Effects of root zone composition and irrigation regime on performance of velvet bentgrass putting greens. II. Thatch, root development and playability

Abstract

The use of velvet bentgrass (Agrostis canina L.) on putting greens is limited by sparse knowledge on optimal maintenance. The objectives of this study were to determine the effects of rootzone composition (SS, straight sand, or GM, sand amended with 20% v/v garden compost) and irrigation regime (LF, light and frequent, or DI, deep and infrequent) on thatch formation, root development and playability. A study was conducted from August 2007 to October 2009 on a USGA green seeded in June 2007 with velvet bentgrass ‘Legendary’ at a coastal location in Norway (Landvik, 58° N). Compared with light and frequent irrigation, deep and infrequent irrigation improved root development in the 10- to 20-cm soil layer in the second year after sowing. A decrease in turf visual quality on SS receiving light and frequent irrigation in the second year was associated with low infiltration rate. Neither thickness nor content of organic matter in the mat layer were significantly affected by the treatments. A 20% higher surface hardness on SS vs. GM persisted only during the first year of the study.

Keywords:

• golf greens
• turfgrass
• turfgrass management

Abbreviations

DI	=	deep and infrequent
GM	=	organic amendment ‘Green Mix’
LF	=	light and frequent
SS	=	straight sand.

Introduction

A special concern in maintenance of velvet bentgrass putting greens is thatch control (Espevig et al. 2012). Thatch is defined as “an intermingled organic layer of dead and living shoots, stems, and roots of grasses that develops between the turf canopy of green vegetation and the soil surface,” while mat is used for the layer which is formed when thatch is intermixed with sand in the case of topdressing (Beard 2002). Among the problems caused by excessive thatch on putting greens are reduced water infiltration and increased risk for disease injury, scalping, dry spots, and poor playing quality (Jordan 2008). We previously showed that the combination of a low N input and heavy topdressing resulted in acceptable visual turf quality and less than 5% organic matter in the mat of a 2-yr-old velvet bentgrass green (Espevig 2011, Espevig et al. 2012). However, effects of rootzone composition and irrigation management in relation to thatch control on velvet bentgrass putting greens have not been investigated before.

The most widely used guidelines for putting green construction developed by United States Golf Association (USGA), does not require organic amendments to the sand-based rootzone (USGA Green Section Staff 2004). Some authors therefore advocate the use of straight sand (SS) rootzones as they believe organic matter will be accumulated over time. Research, however, shows that this happens essentially at the 5-cm upper layer (Liu 2004, Murphy 2007). The low water-holding capacity and poor nutrient retention of SS rootzones may lead to high leaching losses, especially on young greens (Brauen and Stahnke 1995, Aamlid 2005). Compost or other organic amendments contribute to less drainage and improved quality of sand-based rootzones (Petrovic 1995, Engelsjord and Singh 1997, Bigelow et al. 2000, Murphy et al. 2004, Aamlid 2005, Wu et al 2007). Furthermore, compost used as an amendment increases microbial numbers in sand-based rootzones (Aamlid et al. 2009). However, the studies regarding the specific effect of compost on thatch formation are limited (Liu 2004).

Irrigation amount and frequency affect soil moisture and root development. Deeper rooting as a result of infrequent irrigation was shown to enhance turfgrass survival during dry periods (Qian and Fry 1996, Jordan et al. 2003, Fu and Dernoeden 2009b). Root distribution in different soil depths in response to soil moisture has been inconsistent (Huang et al. 1997, Jordan et al. 2003, Fu and Dernoeden 2009b).

The objective of this study was to determine the effects of rootzone composition and irrigation regime on turfgrass playability, thatch formation, and root development on velvet bentgrass putting green. Our hypothesis was that velvet bentgrass greens can be maintained with less thatch formation under deep and infrequent irrigation and if compost is included in the rootzone.

Materials and methods

The materials and methods used in this study were the same as described earlier (Espevig and Aamlid 2012). Additional methods used were as follows.

Thatch/mat assessments

Thatch/mat depth was determined from four cores taken from each plot in September 2008 and 2009. Two of the four samples were also used for determination of organic matter content. The foliage above the mat layer and roots below it were removed. The organic matter content was calculated individually for each core as: Organic matter (%)=(sample ignition loss/sample dry weight)×100%. Sample ignition loss was determined as the difference between sample dry weight (105 °C for 48 h) and sample ash weight (550 °C for 3 h). The data were averaged for each plot prior to statistical analyses.

Soil moisture was measured on all plots (three measurements per plot) prior to irrigation of at least one of four treatments using a portable time-domain reflectometer (TDR) “HydroSence™” (Campbell Scientific, Ltd., Thuringowa, Australia) with 20-cm long probes except in September 2009 due to equipment failure.

Root assessments

On 13 June (by the end of the 6-week drought period) and 8 Sep. 2008 and 10 Oct. 2009, a soil core was taken from each plot using a root auger, 30 cm long and 5 cm in diameter. The cores were separated into the following layers: 0–6 cm, 6–10 cm, and 10–20 cm. The depths were measured from the green surface, and the mat layer removed from the upper samples. Roots from each layer were washed free from soil and dried for 48 h at 105 °C. Root densities were calculated by dividing weight of dry roots by the volume of the sample taken at a certain rootzone depth.

Water infiltration rates were measured on 12 June 2008 and 12 Oct. 2009 at two sites per plot using a double-ring infiltrometer with an outer ring diameter of 12.8 cm and an inner ring diameter of 4.5 cm. The infiltrometer was inserted 2 cm into the turf after the rootzone had been saturated. Water levels in the inner ring were measured after 3 minutes of infiltration. The infiltration rate was expressed as mm per hour.

Playing quality was recorded monthly and assessed as surface hardness and ball roll distance. Surface hardness was measured from 23 Apr. until 16 Sept. 2008 and from 30 Apr. until 16 Oct. 2009 using a Clegg Soil Impact Tester (Lafayette Instrument Co., Lafayette, IN). Readings were taken after each of two successive blows by the 2.25-kg hammer from 0.46-m height at two places per plot. Ball roll distance was determined from 22 July until 15 Oct. 2008 and from 29 Apr. until 16 Oct. 2009 using a stimpmeter modified for research plots (Gaussoin et al. 1995). Measurements were always conducted the day after mowing. The stimpmeter had its ball release notch 38 cm rather than 76 cm from the bevelled end, and measurements were always taken in two directions. Data from each experimental year were pooled into three consecutive periods with 2–5 registrations each: spring (March–May), summer (June–August), and fall (September–October).

The data were analyzed by the SAS procedure PROC ANOVA using statements providing one-way analysis for a block design (SAS Institute 2008). The Fisher's protected least significant difference (LSD) at the 5% probability level was used to identify significant differences among treatments.

Results

Thatch/mat assessments

Neither thickness nor content of organic matter in the mat layer were significantly affected by treatments in any year (Table I). On average for all treatments mat thickness doubled from 10 mm in the fall of 2008 (16-month-old green) to 20 mm in the fall of 2009 (28-month-old green), while the average content of organic matter in the mat layer dropped from 9.8% to 8.3%.

Table I. The effect of root zone composition and irrigation regime on mat characteristics of velvet bentgrass green.

	September 2008		September 2009
Treatment	Mat depth	Organic matter in mat	Mat depth	Organic matter in mat
	cm	% (w/w)	cm	% (w/w)
SS + LF	1.0	10.3	2.0	8.0
SS + DI	0.9	9.1	1.9	7.9
GM + LF	1.0	10.1	2.2	8.8
GM + DI	1.0	9.7	2.0	8.4
p	0.812	0.735	0.631	0.610
SS, straight sand; GM, ‘Green Mix’; LF, light and frequent irrigation; DI, deep and infrequent irrigation.

Soil moisture

Volumetric soil water content measured by TDR was generally lower on DI-irrigated plots than on LF-irrigated plots (Figure 1), but differences were significant only for the GM rootzone. On average for measurements just before irrigation events in 2008 and 2009, soil moisture in the 0–20 cm layer amounted to 8.3% under LF irrigation vs. 6.7% under DI irrigation in the SS rootzone, and 14.5% under LF irrigation vs. 9.9% under DI irrigation in the GM rootzone (data not shown).

Figure 1.  Daily soil water deficit as calculated from the difference between rainfall and pan evaporation in 2008 and from 1 May to 20 July 2009, and from pan evaporation after 20 July 2009 (rain-out shelter installed). The plots were irrigated to field capacity each time the calculated soil water deficit exceeded 5 mm, 10 mm, and 20 mm in the treatments SS + LF, SS + DI & SS + LF and GM + DI, respectively. Volumetric soil moisture content in the 0–20 cm rootzone shown with circles and was measured with a time-domain reflectometer (TDR) prior to irrigation of at least one of the four treatments. SS, straight sand; GM, ‘Green Mix’; LF, light and frequent irrigation; DI, deep and infrequent irrigation.

Root density

The average root dry weight per unit area was 1.4, 1.6, and 2.0 times higher on SS than on GM plots in June 2008, September 2008, and October 2009, respectively (Table II). The effect of irrigation frequency on the total root dry weight was inconsistent within each rootzone.

Table II. Effect of root zone composition and irrigation regime on total weight of dry roots per one square meter.

Treatment	June 2008	September 2008	October 2009
	g m^−2
SS + LF	376 a	295	497 a
SS + DI	329 ab	278	515 a
GM + LF	229 c	235	274 b
GM + DI	263 bc	129	238 b
p	0.036	0.072	<0.001
SS, straight sand; GM, ‘Green Mix’; LF, light and frequent irrigation; DI, deep and infrequent irrigation.

Root densities at different depths are shown in Figure 2. The highest root density was always recorded in the upper 4.0–5.5 cm under the mat layer, but differences in root density between rootzones were not consistent in 2007, 2008, and 2009. The decrease in root density from upper to lower layers occurred more rapidly on GM plots compared with SS plots. Significant effects of irrigation regime only appeared in October 2009. On SS plots, DI irrigation almost doubled root density in the 10–20 cm layer, but reduced root density by 30% in the 6–10 cm layer compared with LF. On GM plots, DI irrigation reduced root density by about 50% in the 2-cm topsoil layer (just under the mat).

Figure 2.  Root density at different soil depths on velvet bentgrass green with root zone compositions straight sand (SS) or ‘Green Mix’ (GM) and irrigation regime light and frequent (LF) or deep and infrequent (DI), measured by the end of dry period in 2008 and the end of both experimental years. Mean values followed by the same letter within the same depth and measured at the same time are not significantly different based on Fisher's protected least significant difference (LSD) test (α = 0.05).

Water infiltration rate

The lowest infiltration rate was in SS plots receiving LF irrigation. Deep and infrequent irrigation improved the infiltration rate by 26% and 449% on SS plots in June 2008 and in September 2009, respectively (Table III). Effects of irrigation on GM plots were not significant.

Table III. Effects of root zones and irrigation regimes on infiltration rates measured by double-ring infiltrometer.

Treatment	June 2008	September 2009
	mm hr^−1
SS + LF	82 b	63 b
SS + DI	103 a	343 a
GM + LF	104 a	267 ab
GM + DI	99 a	333 a
p	0.003	0.051
SS, straight sand; GM, ‘Green Mix’; LF, light and frequent irrigation; DI, deep and infrequent irrigation. Mean values followed by the same letter in the same column have no significant difference based on Fisher's protected least significant difference (LSD) test (α = 0.05).

Playing quality

Ball roll distance increased during summer and fall in 2008 and 2009, but was never significantly affected by treatments (Figure 3a). As indicated by the first blow with the Clegg hammer, playing surfaces were on average 20% harder on SS plots than on GM plots in 2008 (Figure 3b). This difference became even more evident in the second blow at the same position (Figure 3c). From April to October 2009, differences between the two rootzones were significant in the second blow only.

Figure 3.  Effects of root zone composition and irrigation regimes on playing quality of velvet bentgrass green in 2007, 2008, and 2009. Vertical bars are Fisher's protected least significant difference (LSD) values indicating significant differences between treatments at 5% probability level. SS, straight sand; GM, ‘Green Mix’; LF, light and frequent irrigation; DI, deep and infrequent irrigation.

Discussion

The content of organic matter in the mat varied from 9.1% to 10.3% already by the end of 2008, i.e., considerably higher than the optimal organic matter content in a sand medium which is considered not to exceed 4% (McCoy 1992, Murphy et al. 1993). This was not surprising as far as velvet bentgrass possesses high thatch accumulation (Rinehart et al. 2005, Aamlid et al. 2006). As the nitrogen input was reduced 27% on SS plots and 21% on GM plots, the doubling of mat thickness from 2008 to 2009 must have been due to an increase in topdressing rate from 4 mm to 9 mm. In accordance with earlier investigations (Murphy 1983, McCarty et al. 2005, 2007), this increase in topdressing also led to a 23% decrease in the content of organic matter in the mat layer (average for all treatments) from 2008 to 2009 (p=0.006). In contrast to our results showing no significant effect of treatments on thatch/mat characteristics, Fu and Dernoeden (2009a) found that DI irrigation reduced the content of organic matter in the mat on a creeping bentgrass green compared with LF irrigation by the end of their 2-yr study (24.7% under LF irrigation vs. 20.0% under DI irrigation).

By the end of our study the inferior turfgrass visual quality on SS plots that had received LF irrigation (Espevig and Aamlid 2012), could be due to the more or less continuous wetness of the mat layer on those plots. This, in turn, may have led to oxygen deficiency and N losses due to denitrification from these plots (Mancino et al. 1988, Engelsjord et al. 2004). A more compact thatch layer with less favourable conditions for aerobic bacteria may also be indicated by the strongly reduced infiltration rate (Table III) and by the more severe outbreak of microdochium patch on these plots than on other plots in September 2009.

Deep and infrequent irrigation led to an increase in root density at a depth of 10–20 cm, but the effect was significant only when measured in October 2009 on SS plots. At the same time, root density was reduced in the upper layers on both rootzones. Most likely, effects of irrigation on root development in deeper layers in 2007 and 2008 were masked by uncontrolled natural precipitation. Jordan et al. (2003) also reported no differences in rooting on a creeping bentgrass green in response to irrigation at 1, 2, or 4 day intervals during the first year of a 2-yr study under natural rainfall. In our study, the high water-holding capacity of the GM rootzone probably also contributed to negligible effect of reduced irrigation frequency on turfgrass root development.

Although several studies showed an increase in root length, root number, or root density at lower depths in response to limited soil moisture in the upper layer (Huang et al. 1997, Jordan et al. 2003, Fu and Dernoeden 2009b), effects on root distribution in the upper layer have been inconsistent. Huang et al. (1997) reported a decrease in root length of most cultivars of four warm-season turfgrasses in the upper 20 cm soil in response to drying. By contrast, Fu and Dernoeden (2009b) observed an increase in the total root length and total root surface even in the upper 6-cm of soil when irrigating at leaf wilt compared with LF irrigation. Jordan et al. (2003) also showed a higher root density at 1–7.5 cm depth on creeping bentgrass green after irrigating every 4 days than every 1 or 2 days, but only at the end of a 2-yr study.

Until construction of the rain-out shelter in July 2009, root development was more affected by rootzone composition than by irrigation treatment. Although turf visual quality was generally lower, there always were more roots at 6–10 and 10–20 cm depth on SS than on GM plots. The higher root density in the lower layer most likely maximized water and nutrient uptake due to the lower soil moisture content in SS than in GM rootzones (7.8% vs. 13.9% in 2008 and 8.5% vs. 15.2% in 2009) (Huang et al. 1997, Leinauer et al. 1997).

The negligible effect of rootzone in our study on ball roll is in accordance with earlier investigations showing that ball roll depends on the quality of the immediate green surface and not on rootzone composition (Baker and Richards 1995, Baker et al. 1999, Gibbs et al. 2000). The measurement of surface hardness in our study was important both from a player's perspective and since it reflected thatch accumulation by the species. Similar to earlier investigations, our results showed that the higher surface hardness on SS vs. GM plots persisted only during the first year of the study (Baker and Richards 1995, Gibbs et al. 2000, McCarty et al. 2005, 2007). Most probably the thick but similar mat depth and equal content of organic matter among our treatments resulted in the minimal differences in hardness between rootzones on the mature green.

Under Scandinavian climatic conditions, neither rootzone composition nor the intervals between irrigation to field capacity showed significant effects on mat thickness or organic matter content. Deep and infrequent irrigation resulted in a higher root density in the lower layer. A disadvantage of using the compost-amended rootzone was a softer green surface during the first year after establishment.

Acknowledgements

We thank the Scandinavian Turfgrass and Environment Research Foundation (STERF) and the Norwegian Research Council (NFR) for funding. We are grateful to Trond Olav Pettersen, Åge Susort, and Anne A. Steensohn for their excellent technical assistance and to Agnar Kvalbein for helpful discussions.