In cotton production, delaying defoliation exposes open bolls to rainfall, reducing lint yield and fiber quality. However, premature defoliation has detrimental effects on both yield and quality. Currently, one management strategy for defoliation is based on heat-unit (HU) accumulation after physiological cutout or five nodes above white flower (NAWF = 5). Results have been inconsistent across environments when utilizing HU accumulation; therefore, adoption of this method has been limited. Studies were conducted in the Brazos River bottom and upper Texas Gulf Coast to identify an upper temperature threshold (UTT) for calculating degree days for defoliation timing. Experimental design was a split-plot with main plots of temperature thresholds (32°C, 35°C, and no upper limit) and subplots of HU timings (361, 417, 472, 528, and 583) accumulated from cutout. Utilizing an UTT to calculate daily HU failed to explain differences in the optimum time to defoliate based on accumulated HU; however, accumulated HU had an impact on defoliation timing. Comparison of locations showed that maximum lint yield was obtained at 472 HU and 52% open boll in Wharton County versus a maximum of 528 HU and 62% open boll for Burleson County. Employing the NACB = 4 method to time defoliation at both locations would have resulted in premature application of harvest aids and reduced lint yields. There were no differences in adjusted gross income values at Wharton County among the 417, 472, 528, and 583 HU treatments. For Burleson County, adjusted gross income peaked in value at 528 HU.