Evaluating Bentgrasses for Quality, Ball-Roll Distance, Thatch/Mat Development and Annual Bluegrass Invasion

The industry trend to golf putting greens which produce greater ball-roll distances has led to development of high density creeping bentgrass (Agrostis palustris, Huds.) cultivars that tolerate very low mowing heights. These cultivars also have a finer leaf texture and improved competitiveness with the invasive weed, annual bluegrass (Poa annua L.). In this multi-year field study, newer cultivars showed superior quality, color, leaf texture, density and resistance to P. annua invasion than older cultivars; however, there were no significant differences among cultivars in ball roll distance (as determined by a Stimpmeter) or thatch/mat depth. INTRODUCTION On any golf course, the demand for quality turf is greatest on putting greens. Throughout temperate and transitional zone regions of the world, the finest greens are seeded with cultivars of creeping bentgrass, which range from yellow-green to dark bluegreen (Turgeon, 1996; Christians, 1998). Creeping bentgrass is also the choice for bowling greens, lawn tennis and croquet courts. In transition zones, creeping bentgrass is a poor choice for home lawns due to its low tolerance for drought, heat, and wear, as well as a high susceptibility to diseases, high rate of thatch buildup, and high nitrogen requirements (Beard, 1973; Dernoeden, 2000). Under low mowing heights, creeping bentgrass forms a fine-textured, dense, and low-growing turf. The finer textured cultivars tend to be compact and low growing, while the coarser textured cultivars are usually more upright and open (Beard, 1973). Golf greens, bowling greens, and tennis and croquet courts planted with creeping bentgrass are mostly cut to a height of less than 6 mm (Beard, 1982). Heavy thatch, scalping, and overall-quality decline result from cutting bentgrass at higher mowing heights. In transition zone regions, such as much of California’s Pacific Coast, moderate summer temperatures, coupled with cool, moist and overcast conditions in fall, winter and early spring, bentgrass greens are often invaded by weeds, especially P. annua (Harivandi and Hagan, 1994). When this climate is combined with low mowing heights and frequent irrigation, P. annua often becomes the dominant species. Most greens managers choose to live with P. annua under these conditions, as it is so prevalent and difficult to control. However, there are inherent problems with P. annua-infested greens, primarily due to the wide variety in color, habit and rate of growth, seedhead production, heat and drought sensitivity and disease susceptibility within P. annua strains themselves. Also, the irregularity that P. annua patches introduce to greens often negatively affects surface uniformity and reduces putting quality. Some private seed companies and universities claim to have developed several new bentgrass cultivars that, compared to older cultivars (e.g. ‘Penncross’), are denser and therefore more resistant to P. annua invasion, have finer leaf texture, and are more tolerant of very low mowing heights, which in turn may be more conducive to greater ball roll distances. (Croce et al., 1998; Toubakaris and McCarthy, 2000). The objective of this multi-year field study was to evaluate 1) overall quality, color, leaf texture and density; 2) golf ball roll characteristics; 3) thatch/mat depth; and 4) P. annua invasion in 18 commercially available old (e.g. ‘Penncross’) and new creeping bentgrass cultivars. 309 Proc. II IC on Turfgrass Eds.: J.C. Stier et al. Acta Hort. 783, ISHS 2008 MATERIALS AND METHODS The study took place at Crystal Springs Golf Course (CSGC) in Burlingame, located on California’s Central Coast a few miles south of San Francisco. The 18 creeping bentgrasses were seeded in September, 1997 on a practice golf green, newly built according to United States Golf Association (USGA), 2-tier (no intermediate “choker” layer) golf green root zone construction specifications (USGA Green Section Staff, 1993). The green and its surrounds were designed specifically for this research project by the Robert Trent Jones Jr. Golf Architects Group of Palo Alto, California. All cultivars (Table 1), supplied by the National Turfgrass Evaluation Program (NTEP), were planted at the rate of 53 kg ha in a randomized complete block design, on 3 m by 1.5 m plots with three replications. Plots were hand seeded and were lightly raked into the surface of the root zone mix. A complete fertilizer, providing 48 kg ha nitrogen (N), 96 kg ha phosphorous (P) and 96 kg ha potassium (K) was applied at planting. The area of the green outside the research plots was seeded at the same time with ‘Cobra’ creeping bentgrass. The entire green was irrigated immediately after planting. The green was maintained for 6 months until grasses were well established, then opened for use by golfers in spring, 1998 as a practice putting green. To prevent contamination and preserve plot integrity, target flags (stick pins) were used on this practice green instead of cups. Target flags were regularly and often moved around to distribute traffic as evenly as possible. The green has been open for play continuously since then. The use of soft-spike golf shoes was encouraged on this green. During the term of this study (1998-2003) plots were mowed five times weekly at 4 mm, with a walk-behind mower. No rollers or groomers were used. Spiking and 6.5 mm diameter hollow-tine aeration were performed several times per year. No verticutting was done, but plots were occasionally top-dressed with sand. Both inorganic and organic fertilizers provided a combined average rate of 300 kg hay N, 100 kg hay P, and 150 kg ha y K. Fungicides and broadleaf herbicides were applied only when necessary. No insecticides or other pesticides were ever applied to the test site. Data collection began in January, 1998. Each plot was rated visually monthly for overall quality on a scale of 1-9 (9=best). Color and leaf texture were rated visually on a scale of 1-9 (9=darkest green or finest leaf texture) once a year in October, when environmental stress was lowest and the full genetic color of any cultivar was most likely to be expressed. Turf density was usually rated visually twice per year on a scale of 1-9 (9=the most dense stand). Also, two times per year, ball roll distance readings were taken using a Stimpmeter modified for use on small research plots (Gaussion et al., 1995; Radko, 1980). Plots were mowed at 4 mm just prior to data collection. Three golf balls were rolled in the same direction, then rolled back at 180 degrees to be able to discount slope. Final readings are the average of the six measurements. In December 2000 and 2001, and again in August 2003, all plots were visually evaluated for P. annua invasion (Table 2), and visual percent coverage was recorded. In August 2003, non-compressed thatch/mat thickness was measured to assess thatch development tendencies of the 18 cultivars after six years of growth. A standard 10-cm diameter golf green cup cutter was used to take three random samples from each plot. On each sample, thickness of the thatch was defined as the distance from the original seedbed to the base of green tissue. Using a ruler, measurements were made at two opposite points on each core and the average of the six measurements was recorded. On the day of thatch measurement, stand density for each plot was also rated. This trait was rated visually on a scale of 1-9, with 9 representing the densest turf stand (Table 3). All data was analyzed using the analysis of variance (ANOVA) procedure as provided by Statistical Analysis System (SAS Institute, 1999). Mean separation was performed using Waller-Duncan k – ratio, t test at a 95% confidence level (Steel and Torrie, 1980) if the ANOVA F test indicated that source effects were significant. RESULTS AND DISCUSSION Tables 1, 2 and 3 summarize data for all parameters evaluated in this study. Over