at 60°C. Following harvest, the entire plot was mowed to 7.5 cm. Seedhead inhibition was evaluated by counting the total number of seedheads in a randomly selected 1.0-m² area of each plot.

Statistical Design and Analysis
The experiment design was a randomized complete block with 11 treatments and four replications. Analyses of the 2 yr of data did not reveal meaningful treatment x year interactions, so the combined results were used for further analyses, presentation, and discussion. Factorial analysis of variance was performed to examine main and interaction effects of the PGRs and applications (SIA vs. TSA). Single degree-of-freedom linear contrasts were used to compare application effects for each PGR when interaction of the two factors was detected. Marginal means of the two factors were examined when there was no interaction. Dunnett's test (Dunnett, 1955) was used to compare PGR treatments with the untreated. A square root transformation of seedhead number was used to achieve homogeneous variances.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Seasonal Variation
Rainfall for the 12-wk period in 1995 and 1996 are presented in Fig. 1 . Differences in total rainfall (55 cm for 1995 vs. 69 cm for 1996) affected St. Augustinegrass growth and quality for the two years. Less rainfall in 1995 resulted in lower turfgrass quality, mowing frequency, cumulative turfgrass clippings, and seedhead number than in 1996.

View larger version (19K): [in this window] [in a new window]   Fig. 1. Seasonal precipitation from 23 June to 14 Sept. 1995, and 22 June to 13 Sept. 1996, at the G.C. Horn Memorial Turfgrass Field Laboratory, Gainesville, FL.

The injury observed for trinexapac-ethyl in this study is consistent with findings reported by Johnson (1992a)(1992c) for centipedegrass and Tifway bermudagrass. The effects of mefluidide on St. Augustinegrass TQ were similar to previously reported results in centipedegrass (Johnson, 1989; Johnson, 1990). The acceptable TQ ratings obtained for flurprimidol and paclobutrazol are consistent with previous research on centipedegrass (Fry, 1991) and ‘Tifway’ bermudagrass (Johnson, 1992b).

Lateral Stolon Growth and Percent Cover
Effects of PGR and application type on lateral stolon growth following initial PGR applications were independent through Week 6 (Table 4). Lateral growth was most affected by imazapic with reductions of about 60% compared with the untreated (Table 5). Trinexapac-ethyl and mefluidide reduced lateral growth about 30% during this period. Flurprimidol and paclobutrazol were generally ineffective at controlling lateral stolon growth. An effect of application rate was detected at week 4 with lateral growth affected more by SIA than TSA (Tables 4 and 5).

Percent cover was evaluated in the stolon regrowth area on Weeks 10 and 12 following initial PGR applications to assess regrowth and recuperative capacity. A main effect of PGR on percent cover was detected on Week 10 (Table 4) with 35 and 17% less regrowth for imazapic and mefluidide, respectively, compared with the untreated (Table 5). An interaction of PGR and application type was detected on Week 12 because of lower regrowth for TSA (40%) than for SIA (25%) of imazapic.

Seasonal Mowing Frequency and Cumulative Turfgrass Clippings
A main effect of PGR on MF was detected (Table 6), and the greatest reduction (50%) was provided by trinexapac-ethyl (Table 7). Flurprimidol and mefluidide reduced the number of mowings by 26 and 20%, respectively, compared to the untreated.

Johnson (1994) noted a 69% reduction in Tifway bermudagrass mowing frequency for a 12-wk season following an initial trinexapac-ethyl application at 0.2 kg ha^–1 with sequential applications at 4 to 8 wk of 0.1 kg ha^–1. However, no differences in seasonal mowing number were detected when the initial and sequential trinexapac-ethyl application rates were reduced by 50%. Johnson (1994) also reported that flurprimidol and paclobutrazol provided little seedhead control activity and did not reduce season-long mowing frequency because of the number of trim mowings required.

Seedhead Inhibition
Seedhead production was determined 3, 5, 7, and 9 wk following initial PGR applications. It has been suggested that for a PGR to be commercially acceptable, seedhead inhibition should be >70% (Johnson, 1994). No meaningful differences were observed until Week 5, when an interaction between PGR and application type was detected because of lower seedhead production for SIA of trinexapac-ethyl than for TSA (Tables 6 and 7). Comparisons to the untreated revealed that SIA of imazapic reduced seedhead production by 40%, while SIA of flurprimidol increased production by 41% (Table 7).

St. Augustinegrass seedhead production appeared to peak on Week 7 (early August) each year (Table 7). There was a main effect of PGR on seedhead production for Weeks 7 and 9 (Table 6). Imazapic and mefluidide provided greatest reductions (about 35%) in seedhead production compared to the untreated on week 7 (Table 7). Therefore, the commercially acceptable level of seedhead inhibition (>70%) suggested by Johnson (1994) was not achieved in this study. The general ineffectiveness of flurprimidol, paclobutrazol, and trinexapac-ethyl on seedhead inhibition can be attributed to their gibberellin biosynthesis inhibition (Type II PGR activity) and not cell division inhibition as previously documented (Johnson, 1992a, 1994; Spak et al., 1993) and reduced mowing frequency that limited mechanical seedhead removal. Gibberellin biosynthesis inhibitors do not prevent seedhead formation as efficiently as Type I PGRs which are cell division inhibitors (Murphy et al., 2001). Johnson (1994) noted similar reproductive suppression followed by a vegetative growth flush for paclobutrazol and trinexapac-ethyl when evaluating Tifway bermudagrass seedhead suppression to Type II PGRs.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Flurprimidol and paclobutrazol were generally ineffective PGRs for St. Augustinegrass growth management. Trinexapac-ethyl effectively reduced mowing frequency and CTC regardless of application type but provided minimal reduction in lateral stolon growth. Successful seedhead control with this product would require trim mowings, but Johnson (1989) indicated this cultural practice stimulates vegetative growth. Additional sequential applications may then be required in conjunction with trim mowings to provide continued turfgrass clipping and seedhead suppression. Imazapic provided excellent control of seedhead production and lateral stolon growth with the TSA providing greater long-term regrowth control than the SIA. Mefluidide demonstrated potential for controlling lateral growth and reducing mowing frequency. Combinations of these three products could be examined in future PGR studies on St. Augustinegrass.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Technical Contribution of the South Carolina Agricultural Experiment Station, Clemson, SC.

Received for publication August 30, 2003.

	REFERENCES

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