Coring is a common cultural practice used on golf courses. The reported effects of coring alone on thatch-mat accumulation have been mixed. The objectives of this field study were to examine the effects of spring and summer coring on thatch-mat thickness and organic matter accumulation in creeping bentgrass (Agrostis stolonifera L.). Turfgrass color and quality and chlorophyll content were also monitored. The study site was 'Providence' creeping bentgrass grown on a sand-based rootzone maintained as a putting green. Three coring regimes were assessed as follows: spring-only coring (1.27-cm-diam. tines), spring plus three summer corings (0.64-cm-diam. tines), and a noncored control. At the end of the second year, spring-only and spring-plus-summer cored plots had developed a 66 and 89%, respectively, thicker thatch-mat layer compared with noncored bentgrass. The total organic matter content (weight loss-on-ignition) in thatch-mat layers, however, generally was similar among all three regimes in both years. This indicated that the organic matter was diluted by inclusion of sand from topdressing or reincorporation of cores. Thus, organic matter concentration (i.e., gravimetric organic: dry weight of the cores) in the thatch-mat layer was much lower in plots of both coring regimes vs. noncored plots. Both spring-only and especially spring-plus-summer coring caused substantial reductions in turf quality for a 2-wk period. Spring-plus-summer coring resulted in increased chlorophyll levels as well as improved turf color and quality in late summer.