Hyperspectral species mapping for automatic weed control in tomato under thermal environmental stress

Highlights? Thermal stress affects tomato canopy reflectance in 480-670 and 720-810nm. ? Temperature-dependent classification performances ranging from 62.5% to 91.6%. ? Omission of 480-670nm stabilizes crop/weed discrimination with 86.4% accuracy. ? Site-specific calibration mitigates extreme temperature effects with 90.3% accuracy. ? Global calibration achieves optimum accuracy of 92.2% for robust plant recognition. This work studied the impacts of variations in environmental temperature on hyperspectral imaging features in the visible and near infrared regions for robust species identification for weed mapping in tomato production. Six major Californian processing tomato cultivars, black nightshade (Solanum nigrum L.) and redroot pigweed (Amaranthus retroflexus L.) were grown under a variety of diurnal temperature ranges simulating conditions common in the Californian springtime planting period and one additional treatment simulating greenhouse growing conditions. The principal change in canopy reflectance with varying temperature occurred in the 480-670 and 720-810nm regions. The overall classification rate ranged from 62.5% to 91.6% when classifiers trained under single temperatures were applied to plants grown at different temperatures. Eliminating the 480-670nm region from the classifier's feature set mitigated the temperature effect by stabilizing the total crop vs. weed classification rate at 86.4% over the temperature ranges. A site-specific recalibration method was also successful in alleviating the bias created by calibrating the models on the extreme temperatures and increased the classification accuracy to 90.3%. A global calibration method, incorporating all four temperature conditions in the classifier feature space, provided the best average total classification accuracy of 92.2% out of the methods studied, and was fairly robust to the varying diurnal temperature conditions.