Mapping peak flowering phenology of rapeseed (Brassica napus) in North Dakota using Landsat 8 and Sentinel-2

Detecting the peak flowering time of rapeseed (Brassica napus) is essential for optimizing crop management, improving yield forecasts, and enhancing pollination services. This study aimed to map and analyze the spatial and temporal patterns of peak flowering in spring rapeseed across North Dakota using satellite remote sensing. We utilized multi-year optical imagery from Landsat 8 (2013–2024) and Sentinel-2 (2018–2024), applying the Normalized Difference Yellow Index (NDYI)—an index sensitive to yellow floral reflectance—to identify peak flowering dates. Our results indicate that peak flowering dates generally cluster around Julian day 200 (mid-July). Sentinel-2 provided more temporally consistent and spatially detailed estimates than Landsat, owing to its higher revisit frequency (5–7 days versus 16 days) and finer spatial resolution (10 m versus 30 m). On average, Sentinel-2 detected slightly later flowering dates (Julian days 203–230) compared to Landsat (196–213). Median peak flowering dates from Landsat ranged between Julian days 191 and 221, whereas Sentinel-2 medians were narrower and more closely aligned with the mid-July window, spanning Julian days 202 to 216. These differences underscore Sentinel-2’s superior capacity to capture flowering events precisely, while Landsat’s coarser temporal resolution occasionally resulted in earlier or delayed detections. Overall, the NDYI-based approach effectively captured flowering dynamics, revealing both inter-annual variability and field-level heterogeneity. These findings demonstrate the value of NDYI remote sensing for monitoring rapeseed phenology and suggest its broader applicability in agricultural and ecological contexts.