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© 2007 Plant Management Network.
Accepted for publication 15 December 2006. Published 23 April 2007.

Turf Covers for Winter Protection of Bermudagrass Golf Greens

J. Michael Goatley, Jr., Turfgrass Specialist, Crop and Soil Environmental Sciences Department, Virginia Tech, Blacksburg 24061-0403; J. Pat Sneed, Victor L. Maddox, Barry R. Stewart, D. Wayne Wells, and H. Wayne Philley, Plant and Soil Sciences Department, Mississippi State University, Mississippi State 39762

Corresponding author: J. Michael Goatley, Jr. Goatley@vt.edu

Goatley, J. M., Jr., Sneed, J. P., Maddox, V. L., Stewart, B. R., Wells, D. W. and Philley H. W. 2007. Turf covers for winter protection of bermudagrass golf greens. Online. Applied Turfgrass Science doi:10.1094/ATS-2007-0423-01-RS.

Abstract

Research trials were conducted on a bermudagrass (Cynodon magennissii Hurc. ‘MS-Express’) putting green at the Mississippi State University Golf Course over the winter months of 2000-2001, 2001-2002, and 2002-2003 to evaluate how various turf covers modify surface temperatures and turfgrass growth when applied for temporary cold temperature protection. Average daily maximum, minimum, and mean temperatures were recorded, as well as the average daily range in temperatures under covers. Temperature responses varied with cover composition, permeability, color, and to a lesser extent, thickness. An experimental translucent overwintering blanket provided the highest average daily maximum temperatures, but also had the greatest temperature range, indicating the potential for excessive heating under the cover. A commercially available interwoven polyethylene cover also provided high daily mean soil surface temperatures, but its mean daily minimum temperatures were not significantly different from the uncovered control in two of the three years, apparently indicating much of the energy acquired during the day was lost after sundown. Doubled layers of commercially available white or black polypropylene covers had only slightly increased mean daily minimum temperatures as compared to single layers. All covers provided some degree of potentially desirable temperature modifications, but selection and use would depend on the particular needs of the turf manager.

Introduction

Winterkill is one of the greatest challenges facing golf superintendents managing bermudagrass putting greens in the highly variable climates of the transition zone and mid-south regions of the United States. Recent improvements in bermudagrass density and tolerance to cutting heights ≤ 3 mm have increased the use of these grasses for putting green turf (14). However, these grasses’ tolerance to low mowing heights is also coupled with a reduction in freeze tolerance, suggesting increased winterkill potential (1,3,11,12).

Turf covers perform in much the same way that a cloud canopy does in modifying the levels of solar radiation transmitted to the earth and trapping radiant energy that is trying to escape to the atmosphere (18). The use of a pine straw cover for winter protection of bermudagrass golf greens was first reported in 1929 (4). Numerous researchers have documented enhanced fall color retention, survival, and post-dormancy regrowth of covered bermudagrass maintained at cutting heights of ≥ 19 mm (8,15,16,17). Winter-long installation of natural covering products such as wood mat or straw mulch, or the use of a synthetic, impermeable cover have been shown to improve winter survival of annual bluegrass (Poa annua L.) and creeping bentgrass (Agrostis palustris Huds.) putting greens, although local conditions such as the specific location of the greens played a role in cover performance (6,10). For all of these research reports, the covers were installed for a period of consecutive months, removing the turf from play. Since golf can be played throughout much of the winter in areas where bermudagrass is adapted, turf covers are often applied to putting greens on a temporary basis to provide short duration, low temperature protection.

Little published research exists detailing how covers modify surface temperatures of bermudagrass golf greens. The objectives of this research were: (i) to determine how various commercially available and experimental cover materials modify surface temperatures of a bermudagrass putting green when applied prior to predicted severe winter temperatures; and (ii) to determine the extent that the turf covers reduce the levels of photosynthetically active radiation (PAR) at the turf surface.

Cover Sources and Temperature Recording Techniques

The research was conducted on a ‘MS-Express’ (Cynodon magenissii Hurc.) practice putting green maintained at a 4.5-mm cutting height on a 90% USGA specification sand/10% composted rice hull (v/v) soil mix at the Mississippi State University Golf Course during the winter months of 2000-2001, 2001-2002, and 2002-2003 (9). The turf received N and K at approximately 488 kg/ha per growing season, and other nutritional adjustments were applied according to soil test recommendations or as the golf course superintendent deemed necessary (soil pH during the trials ranged from 6.8 to 7.1). Irrigation and other cultural programs (core cultivation, topdressing, rolling, etc.) were applied according to the superintendent’s discretion in maintaining healthy turfgrass growth and desirable playing characteristics. The materials presented in Table 1 (listed both as standard commercially available covers and experimental covers specific to this research) and shown in Figures 1 and 2 were evaluated across the three yearly trials. Each cover measured approximately 3 × 3 m and was installed with sod staples. A 3- × 3-m area of uncovered turf was used as the control treatment.

Table 1. Turf cover source, abbreviations used in text, cover description, and product supplier.

 ^x Commercially available.

 ^y Experimental cover.

Fig. 1. Commercially available cover sources used in this trial were (left to right, abbreviations defined in Table 1): black woven polypropylene (BX); white and black spunbonded polypropylene (WT1 and BT1, respectively); an interwoven translucent polyethylene (EG) cover; and a gray, nonwoven geotextile (SL).		Fig. 2. Experimental cover sources used in this trial were (left to right, abbreviations defined in Table 1): translucent horticultural thermal blanket (OB); three polypropylene microfoam sheets with a laminated backing (PPS); a green polyethylene tarpaulin (Tarp); a single polypropylene microfoam sheet with laminated backing (PPF); and a 2-mm thick reflective film attached to a nonwoven geotextile (SLR).

Covers were applied at the discretion of the Mississippi State University Golf Course Superintendent (Pat Sneed, Certified Golf Course Superintendent) when minimum predicted temperature of ≤ -4°C (based on area National Weather Service forecasts) were forecast for at least two consecutive evenings. Based on this criterion, covering event dates for the three yearly trials were 2 December 2000 through 6 January 2001, and 20 January through 23 January 2001 during the 2000-2001 trial; 29 December 2001 through 4 January 2002, 7 January through 9 January 2002, and 26 February through 1 March 2002 during the 2001-2002 trial; 11 January through 14 January 2003 and 17 January through 28 January 2003 during the 2002-2003 trial.

Hobo Pro series data loggers (Onset Computer Corp., Bourne, MA) were used to record temperatures under each cover at 15-min intervals. The temperature data are presented in the following four categories: mean surface temperature over all covering days, mean of the range in daily surface temperatures, mean of the daily maximum surface temperature, and mean of the daily minimum surface temperature.

Soil surface PAR (µmol/m²/s) measurements for all cover treatments were made with a photometer (Li-Cor Corp., Lincoln, NE). The data were collected under a cloudless sky from 1100 to 1115 h on 21 October 2003.

The experimental design was a randomized complete block with three replications per treatment. Analyses of variance were performed and means were separated using Fisher’s Protected LSD test at the P ≤ 0.05 level when appropriate (14). Due to a significant interaction between cover treatment and year, data are presented for each seasonal trial.

Cover Effects on PAR and Spring Greening

Reductions in PAR were ≥ 87% for all black covers (BT1, BT2, BX) and the experimental SLR and PPS (Table 2). The standard Tarp, EG, PPF, and SL resulted in moderate reductions in PAR (64 to 79%) that still might be of concern for turf growth, but would offer greater flexibility in covering duration. The white covers (WT1 and WT2) and clear OB resulted in the least reduction in PAR with values of 36, 49, and 34%, respectively (Table 2).

Table 2. Percent reduction in photosynthetically active
radiation (PAR, µmol/m²/s) under turf covers as
compared to the uncovered control.

 ^x Measurements made between 1100 and 1115 hrs in
full sunlight on 23 October 2003 with a Li-Cor
photometer (Li-Cor Corp., Lincoln, NE).

Covers that significantly restrict PAR will likely limit rapid spring bermudagrass regrowth from dormancy. Translucent covers have been shown to accelerate spring transition from dormancy to active growth when applied for the duration of the winter months (8,15,17). However, significant visible greening enhancement was not evidenced in our trials, possibly because of the limited duration of cover applications based on temperature fluctuation and relatively mild winter weather patterns (data not shown). Further research is required to determine how bermudagrass growth is affected by reduced PAR levels as a result of longer term covering.

Moderation of Temperatures by Covers

The OB provided the highest daily mean maximum, minimum, average, and range in temperatures of all cover treatments (Tables 3 to 6). This cover source, a translucent material similar to commercial "bubble wrap," performed comparably to clear plastic covers previously described in Virginia and Maryland (15,17). The OB resulted in daily mean maximum temperatures that were at least 3°C higher than the next highest cover treatment temperature in any year and 6 to 8°C warmer on average than the control. These data indicate that the OB provided the best low temperature protection as well (Table 4), but the capacity for a rapid change in temperature (as indicated by the range data in Table 6) indicate concerns for leaving the OB on for extended time periods in a climate where rapid temperature fluctuations are likely. A maximum temperature of 45°C was recorded under the OB treatment on 11 January 2003 as compared to 11°C for the uncovered control (data not shown), a temperature that could possibly result in an untimely break in bermudagrass winter dormancy and/or loss of cold temperature acclimation if covers remained in place during warm weather conditions. The OB was not modified to provide air and moisture exchange and it is likely that venting the product could improve the flexibility in scheduling this cover’s installation and removal frequency. Vented clear plastic sheeting delayed autumn dormancy, reduced freezing injury, and hastened postdormancy growth of bermudagrass in Virginia (15).

Table 3. Daily maximum temperature means under turf covers applied
to a bermudagrass golf putting green for three years of research trials.

 ^x See abbreviations and descriptions of covers in Table 1.

 ^y Means within a column followed by the same letter are not
significantly different at the P ≤ 0.05 probability level as determined
by Fisher’s Protected Least Significant Difference (LSD) test.

 ^z Coefficient of variation.

Table 4. Minimum daily temperatures under turf covers applied to a bermudagrass golf putting green for three years of research trials.

 ^x See abbreviations and descriptions of covers in Table 1.

 ^y Means within a column followed by the same letter are not
significantly different at the P ≤ 0.05 probability level as determined
by Fisher’s Protected Least Significant Difference (LSD) test.

 ^z Coefficient of variation.

Table 5. Daily average temperatures under turf covers applied to a
bermudagrass golf putting green for three years of research trials.

 ^x See abbreviations and descriptions of covers in Table 1.

 ^y Means within a column followed by the same letter are not significantly different at the P ≤ 0.05 probability level as determined by Fisher’s
Protected Least Significant Difference (LSD) test.

 ^z Coefficient of variation.

Table 6. Daily range in temperatures under turf covers applied to a bermudagrass golf putting green for three years of research trials.

 ^x See abbreviations and descriptions of covers in Table 1.

 ^y Means within a column followed by the same letter are not significantly different at the P ≤ 0.05 probability level as determined by Fisher’s Protected Least Significant Difference (LSD) test.

 ^z Coefficient of variation.

The other three experimental covers (SLR, PPF, and PPS) had the lowest range in daily temperatures (Table 6). It was hypothesized that the reflective film backing added to the underside of the commercially available SL cover to make the SLR cover treatment would aid in trapping additional thermal radiation. The addition of the reflective material resulted in a significant mean minimum temperature increase of 2.7 and 2.5°C over the SL treatment in 2001-2002 and 2002-2003, respectively (Table 4). The SLR provided the highest average daily minimum temperatures, and consistently had some of the highest average daily mean and lowest average daily ranges in temperatures across the three years (Tables 4 to 6). The lowest mean maximum temperature values were recorded for the experimental PPS, having mean temperatures similar to control in all years (Table 3). PPS and PPF were effective for low temperature protection and would likely not require removal during warm weather patterns. However, they probably will not promote postdormancy growth of bermudagrass as indicated by their low daily maximum temperature means (Table 3) and their low PAR transmission values (Table 2).

Among commercially available covers evaluated, WT1 and WT2 tended to have the highest mean daily maximum temperatures after the OB in each of the three years (Table 3). Doubling the cover material in WT2 versus WT1 resulted in significant increases in average daily minimum and mean temperatures in 2001-2002 and 2002-2003, but the differences were still only 1.1 to 1.7°C (Tables 4 and 5). WT2 tended to have a lower average daily temperature range than WT1 across all three years, but the differences were not significant (Table 6). The additive effects of a second layer of white Typar on temperature modification were minimal under these experimental conditions.

The EG cover tended to increase the mean daily maximum temperatures as compared to the control (Table 3), but average daily minimum temperatures were not significantly different from the control in two of the three years (Table 4). The EG cover placed in the second-lowest statistical grouping for average daily mean temperatures and the second highest statistical grouping for average daily temperature range in each year (Table 6). The visibly loose interweave of translucent polyethylene in the EG cover allowed for significant day-time heating, but much of the energy captured was lost during the evening hours.

The polyethylene tarp tended to provide some of the most variable temperature responses, with highly fluctuating mean daily minimum and daily average temperatures between years 1 and 3 as compared to year 2 (Tables 4 and 5). We have no explanation for this variation. However, its mean daily maximum temperatures were consistently in the second highest statistical category in each year (Table 3) and its mean daily range temperature was in either the highest or second highest category each season (Table 6). Though widely available at many retail stores, tarps such as these have declined in popularity for temporary cold protection of bermudagrass greens because of their weight and handling characteristics. Still, this material provided desirable temperature modification characteristics comparable to other industry standard and experimental covers evaluated here.

It was anticipated that the commercially available (BT1, BT2, BX) light- impermeable covers (≥ 90% reduction in PAR) might have a significant reduction in mean daily maximum temperatures due to a "shade" effect. However, these covers consistently had some of the highest average daily maximum temperatures across the three years (Table 3). Doubling the black Typar spunbonded polypropylene cover (BT2) did not significantly affect any mean temperature variable measured as compared to the single black cover treatments BT1 or BX (a woven polypropylene) (Tables 3 to 6). There were no consistent patterns in temperature response between white or black Typar covers. Minner et al. (13) reported that dark colored rain covers resulted in the lowest turf quality ratings when applied for winter protection of Kentucky bluegrass (Poa pratensis L.), likely due to inadequate PAR. Research is needed to evaluate bermudagrass response to various colored covers when applying on a temporary basis for either frost or extreme cold protection and long term seasonal applications for the winter months.

The gray-colored SL geotextile reduced PAR an additional 33% as compared to WT1, but its temperature values were not statistically different than WT1 over the three years (Tables 3 to 6). This fabric has traditionally been one of the most popular turf blankets in the mid-south of the United States for winter covering of bermudagrass putting greens (both on a temporary and permanent basis) since the 1980s because of its relative affordability and anticipated performance.

The lowest temperature recorded at the soil surface of the uncovered control plots of ‘MS-Express’ bermudagrass across the three years was -4.8°C. Laboratory trials in Oklahoma showed that standard putting green cultivars of bermudagrass had predicted freeze tolerance levels of -4.8 to -6.5°C (1). Winterkill of bermudagrass is not solely caused by low temperature extremes, as it has also been shown to be affected by the duration of the cold temperatures, limitations in soil physical and chemical properties, grass mowing height and nutritional levels, and slope aspect (2,5,7,12). The MS-Express cultivar used in these trials is suited to regular mowing at 4.5 mm, but other bermudagrasses can be mowed at 3 mm or less. These grasses need to be examined in future research. However, while there were few environmental extremes over the three winter seasons in our trial, we anticipate the trends in temperature modification for the respective covers would be similar under more adverse environmental conditions and for other cultivars. Long-term covering trials that encounter a broad range of severe winter conditions conducive to winterkill are required to further distinguish the performance of the various covers.

Conclusions

Applying any form of cover on a temporary basis prior to anticipated temperatures of ≤ 4°C resulted in an increase in average minimum temperatures as compared to the uncovered control; therefore coverage would likely improve turfgrass survival during periods of extreme low temperatures. Cover selection should also consider the levels of PAR transmission and daily temperature range as both were found to be highly variable depending on cover composition, color, and to a smaller degree, cover thickness. Additionally, practical considerations such as cost, durability, weight (both wet and dry), handling and placement under windy conditions, and storage requirements are also important.

Acknowledgment

The researchers thank the Louisiana-Mississippi Chapter of the Golf Course Superintendent’s Association of America and Xton, Inc. for partial funding of this research; Mr. Wayne Langford for assistance in field research; numerous Mississippi State University undergraduate turfgrass management majors that worked at the MSU Golf Course and contributed to this research; and Dr. Pat Gerard of the Department of Mathematics and Experimental Statistics at Mississippi State University for assistance in data analyses.

Literature Cited

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