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Published online 8 January 2009
Published in Agron J 101:67-74 (2009)
DOI: 10.2134/agronj2008.0180
© 2009 American Society of Agronomy
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Right arrow Turfgrass

Effectiveness of Cultural Thatch-Mat Controls for Young and Mature Kikuyu Turfgrass

L. Barton*, G. G. Y. Wan, R. P. Buck and T. D. Colmer

School of Plant Biology, Faculty of Natural and Agricultural Sciences, The Univ. of Western Australia, 35 Stirling Hwy., Crawley 6009, Western Australia, Australia

* Corresponding author (lbarton{at}cyllene.uwa.edu.au).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Excessive thatch and mat can be detrimental to turfgrass health and management. Mechanical and topdressing techniques to reduce accumulation of thatch and mat were evaluated in a 24-mo field study of kikuyu [Pennisetum clandestinum (Hochst. ex Chiov.)] turfgrass of two contrasting organic matter (OM) contents in the surface 50 mm of soil. Treatments included two kikuyugrass ages (established from 20 wk or 20-yr-old kikuyugrass) and five renovation techniques (none, verticutting, coring, topdressing with sand, coring + topdressing). The renovation techniques varied in effectiveness depending on the initial OM content of the soil immediately underlying the kikuyugrass. Annual verticutting, or twice annual topdressing with or without annual coring a young kikuyugrass were most successful at restricting the accumulation of soil OM (P < 0.05), with OM content <3.5% by 24 mo. Twice annual topdressing with or without annual coring, most rapidly decreased soil OM in the mature kikuyugrass (P < 0.05), with OM content averaging 6.2% by 24 mo. Combining coring with topdressing did not necessarily further decrease OM contents. Topdressing was up to three times more effective at reducing soil OM content than coring alone (P < 0.05). The color and N concentration of both kikuyugrass ages was maintained to local standards by all mechanical and topdressing techniques, although verticutting decreased the incidence of mower scalping in the second year. Verticutting was the most effective approach for restricting the progressive softening of young kikuyugrass with time (P < 0.05), whereas the mature kikuyugrass softened by the same amount irrespective of the renovation treatment.

Abbreviations: LSD, least significant difference • OM, organic matter • +, plus

Received for publication May 21, 2008.
   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
RESTRICTING THE ACCUMULATION of thatch and mat in turfgrass surfaces is critical for maintaining sports and recreational grounds. Thatch is a concentrated layer of dead, decomposing, and living plant tissue that develops between the soil surface and the green turfgrass vegetation (Beard, 1973). Mat is a concentrated layer of decomposing OM intermingled with mineral soil, and develops within the rooting zone forming part of the soil profile. In sandy soils, mat forms a distinct layer above the mineral soil. Thatch and mat accumulate when vegetative matter production exceeds its rate of decomposition (Beard, 1973). Therefore, factors that influence turfgrass growth rates or OM decomposition influence thatch and/or mat accumulation (Ledeboer and Skogley, 1967; Meinhold et al., 1973; Shearman et al., 1980; Waddington, 1992).

Thatch and mat accumulation in turfgrass are considered undesirable; however, a small amount is considered to reduce surface hardness, buffer moderate soil temperature extremes, and improve the resilience and wear tolerance of the turfgrass surface (Beard, 1973). Excessive thatch and mat can cause several problems including shallow rooting, impaired soil hydraulic properties, localized dry spot, mower scalping, disease, and pests (Waddington, 1992). Excessive mat in sports playing fields is particularly problematic as low infiltration rates cause soil water contents to be elevated for extended periods following rain, increasing the incidence of traffic damage while the surface is still wet. Maintaining low amounts of thatch and mat in turfgrass is critical for ensuring playability and safety of sports playing fields.

Mechanical and topdressing practices are commonly used for removing or diluting the amounts of thatch and mat in turfgrass. Mechanical techniques include core cultivation (aeration) with hollow or solid tines, vertical mowing, and verticutting. Much of the research investigating the effectiveness of turfgrass renovation techniques has been conducted on newly established turfgrass surfaces managed as golf greens in the United States, without excessive thatch or mat contents (Callahan et al., 1998; Eggens, 1980; McCarty et al., 2007; McCarty et al., 2005; Smith, 1979; White and Dickens, 1984), with a limited number of studies examining less intensively managed turfgrass surfaces such as home lawns (Carrow et al., 1987; Dunn et al., 1981, 1995; Murray and Juska, 1977). Comparisons between studies are confounded due to differences with cultivation techniques, and varying approaches for measuring thatch and/or mat accumulation. It is apparent, however, that coring, vertical mowing, and topdressing have all had success at restricting the accumulation of thatch and mat across a variety of turfgrass types and surfaces. It remains unclear, however, what techniques are most appropriate for decreasing preexisting thatch or mat in mature turfgrasses.

Local government municipalities are often responsible for managing large areas of turfgrass (i.e., broadacre parks and multipurpose sports fields), and without the resources available to high profile sports grounds and golf courses for regular turfgrass renovation. Thus in broadacre turfgrass management, excessive thatch and mat can accumulate before renovation. The aim of the following 2-yr study was to investigate the effectiveness of mechanical and topdressing techniques to reduce the accumulation of thatch and mat in kikuyugrass of two contrasting OM contents in the surface soil.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Soil and Site
Kikuyugrass plots (10 m2) were established at The University of Western Australia's Turf Research Facility located at Shenton Park (31°56' S, 115°47' E), approximately 8 km west of Perth city. Perth has a Mediterranean-type climate, and experiences hot, dry summers, and mild, wet winters. In the last 13 yr, Perth has had an annual rainfall of 746 mm (mainly in winter), a mean annual maximum temperature of 24.4°C, and a mean annual minimum temperature of 12.6°C (Commonwealth Bureau of Meteorology, http://www.bom.gov.au/climate/averages). The soil at the experimental site is known locally as Karrakatta Sand (McArthur and Bettenay, 1960) (Dystric Xeropsamments (USDA, 1992)) and was cleared of vegetation (Banskia woodland) in 1996. The soil is free-draining and has a low chemical fertility. The surface soil (0–150 mm) has a pH of 4.7 (1:5 soil: 0.01 M CaCl2 extract), electrical conductivity of 0.01 dS m–1 (1:5 soil/water extract), cation exchange capacity of 3.22 cmol (+) kg soil–1, C content of 6.5 mg g–1 and N content of 0.4 mg g–1. The subsurface soil ( >150–1000 mm) has a pH of 5.6, electrical conductivity of 0.003 dS m–1, cation exchange capacity of 1.33 cmol (+) kg–1 dry soil, C content of 0.9 mg g–1 and N content of 0.2 mg g–1. The surface soil contains 92% coarse sand, 2% fine sand, 2% silt, and 4% clay (Pathan et al., 2003).

The site included a variable-speed travelling irrigator with a fixed-boom (Short and Colmer, 2007) coupled with a weather station (WeatherMaster 2000, Environdata, Warwick, Queensland, Australia). The amount of water applied to the kikuyugrass was adjusted by varying the velocity of the irrigator. The median daily efficiency of discharge value ([actual irrigation depth/programmed irrigation depth] x 100) was 97% (data not shown).

Experimental Design and Approach
The experimental design was completely randomized, consisting of two kikuyugrass ages by five renovation treatments by three replicates. The two kikuyugrass ages were plots (10 m2) established from 20-wk-old (‘young’ kikuyugrass) or from 20-yr-old kikuyugrass (‘mature’ kikuyugrass). The young kikuyugrass was newly grown sod, cut to a depth of 15 mm; while the mature kikuyugrass was cut from a golf course fairway to a depth of 50 mm so as to include a mat layer of high OM content (36% by dry mass). The renovation treatments were none, verticutting, coring, topdressing with sand, or coring + topdressing (Table 1 ). Verticutting involved the removal of above and belowground biomass using a series of vertically mounted rotating blades that were set to a depth that penetrated 20 mm into the soil. Verticutting and coring treatments were swept of resultant material following renovation. The amount of sand applied to the coring + topdressing treatments in November (following coring) was 5 mm plus that needed to fill the core holes. Particle size analysis (McKenzie et al., 2002) of the topdressing sand showed it to contain 99.4% sand by mass, with the remainder made up of clay and silt. Treatments were applied annually each spring (Year 1, 18 Nov. 2005; Year 2, 17 Nov. 2006), with sand also applied to the topdressing, and coring + topdressing treatments, in late autumn (Year 1, 5 May 2006; Year 2, 4 May 2007).


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  		Table 1. Description of renovation treatments applied to kikuyugrass plots.

 
Plots were planted on 19 Jan. 2005, with renovation treatments first applied on 18 Nov 2005; allowing approximately 10 mo for the kikuyugrass to become established. All plots were irrigated at 60% replacement of net evaporation, and this replacement of cumulative net evaporation for 2 d was given every second day; which is sufficient to maintain newly planted kikuyugrass in southwestern Australia (Short, 2002). All plots received 200 kg ha–1 yr–1 of N from the application of ammonium sulfate that was divided evenly across four applications per year (two in spring, two in autumn). Plots were mostly mown weekly, except in winter (June–August) when turfgrass growth slowed and plots were mown every 2 wk; at a cutting height of 15 mm, with clippings returned to the plots. The kikuyugrass was found to be mildly deficient of Fe and Mn (data not presented), and received two foliar applications of Fe and Mn sulfate (25 kg ha–1 per application of Fe and Mn) each spring. In summer, a wetting agent was also applied to all plots to prevent the development of soil hydrophobicity. The frequency of renovation techniques, turfgrass fertility, and management regimes represented those commonly used by local government authorities in southwestern Australia.

Turfgrass Growth
Kikuyugrass growth was assessed using the dry mass of mowing clippings. The plots were mown, at a height of 15 mm, and the mass of the fresh clippings from each plot weighed. A subsample (20–25 g) of the fresh clippings from each plot was collected and weighed, dried (60°C for 1 wk) and reweighed to determine the water content (%). After collecting the subsample, the remaining fresh clippings were immediately redistributed across the surface of the respective plot. The dry mass of clippings from the plot was calculated from the fresh/dry mass ratio.

Turfgrass Quality
Kikuyugrass quality was assessed by measuring color, surface hardness, tissue N concentration, and the incidence of mower scalping. Kikuyugrass color was measured monthly following the application of renovation treatments using a Chroma Meter (Minolta, CR-310, Osaka, Japan), an instrument previously shown to enable quantitative assessments of turfgrass color (Barton et al., 2006; Landschoot and Mancino, 2000). The meter describes color in three coordinates: L*, lightness, from 0 (black) to 100 (white); a*, from –60 (green) to 60 (red); and b*, from –60 (blue) to 60 (yellow). As recommended by Landschoot and Mancino (2000), data used to assess turfgrass greenness were the hue angles for the CIELAB color space (Hamill and Camlin, 1984). At any level of lightness, the hue angle is calculated as arctangent (b*/a*); the greater the hue angle the greater the greenness. Measurements were taken in three positions along a transect in each plot by pressing a 50 mm diam. measuring cylinder firmly down onto the canopy surface to exclude external light. The Chroma Meter was calibrated after every 36 readings, using a calibration plate (CR-A44, Minolta, Osaka, Japan) and following the instructions provided by the manufacturer. Kikuyugrass hardness was measured every 3 mo, and following mowing, using a 2.25 kg Clegg impact hammer (Dr. Baden Clegg Pty Ltd, Perth, Australia). Measurements were taken in three positions per plot by allowing the hammer to drop from a height of 450 mm three times and recording the values (in units of gravities) following the third drop. Total N in the dried kikuyugrass clippings was measured every 3 mo by fine grinding a subsample using a ball grinder, and analyzing the tissue powder using a CHN analyzer (LECO CHN 1000, MI, USA). Plant tissue nutrient concentrations were validated against plant tissue standards analyzed using the same procedures. Furthermore, the incidence (presence or absence) of mower scalping was recorded for each plot following weekly mowing.

Organic Matter Content, Thatch Height, and Soil pH
The accumulation of thatch in each plot was measured every 3 mo by collecting a core (70 mm in diameter, 100 mm depth) and recording the height of the thatch plus shoots above the surface soil. Cores were collected the day following mowing. Every 6 mo, two cores (70 mm in diameter, 50 mm depth) were collected from the soil surface for OM determination. Live rhizomes were removed from the OM cores, and then the remaining sample dried (105°C), weighed, and ground before analyzing for OM. Organic matter content was determined as the difference between dry mass (105°C for 48 h) and ash mass (600°C for 24 h). These soils contained no free CaCO3. Soil pH (surface 0–50 mm) was measured annually. Soil samples were air-dried, sieved (<2 mm) and shaken in 0.01 M CaCl2 (1:5 soil/extract) for 16 h. Samples were centrifuged (10 min at a relative centrifugal force of 850 G) before measuring pH with a glass electrode.

Industry Benchmarking
Critical values for kikuyugrass color are not established for maintained turfgrass grown in southwestern Australia. Consequently, on six occasions during the study (21 Oct. 2005, 27 Jan. 2006, 21 Apr. 2006, 14 July 2006, 3 Nov. 2007, 25 Jan. 2007) we measured kikuyugrass color at six kikuyugrass sports fields (three measurements per site) managed by local government so as to benchmark findings from our experimental site with industry sites. Kikuyugrass color was measured using the instrument and technique described above.

Data Analyses
All data were statistically analyzed using Genstat (2007). A general analysis of variance was used to determine if renovation treatment affected kikuyugrass growth, quality, or accumulation of thatch and mat, for each kikuyugrass age. Furthermore, repeated measures ANOVA was used to determine whether parameters varied with time. Post-hoc pair-wise comparisons of means were made using least significant difference (LSD, significance level of 5%) calculated for each of the treatments and treatment interactions.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Turfgrass Growth
By the end of 24 mo, cumulative growth (clipping dry mass) ranged from 4265 kg ha–1 (young kikuyugrass, verticut) to 13,998 kg ha–1 (mature kikuyugrass, no renovation) (Fig. 1 ). The cumulative dry mass of the clippings by the end of 24 mo was dependent on kikuyugrass age (P < 0.001) and renovation treatment (P < 0.001) (Fig. 1). The mature kikuyugrass produced, on average, 62% more total clipping dry mass than the young kikuyugrass. Verticutting produced the least amount of clippings for both kikuyugrass ages (P < 0.05). Cumulative 4-weekly growth (clippings dry mass) was not consistent throughout the year (P < 0.001), and ranged from 40 to 1723 kg ha–1 4-wk–1 (topdressed, mature kikuyugrass in autumn) (Fig. 2 ). Growth tended to be greatest for all treatments in autumn (March–May), and in response to N fertilizer applications (Fig. 2). Furthermore, verticutting the young kikuyugrass lessened 4-weekly growth in comparison with the other treatments from January to May each year (Fig. 2).


Figure 1
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  		Fig. 1. Influence of renovation treatment on cumulative (i.e., total) dry matter production of (a) young and (b) mature kikuyugrass over 24 mo. Values are means (and standard errors) of three replicates. LSD for comparing values within the young kikuyugrass renovation treatments, 1353. LSD for comparing values within the mature kikuyugrass renovation treatments, 4620.

 

Figure 2
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  		Fig. 2. Dry matter production with time for clippings from the (a) young and (b) mature kikuyugrass receiving different renovation treatments. Values represent means (± standard errors) of three replicates. Timing of fertilizer applications indicated by filled circles at the top of (a) (added to both kikuyugrass ages). LSD for comparing young kikuyugrass treatments with time is 123 (or 113 if comparing same renovation treatment with time); LSD for comparing mature kikuyugrass treatments with time is 322 (or 280 if comparing same renovation treatment with time).

 
Turfgrass Quality
Color
The mature kikuyugrass was greener than the young kikuyugrass (P < 0.001) (Fig. 3 ). Color (hue angle) varied during the year (P < 0.001), with greenness often lowest in winter (Fig. 3). In addition, color often declined in autumn (March–May) following N fertilizer applications, as additional growth at these times of the year (Fig. 2 and 3) caused large sections of green shoots to be removed by mowing and the underlying (brown) thatch revealed (i.e., ‘scalped’). Scalping strongly affected kikuyugrass color in late summer and early autumn during the second year of the study (Fig. 3). Further details on the incidence of scalping are given below.


Figure 3
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  		Fig. 3. Influence of renovation treatment on the color of the (a) young and (b) mature kikuyugrass with time. Increasing hue angle indicates increasing "greenness". Measurements were taken with a Chroma Meter. Values represent means (± standard errors) of three replicates. Timing of fertilizer applications indicated by filled circles at the top of (a) (added to both kikuyugrass ages). LSD for comparing young kikuyugrass treatments with time, 5.6 (or 5.5 if comparing same renovation treatment with time); LSD for comparing mature kikuyugrass treatments with time, 5.9 (or 5.2 if comparing same renovation treatment with time).

 
Kikuyugrass color differed among renovation treatments (P < 0.05), but the effect of renovation treatment depended on kikuyugrass age (P < 0.05). For the young kikuyugrass averaged across time, the verticut treatment was less green than the other treatments (P < 0.05), with color tending to take up to 3 mo to recover following renovation. Furthermore, the verticut young turfgrass was less green than the other treatments in the winter of 2007 (Fig. 3). For the mature kikuyugrass averaged across time, no renovation and coring had lower color than coring + topdressing and the topdressing treatments; while the average color of the verticut mature kikuyugrass did not differ from any of the other treatments (P < 0.05). Coring + topdressing and topdressing the mature kikuyugrass tended to increase color relative to no renovation, throughout the year. Kikuyugrass color (i.e., hue angle) at the six industry benchmarking sites ranged from 97° to 120°, and averaged 111°. If we nominate a minimum critical value of 97°, then all the treatments met the industry requirements throughout the study (Fig. 3).

Surface Hardness
The young kikuyugrass was harder than the mature kikuyugrass during the study (P < 0.05). Surface hardness ranged from 58 to 93 gravities for the young kikuyugrass (Table 2 ), and 31 to 50 gravities for the mature kikuyugrass (Table 3 ). For the young kikuyugrass, surface hardness progressively softened with time (P < 0.05); however, by the end of 24 mo the control and verticut treatments had a greater surface hardness than the other treatments (Table 2). By the end of the study, the surface hardness of the verticut young kikuyugrass was considered to be within the ‘ideal’ range (70–89 gravities) for Australian football grounds, while the remaining treatments were considered acceptable, but too "soft" to be considered ideal (Chivers and Aldous, 2003). For the mature kikuyugrass, all treatments progressively softened with time, and at the end of the study there were no differences in surface hardness of the mature kikuyugrass under the various treatments (Table 3, P < 0.05). The surface hardness of the mature kikuyugrass was acceptable, but not in the ideal range as it was too soft.


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  		Table 2. Effect of renovation treatment on surface hardness of young kikuyugrass with time, measured using a 2.25-kg Clegg Hammer.

 

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  		Table 3. Effect of renovation treatment on surface hardness of mature kikuyugrass with time, measured using a 2.25-kg Clegg Hammer.

 
Tissue Nitrogen Concentrations
Nitrogen concentration in clippings ranged from 18 to 31 g kg–1 in the mature kikuyugrass, and 20 to 39 g kg–1 in the young kikuyugrass (data not shown). The mature kikuyugrass tended to have a greater N concentration (29 g kg–1 over time and renovation treatments) than the young kikuyugrass (23 g kg–1) (P < 0.001). The N concentration of the mature kikuyugrass was not affected by renovation treatment (P < 0.05). However, the N concentration in the verticut young kikuyugrass (21 g kg–1, averaged over time) tended to be lower than the topdressed young kikuyugrass (24 g kg–1). Leaf N concentration should be ≥20 g kg–1 to maintain kikuyugrass (Johnston, 1996). Given this critical value, then apart from May 2006, treatments were mainly above the critical value.

Mower Scalping
In autumn of each year (March–May), increased growth (Fig. 2) and mower scalping was observed (Table 4 ). Autumn scalping was more frequently observed in the mature kikuyugrass than the young kikuyugrass (P < 0.05), and was observed for all renovation treatments except verticutting (Table 4). The observance of mower scalping only varied with renovation treatment in 2007 (P < 0.05). For the young kikuyugrass in 2007, only verticutting decreased the incidence of scalping during autumn in comparison with the untreated kikuyugrass, and from an average of three incidences (no renovation) to no incidences (verticut); whereas for the mature kikuyugrass, all renovation treatments decreased autumn mower scalping from eight to at most three incidences (Table 4).


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  		Table 4. Effect of renovation treatment on the incidence{dagger} of autumn mower scalping in young and mature kikuyugrass.

 
Organic Matter and Thatch Accumulation
Organic Matter Content
Organic matter content in the surface (0–50 mm) soil was determined, as this indicates levels of mat in turfgrass (Carrow et al., 1987). During the study OM (% by oven-dried mass) content ranged from 2.5 to 4.8% in the young kikuyugrass, and 5.6 to 32% in the mature kikuyugrass (Fig. 4 ). The mature kikuyugrass tended to have greater OM content than the young kikuyugrass; however, by 18 mo, coring and topdressing the mature kikuyugrass reduced OM to values similar to those measured in the young kikuyugrass (P < 0.05). By 24 mo, topdressing and coring + topdressing the mature kikuyugrass produced similar OM contents to that measured on the untreated young kikuyugrass.


Figure 4
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  		Fig. 4. Influence of renovation treatment on organic matter content of the surface 50 mm of soil under the (a) young and (b) mature kikuyugrass with time. Values represent means (± standard errors) of three replicates. Renovation treatments applied at 0 (all), 6 mo (topdressed treatments only), 12 mo (all), and 18 mo (topdressed treatments only). Note differences in y axis between (a) and (b). LSD for comparing young kikuyugrass treatments with time is 0.80 (or 0.77 if comparing same renovation treatment with time); LSD for comparing mature kikuyugrass treatments with time is 5.61 (or 5.57 if comparing same renovation treatment with time).

 
For the young kikuyugrass, OM contents did not vary with time for the verticutting, topdressing and coring + topdressing treatments (P < 0.05); however by the end of 24 mo, only the verticut and topdressed treatments had lower OM contents than young kikuyugrass not renovated (P < 0.05) (Fig. 4). Furthermore, there was no difference between the cored + topdressed and the topdressed treatments (P < 0.05). The OM content in the untreated and cored young kikuyugrass treatments increased with time (P < 0.05), so that by the end of 24 mo the cored treatment contained more OM than the cored + topdressed, topdressed, or verticut treatments (P < 0.05). Despite these differences for treatments applied to the young kikuyugrass, the OM contents of the young kikuyugrass were below a critical value of 3.5% proposed by McCoy (1992), with the exception of the coring treatment (Fig. 4).

For the mature kikuyugrass, topdressing and coring + topdressing decreased OM content with time, and by similar amounts (P < 0.05); whereas the remaining treatments did not vary with time (P < 0.05) (Fig. 4). By the end of 24 mo, the cored + topdressed and topdressed treatments contained less OM than the other treatments applied to the mature kikuyugrass (P < 0.05), and the OM contents were approaching the critical value of 3.5%. Coring and verticutting the mature kikuyugrass also decreased the OM content with time in comparison with the control (P < 0.05), however contents were still unacceptable (~20% OM) (Fig. 4).

Thatch Height
Thatch + turfgrass leaf heights (the day after mowing) differed on average <2 mm between the kikuyugrass ages, despite being statistically different (P < 0.05) (data not shown). For the young kikuyugrass, heights ranged from 16 to 37 mm and averaged 23 mm across time and renovation treatments; while for the mature kikuyugrass, heights ranged from 17 to 37 mm and averaged 25 mm. Averaged across turfgrass age and time, thatch + leaf heights decreased in the order: control (27 mm) = cored (27 mm) > verticut (24 mm) > topdressed (22 mm) = cored + topdressed (21 mm) (P < 0.05). Renovation treatment effects, however, were not consistent for each measurement period. Differences in thatch + leaf height between renovation treatments were only evident in August 2006, November 2006, February 2006, and August 2007. Furthermore, the height of the thatch + leaf did not change consistently. Instead, thatch + leaf height appeared to increase for the first 12 mo in both kikuyugrass ages, before declining. Consequently by the end of 24 mo, there was neither an effect of renovation treatment on thatch + leaf height, nor a change in height between the first (November 2005) and last (November 2007) measurements.

Soil pH
Soil pH (0–50 mm) was unaffected by renovation, but tended to be less acidic in the young kikuyugrass than the mature kikuyugrass (P < 0.001) (data not shown). Soil pH decreased from 5.9 to 4.0 after 24 mo in the surface (0–50 mm) soil of the young kikuyugrass (P < 0.05), but remained unchanged in the mature kikuyugrass (averaged 3.9).


   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Recommended turfgrass renovation techniques will vary depending on the initial OM content of the soil immediately underlying the turfgrass. In the case of young turfgrass stands, where the initial OM content is less than a critical value, renovation techniques that restrict the accumulation of OM with time, without compromising turfgrass quality, will be favored. By contrast where OM contents are above a critical value, renovation techniques that quickly decrease the soil OM content will be preferred. In our study, verticutting or topdressing a young kikuyugrass with low initial soil OM content were most successful at restricting the accumulation of OM after 24 mo. Whereas for the mature kikuyugrass, with a high initial OM content, topdressing or coring + topdressing were the most successful approaches for rapidly decreasing soil OM content while maintaining kikuyugrass quality. Other studies investigating turfgrass renovation have mainly been interested in techniques that restrict, rather than decrease, further accumulation of OM in newly established ‘Tifway’ bermudagrass [Cynodon dactylon (L.) Pers x C. transvaalensis (Burtt-Davis)] (Carrow et al., 1987) and ‘L-93’ creeping bentgrass [Agrostis stolonifera L. var. palustris (Huds.) Farw.] (McCarty et al., 2007). To our knowledge this is the first study to compare the ability of turfgrass renovation techniques to restrict the accumulation of soil OM in a newly established turfgrass, as well as decrease the OM content in a mature turfgrass.

Topdressing turfgrass twice a year with sand was a successful approach for restricting the accumulation of soil OM in newly planted turfgrass, and for decreasing the OM content of the mature kikuyugrass (i.e., topdressing alone, or coring + topdressing). After 24 mo, with topdressing two times per year, the OM content of the young kikuyugrass was unchanged; while the mature kikuyugrass plots contained 76% less OM in the surface soil than the untreated mature kikuyugrass. Others have also reported topdressing decreased the accumulation of thatch and mat in Tifway bermudagrass (Carrow et al., 1987) and ‘Penncross’ creeping bentgrass [Agronstis palustris Huds.] (Eggens, 1980). For example, Carrow et al. (1987) found topdressing was up to three times more effective than coring or vertical mowing at restricting the accumulation of OM in Tifway bermudagrass maintained under home-lawn conditions. By contrast, McCarty et al. (2005) concluded that topdressing failed to prevent thatch-mat accumulation in newly seeded Penncross creeping bentgrass after 2 yr; although treatment differences would have been difficult to detect given that OM contents were low throughout the study (1.38% for untreated vs. 1.30% topdressed, after 2 yr). The success of topdressing to reduce the accumulation of thatch-mat in turfgrass has been attributed to an altered microenvironment favoring decomposing microorganisms, but is principally thought to decrease the OM content via "dilution" (McCarty et al., 2005).

Combining coring with topdressing did not necessarily further decrease OM contents. Topdressing was up to three times more effective at reducing soil OM content than coring alone; however, coring plus topdressing plots did not further decrease OM contents. The benefit of combining coring and topdressing turfgrass on soil OM has rarely been investigated. Notably, Eggens (1980) reported monthly topdressing decreased thatch-mat height in Penncross creeping bentgrass relative to untreated turfgrass after 3 yr, however also coring topdressed plots did not further decrease thatch-mat height. Instead, studies have tended to simply compare the relative effectiveness of topdressing or coring alone. As was the case in our study, topdressing was found to be more effective, than coring alone, to decrease thatch-mat (Carrow et al., 1987; Eggens, 1980). By contrast McCarty et al. (2007) concluded that coring and topdressing were equally effective in an established creeping bentgrass golf green after 2 yr. It is worth noting that topdressing turfgrass following coring is often recommended so as to level the surface for mowing (Beard, 1973), rather than to further decrease OM content.

Surface soil pH (0–50 mm) was measured annually in the present study as it has been suggested that decreases in soil pH resulting from N fertilizer applications may lead to decreased microbial decomposition rates and the accumulation of thatch and mat (Waddington, 1992). In the present study, soil pH did not vary between renovation treatments, and so the relative performance of the renovation treatments was not attributed to variations in soil pH. However, the mildly acidic soil conditions may have facilitated the accumulation of OM in all plots during the study. Further research is required to isolate the acidifying effect of some N fertilizers on the accumulation of mat and thatch in turfgrass, and at different N fertilizer application rates.

Color and growth of both kikuyugrass ages were maintained or improved by topdressing or coring + topdressing. By contrast, verticutting the young kikuyugrass resulted in poorer growth and color. Carrow et al. (1987) also reported topdressing improved the color of established Tifway bermudagrass, which was attributed to higher surface soil temperatures. Others, however, have reported coring to negatively affect turfgrass quality by decreasing shoot density and increasing the incidence of scalping (Carrow et al., 1987; McCarty et al., 2005, 2007), although this has been overcome by returning sand from cores to the turfgrass, topdressing, and by coring less frequently (Carrow et al., 1987; Dunn et al., 1981, 1995; Murray and Juska, 1977). Interestingly, verticutting decreased the incidence of autumn mower scalping in the young kikuyugrass, while all renovation treatments decreased the incidence of mower scalping in the mature kikuyugrass. Similarly, White and Dickens (1984) reported topdressing reduced the incidence of scalping in an experimental putting green planted to three types of bermudagrass (Cynodon dactylon (L.) Pers x C. transvaalensis Burtt-Davy, cultivars Tifdwarf, Tifgreen, and the selection, Dothan). Our study, plus those of others, demonstrates renovating turfgrasses need not decrease the overall growth and can subsequently enhance the quality of turfgrass surfaces.

Renovation techniques did not reduce the progressive softening of the turfgrass surface with time. Surface hardness can affect ball bounce and the incidence of player injury (Chivers and Aldous, 2003; Gibbs et al., 2000), however until recently only a few studies have quantified the effect of renovation on turfgrass hardness (Gibbs et al., 2000; McCarty et al., 2005, 2007). In the present study, verticutting was the most effective approach for restricting the softening of a young kikuyugrass with time, whereas the mature kikuyugrass softened progressively with time irrespective of the renovation treatment. McCarty et al. (2007) reported coring softened a 3-yr-old Penncross creeping bentgrass green; whereas topdressing alone, vertical mowing, and grooming had similar surface hardness to untreated kikuyugrass. By contrast, Gibbs et al. (2000) reported verticutting a recently established bentgrass-fescue (Agrostis capillaris L. cultivar Egmont, A. capillaris/A. castellana Bois. & Reut., Festuca nigrescens Lam. ssp. commutate cultivar Enjoy) golf green resulted in a harder surface than verti-draining or coring after 2 yr. As desired surface hardness will vary depending on the turfgrass use and method of measurement, further research is required to assess critical surface hardness values for turfgrass surfaces, and the effectiveness of various turfgrass renovation techniques to achieve and maintain these values.

In conclusion, verticutting (annually) or topdressing (twice annually) kikuyugrass containing <3.5% OM both appear to be suitable approaches for preventing excessive accumulation of OM with time. For kikuyugrass containing a high OM content, topdressing was the best renovation technique in the present study for decreasing the OM content within 24 mo, and without compromising kikuyugrass quality.


   ACKNOWLEDGMENTS
 
Greenacres Turf Farm is thanked for help in the design and maintenance of the irrigator. Murdoch Challenger TAFE, City of Stirling, City of Canning, City of Perth, Lovegroves and the WA Golf Course Superintendents Association for providing staff and students to assist with planting and mowing. Lawn Doctor is thanked for applying renovation treatments to turfgrass plots. Members of the UWA Turf Industries Research Steering Committee is thanked for their support and advice. This project has been facilitated by Horticulture Australia Ltd in partnership with the Australian turf industry. It was funded by voluntary contributions from the Parks and Leisure Association of Australia (representing a consortium of local and state government authorities), CSBP Ltd, Organic 2000, Turf Grass Association of Australia (WA), WA Golf Course Superintendents Association, Baileys Fertilisers, Turf Master Facility Management, Turf Growers Association of Western Australia, Lawn Doctor, Micro Control Engineering, and the Water Corporation.


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 ABSTRACT
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 RESULTS
 DISCUSSION
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