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Fall N fertilizers are applied to turfgrass at predetermined rates instead of being based on plant‐N status. Normalized difference vegetative index (NDVI) and the sufficiency index (SI) concept are used to assess plant‐N status in some agronomic crops but has not been evaluated for turfgrass. This study was conducted across 3 yr in Connecticut to determine fall and spring NDVI SI values in a mixed species, cool‐season lawn. Field experiments were set out with varying fall N rates with NDVI measured weekly in October and November, and in late April. Quadratic and Linear‐Response and Plateau (LRP) models were significant (P <.0001) for all fall and spring SI values in response to fall N rates. Fall SI was maximized between 140 and 155 kg N ha−1 for the quadratic model and plateaued between 79 and 102 kg N ha−1 for the LRP model. Spring SI was maximized at 189 and plateaued at ≥116 kg N ha−1 for quadratic and LRP models, respectively. In relation to commonly recommended fall N rates of 24.5, 49, and 98 kg N ha−1, the models suggest that no supplemental N is needed when SI is ≥92, ≥95, and ≥99% of the N‐rich reference in the fall, respectively. Late April SI >89% of spring N‐rich reference would indicate that excess fertilizer was applied the previous fall at rates >49 kg N h−1. The NDVI SI can provide site‐specific fall N fertilization guidance of cool‐season turfgrass lawns. [ABSTRACT FROM AUTHOR]