Nitrogen Affects the Summer Density of Creeping Bentgrass

Creeping bentgrass (Agrostis stolonifera var. palustris Huds. Farw.) is the preferred turfgrass species for golf greens (Beard, 2002). Creeping bentgrass is a cool-season grass that forms an extremely dense, fine-textured, persistent turf that tolerates close (less than 0.125 inches), frequent mowing. During summer months, however, shoot density (SD) often declines, resulting in poor stand quality. Various cultural practices, such as mowing height, fertilization regime, topdressing and vertical mowing, can have an influence on SD. Golf course superintendents utilize several management practices to maintain turf vigor during the summer. One practice is light, frequent, nitrogen (N) fertilization. However, annual N rates vary widely.

Recently, high shoot density (HSD) bentgrasses bred to provide superior appearance and stress tolerance compared to the industry standard (Penncross) have been widely planted (Beard et al., 2001; Landry and Schlossberg, 2001). The effect of variable N rates on these cultivars and their seasonal changes in SD is unclear.

Tale of two fertilities

A field study was conducted at the W.H. Daniel Turfgrass Research and Diagnostic Center at Purdue University, West Lafayette, Ind., from 2006 to 2007 on a sand-based research green built to United States Golf Association (USGA) specifications. Three creeping bentgrass cultivars — A-4, L-93 and Penncross — were evaluated because of their commonality on golf greens and noted differences in SD.

The study site was maintained to emulate moderate golf course putting green conditions with modern cutting heights and cultural practices.

Two fertility regimes designated as "low" and "high" (2.3 pounds versus 4 pounds of N per 1,000 square feet annually) were used to assess the varying range of N applied to mature putting greens annually. Granular N was applied in the spring and fall to promote recovery from hollow-tine core cultivation (mid-April and mid-September) and to store carbohydrates in late fall (early November). Spoon feeding N was performed during the summer months (mid-May through September) with roughly half of the total N applied as a liquid.

Seasonal SD was measured by removing two intact cores per plot with a soil probe and counting individual shoots, which were then averaged and used for data analysis. Four SD measurements were taken in 2006 (June, July, August and October) and six times in 2007 (May through October).

Seasonal density changes

Seasonal SD counts ranged from 760 to 2,160 shoots dm² (per square decimeter) throughout the study (Figure 1). The temporal changes in SD followed the cool-season growth pattern with the highest values measured during spring and autumn and a decline during the summer months. Among cultivars, A-4 generally possessed the highest SD (1,400 to 2,160 shoots dm² ) compared to Penncross (760 to 1,470 shoots dm² ) which had the least, while L-93 was intermediate (1,230 to 1,780 shoots dm² ). While each cultivar experienced reduced SD during the summer, the magnitude of the decline varied with cultivar and to a lesser extent N regime. The spring density of Penncross in both years was on average 37 percent less than A-4 at 4 lbs N/1,000 ft² yr^-1 (square feet per year). By comparison, summer density of Penncross averaged over both years was 44 percent less than A-4 under 4 lbs N/1,000 ft² yr^-1 . The higher SD of L-93 and A-4 is consistent with reports that have demonstrated the enhanced ability of HSD cultivars to provide superior turf conditions during the summer when compared to Penncross and many other earlier bentgrass generations (Landry and Schlossberg, 2001).

Figure 1 Seasonal shoot density was measured by removing two intact cores per plot with a soil probe and counting individual shoots.

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