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Crop Science 41:1901-1905 (2001)
© 2001 Crop Science Society of America

TURFGRASS SCIENCE

Plant Growth Regulator and Mowing Height Effects on Seasonal Root Growth of Penncross Creeping Bentgrass

Matthew J. Fagerness*,a and Fred H. Yelvertonb

a Dept. of Horticulture, Forestry, and Recreation Resources, 2021 Throckmorton Plant Sciences Bldg., Kansas State Univ., Manhattan, KS 66506-5507
b Crop Science Dept., North Carolina State Univ., 100 Derieux St. Box 7620, Raleigh, NC 27695-7620

* Corresponding author (mfagerne{at}oznet.ksu.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
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*)-ß-[(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.

Abbreviations: ET, evapotranspiration • PGR, plant growth regulator • RB, root biomass


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
CREEPING BENTGRASS often is regarded as the best species for putting green turf. It has excellent tolerance to low mowing, moderate adaptation to a variety of climates, and good recuperative ability in response to traffic and environmental stress (Cuddeback and Petrovic, 1985; Rosenberg, 1964). The use of creeping bentgrass as a putting green surface has extended into the transition zone of the United States (Callahan, 1978), creating management difficulties when subjected to high temperatures and humidity in summer. High air temperatures and poor soil aeration reduces root growth and root viability in creeping bentgrass, and contributes to summer bentgrass decline (Huang et al., 1998a,b).

Strategies to reduce creeping bentgrass summer root growth decline have included use of soil amendments (Ferguson et al., 1986) and increased mowing height (Huang et al., 2000; Mazur, 1986). Creeping bentgrass cultivars with superior stress tolerance improve the turf manager's ability to maintain a quality turf during summer months. However, heat tolerant cultivars of creeping bentgrass are still susceptible to root growth decline when air temperatures rise above 30 °C (Huang et al., 1998b; Karnok and Kneebone, 1975). Plant growth regulators may reduce turfgrass evapotranspiration (ET) rates (Borden and Campbell, 1987; Devitt and Morris, 1988; Doyle and Shearman, 1985; Fry and Jiang, 1998), but it is unknown if such responses are relevant to creeping bentgrass root growth.

Plant growth regulator uses on creeping bentgrass putting greens in the southern United States have included delaying bermudagrass (Cynodon spp.) encroachment into creeping bentgrass (Johnson and Carrow, 1989), disease prevention (Burpee et al., 1996), and reducing populations of annual bluegrass {Poa annua var. reptans Hausskn. [=Poa annua f. reptans (Hausskn.) T. Koyama]} on putting greens (Johnson and Murphy, 1996). Paclobutrazol may be applied six to eight times annually to creeping bentgrass for the purpose of reducing annual bluegrass. It is unknown how these frequent applications may affect creeping bentgrass root growth. Trinexapac-ethyl is labeled for preparing turf to better tolerate environmental stress in the summer. It is unknown, however, if altered root growth is a means by which enhanced summer stress tolerance is achieved.

While reduced mowing heights on putting greens and in other turfgrass systems are generally believed to be at the expense of root growth, the effects of PGR applications on root growth of bentgrass grown on putting greens are unclear. The objectives of this research, therefore, were to investigate how monthly applications of trinexapac-ethyl or paclobutrazol would affect seasonal RB of Penncross creeping bentgrass maintained at three different mowing heights.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
This experiment was conducted from April 1997 through April 1999 at the North Carolina State University Sandhills Research Station, Jackson Springs, NC. Turf was a 2-yr-old stand of 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. The putting green was free of annual bluegrass. Plots received irrigation to prevent drought stress. Six equal rate applications of greens-grade 25-12-6 fertilizer (three autumn applications from September to November, and three spring applications from March to May) were made to provide a total of 300 kg N ha-1 yr-1. Biweekly chlorothalonil (tetrachloroisophthalonitrile) applications were made between 15 June and 15 September for preventive disease control. All plots were vertically mowed and topdressed in spring of each year to minimize thatch accumulation.

Three walk-behind reel mowers were used. Each mower was set to mow to a height of 3.2, 4.0, or 4.8 mm. Plots were mowed five times weekly. Mowing treatments were initiated 16 March 1997. Plant growth regulators were first applied 17 Apr. 1997 and included paclobutrazol (240 g L-1 suspension concentrate formulation) at 0.14, 0.28, or 0.56 kg a.i. ha-1 and trinexapac-ethyl (120 g L-1 emulsifiable concentrate formulation) at 0.05 kg a.i. ha-1. Treatments were applied to 1.53- x 4.58-m plots in 304 L ha-1. Spray equipment was a CO2–pressurized (179 kPa), wheel mounted boom sprayer with four TeeJet 8002 nozzles (Spraying Systems Co., Wheaton, IL) spaced 25 cm from each other and set 30 cm above the ground. Plant growth regulator treatments were applied every 4 wk for 2 yr. Trinexapac-ethyl was applied before paclobutrazol on each application date to allow for foliar absorption of trinexapac-ethyl before paclobutrazol was watered-in, 2 to 12 h after PGRs were applied.

Because subsurface visual estimation methods for root growth may be inconsistent (Glinski et al., 1993), destructive and repetitive root sampling methods were chosen for this experiment. The process of harvesting, separating, and weighing roots were similar to that reported by Koski (1997). Root biomass was based on measurements from two 1800-cm3 sod and soil cores taken from each plot. Cores initially were harvested on 17 April 1997 to provide a baseline for subsequent seasonal changes in RB. Subsequent cores were taken 2 wk following PGR applications. Because new root growth into previously sand backfilled cores was observed to be greater than into undisturbed soil, monthly cores from each plot were harvested at least 25 cm from those previously harvested. Roots were separated by removing sod and thatch at the thatch/soil interface and then washing the soil through a mesh screen to remove most soil particles. Samples were oven-dried at 70 °C for 96 h, placed in ceramic crucibles, and weighed. Samples in crucibles were placed in a muffle furnace and heated at 500 °C across 12 h to combust organic material. Differences in crucible weights, before and after combustion, were valued as RB.

The design was a split plot, with PGR treatments as subplots superimposed across each mowing height. The experiment had four replications. Root biomass data were subjected to ANOVA for a split plot design using SAS Version 7.0 (SAS Institute, 1998). Main effects of mowing height, PGR treatments, month, and growing season were analyzed. Standard F tests were used to determine significant (P <= 0.05) main effects and interactions. Main effect means and appropriately sorted interaction means were separated using Fisher's Protected LSD test at P <= 0.05.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Month, growing season (1997/1998 vs. 1998/1999), mowing height, and PGR treatments all affected RB. Analysis of month as a separate experimental factor was appropriate since time of year has a profound effect on patterns of creeping bentgrass growth (Rist et al., 1997). Additionally, selected monthly intervals for analysis (May to September, September to January, and February to April) were based on both the strength of the month main effects (P < 0.0001) and on suspected seasonal RB trends. Only the September to January RB recovery period proved to be of interest.

When averaged across growing seasons from non-PGR-treated plots to illustrate annual patterns of root growth, RB was found to be lowest in September, greatest in May, and intermediate in winter and early spring (Fig. 1) . However, RB was 34% greater in the 1997/1998 growing season than in 1998/1999 (P < 0.0001). Large differences across growing seasons were due in part to air and soil temperature differences (Table 1). An interaction between month and growing season (P < 0.0001) also was detected. This interaction was of limited use in terms of the experimental objectives, thereby placing greatest emphasis on the month main effect (Murray et al., 1999).



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  		Fig. 1. Monthly (P < 0.0001) means for non-PGR-treated ‘Penncross’ creeping bentgrass root biomass averaged across the 1997 to 1998 and 1998 to 1999 growing seasons to illustrate seasonal trends. Error bars represent a calculated LSD0.05 value of 0.08. Mowing height treatments were initiated 16 March 1997 and maintained throughout the experiment. Plant growth regulator treatments were first applied 17 April 1997, and monthly thereafter. Root biomass values were based on two 1800 cm3 cores plot-1.

 

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  		Table 1. Sandhills Research Station, Jackson Springs, NC, weather data from January 1997 to April 1999.

 
When averaged across all 24 mo of the experiment, RB levels were 32% greater in turf maintained at 4.8 mm, and 36% greater in turf maintained at 4.0 mm when compared with the 3.2 mm mowing height (Table 2). Reduced RB in turf maintained at 3.2 mm was prevalent during late summer and autumn months, and mowing height interacted with month from September to January (Table 2). Although turf maintained at all three mowing heights had low RB in September, RB from September to October increased 70% in turf mowed to a height of 4.0 or 4.8 mm, while September to October RB increases in turf mowed to 3.2 mm were 55%. Following little increase in RB from October to November at all mowing heights, RB in turf mowed to 3.2 mm increased 141% from November to December, and then leveled. During the November to December period, RB increases at the higher two mowing heights were only 60%, but these increases continued into January. Results showed that creeping bentgrass turf maintained at 3.2 mm produced less overall RB than turf maintained at the higher mowing heights. Also, RB recovery in turf mowed to a height of 3.2 mm may be both initially less in magnitude and ultimately shorter in duration than at higher mowing heights. Such a response following periods of summer stress may be related to reduced quality and shoot density in creeping bentgrass mowed to a height of 3.2 mm during summer months (Fagerness et al., 2000).


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  		Table 2. Mowing height main effect (24-mo average) and month x mowing height interaction means, from September to January, for creeping bentgrass putting green root biomass, averaged over the 1997–1998 and 1998–1999 growing seasons.

 
Across a 12-mo period, PGRs did not enhance RB; however, paclobutrazol applied at 0.56 kg a.i. ha-1 reduced RB by 10%, when compared with all other treatments (data not presented). A PGR by growing season interaction was detected during the September to January period. Areas treated with paclobutrazol at 0.56 kg a.i. ha-1 had 24% less RB, compared with non-PGR-treated areas, during the second growing season, but only 10% less during the first (Table 3). Results therefore suggested continuous paclobutrazol use at 0.56 kg a.i. ha-1 had a negative effect on RB, which was manifested following the most stressful period of the growing season.


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  		Table 3. Growing season x plant growth regulator (PGR) interaction means for creeping bentgrass putting green root biomass averaged from September to January over the three mowing heights.{dagger}

 
A mowing height by PGR interaction was found to be specific to the September to January period. Paclobutrazol applied at 0.56 kg a.i. ha-1 reduced RB 25% in turf mowed to a height of 4.8 mm, when compared with non-PGR-treated turf, but did not similarly affect RB in turf mowed at either 3.2 mm or 4.0 mm (Table 4). Differences in RB between the 0.56 kg a.i. ha-1 and 0.14 kg a.i. ha-1 paclobutrazol treatments were detected in turf mowed to a height of 4.0 mm. No PGR treatment effects on RB were evident when turf was mowed to a height of 3.2 mm.


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  		Table 4. Mowing height by plant growth regulator (PGR) interaction means for creeping bentgrass putting green root biomass, averaged from September to January and over the 1997–1998 and 1998–1999 growing season.{dagger}

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Because March and April are usually excellent months for cool-season turfgrass growth (Beard, 1973), higher average maximum air temperatures for these months in 1997 could have stimulated RB. Soil temperatures were conducive for cool-season turfgrass root growth ({approx}15 °C or higher) in January and February 1997. Similar conditions in 1998 did not occur until April (Table 1), accounting for differences in root growth between growing seasons.

The cultural stress imposed by continuous mowing and PGR treatments also may have contributed to overall decline in RB from one growing season to the next. If this were true, RB would have been expected to continuously decline through to the end of the experiment. Since RB was comparable in April 1998 and 1999, and because air and soil temperatures were comparable across years from November to March, these cultural effects seemed to be less causal to variable RB than temperature differences.

Penncross creeping bentgrass had lower RB when mowed to a height of 3.2 mm than when mowed at either 4.0 or 4.8 mm heights, suggesting that Penncross should not be maintained below 4.0 mm, particularly during the summer (Table 2). Observed summer decline in shoot density of Penncross mowed to a height of 3.2 mm (Fagerness et al., 2000) may explain why autumn RB recovery started slower and ended sooner at the low mowing height (Table 2). Since RB was equal between bentgrass mowed to a height of 4.0 and 4.8 mm (Table 2), and because measured putting green speed is increased when lowering mowing height from 4.8 to 4.0 mm (Fagerness et al., 2000), there was no apparent benefit of mowing bentgrass to a 4.8 mm height. Considering Penncross quality when mowed to a 4.0 mm height is greater than at a 3.2 mm mowing height, the 4.0 mm mowing height may be ideal for this cultivar in areas with severe summer stress (Fagerness et al., 2000; Huang et al., 2000). This is contrary to the suggestion that lower mowing heights may enhance summer survivability (Engelke, 1998).

Plant growth regulators may reduce turfgrass ET rates (Borden and Campbell, 1987; Devitt and Morris, 1988; Doyle and Shearman, 1985; Fry and Jiang, 1998). It is unclear if and how altered ET in creeping bentgrass relates to changes in stress tolerance or root growth. Flurprimidol {{alpha}-(1-methylethyl)-{alpha}-[4-(trifluoro-methoxy)phenyl]-5-pyrimidine-methanol} was shown to reduce creeping bentgrass root length and water use (Lee and Diesburg, 1993). Autumn and spring applications of paclobutrazol enhanced fairway height creeping bentgrass root growth, but did not affect drought resistance (Koski, 1997). Trinexapac-ethyl was shown to increase both root growth and root carbohydrate levels (Han and Fermanian, 1995). However, trinexapac-ethyl did not affect RB in this experiment and was reported previously to have no effect on Penncross quality, shoot density, or ball roll (Fagerness et al., 2000).

This experiment showed that following the most stressful time of the year, recovery of creeping bentgrass RB was unaffected by either trinexapac-ethyl or paclobutrazol applications at or below 0.28 kg a.i. ha-1 (Table 4). Only paclobutrazol applied at a twice labeled rate (0.56 kg a.i. ha-1) reduced RB at any of the mowing heights evaluated. The continuous use of paclobutrazol at this rate also may reduce putting green ball roll and compromise turfgrass quality during summer months (Fagerness et al., 2000), suggesting that the high rate of paclobutrazol in creeping bentgrass should not be considered. Summer decline of creeping bentgrass RB will continue to be a concern where severe summer conditions exist. However, results from this experiment showed conservative mowing height selection for Penncross, and the labeled use of PGRs neither exacerbated summer RB decline nor inhibited autumn RB recovery.


   ACKNOWLEDGMENTS
 
The authors would like to thank Mr. Tom Garrett for on-site assistance with this experiment and both Dr. Cavell Brownie, Department of Statistics, North Carolina State University, and Dr. Bingru Huang, Division of Horticulture, Kansas State University, for their assistance with statistical analyses.

Received for publication January 18, 2001.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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