Temporal remote sensing measurements of plant growth may give breeders a better understanding of crop growth habits, yield, fiber quality, and maturity in cotton (Gossypium hirsutum L.) genotypes. The objective of this research was to derive a spectral index maturity scoring method based on the normalized difference red edge (NDRE) index. Seasonal NDRE measurements were collected from 2015 to 2017. A growth inflection point (GIP) was generated based on a quadratic fit of the NDRE growth curves for nine commercial cotton cultivars under three irrigation treatments. This GIP was the number of heat units associated with the inflection point of the NDRE values during the season. It was compared with manual measurements of crop maturity, including nodes above white flower (NAWF), percentage open boll (POB), and end-of-season plant mapping indices. Each year had environmental conditions that changed growth habits and maturity of the cultivars. Cultivar and irrigation affected maturity in all 3 yr. The GIP correlated with each maturity assessment, with the highest correlations found with NAWF (r2 from 0.38 to 0.88) by irrigation. In most cases, the relationship between NAWF and GIP was not cultivar specific, suggesting that GIP may be used across multiple cotton genotypes within multiple growing environments. The GIP method provides a method to more rapidly and objectively evaluate maturity characteristics of cotton cultivars, as well as the effects of management on these characteristics.