Coupling Satellite-Derived Vegetation Indexes and Ground-Truth Data in Hazelnut Cultivation to Assess Biostimulants’ Efficacy

Hazelnut (Corylus avellana L.) cultivation in Italy is facing constraints related to climate change, causing decreases in production as a consequence of summer droughts and late spring heatwaves. This two-year study (2024–2025, i.e., Y1 and Y2) evaluated the effectiveness of two biostimulant protocols on the eco-physiological and productive performance of a hazelnut orchard (cv ‘Tonda Gentile Romana’) in Central Italy. Treatment A included a mixture of formulations (silicon, Ecklonia maxima and microalgae), while Treatment B featured an Ecklonia maxima-containing biostimulant. Data-gathering combined ground-level measurements and remote-sensing technologies, which allowed for the extraction and assessment of vegetation indexes such as the Normalized Difference Vegetation Index (NDVI), the Normalized Difference Red Edge Index (NDRE) and the Normalized Difference Moisture Index (NDMI). Treatments A and B successfully maintained higher chlorophyll content; this beneficial effect was validated by the NDVI, but the NDRE might have suffered from soil interference due to its high sensitivity. The NDMI was positively influenced by both treatments. Treatment A brought to a remarkable production increase in both seasons, especially in Y1 with 7.75 kg plant−1 (+40% vs. Control) and without negatively affecting the shell/nut ratio. These findings suggest that biostimulants could represent an effective strategy for improving productivity and enhancing abiotic stress resilience in hazelnut cultivation.