Plant Colorants Interfere with Reflectance-Based Vegetation Indices

Reflectance-based vegetation indices are commonly used to quantify turfgrass color, estimate chlorophyll content, and make inferences about plant health. These methods are often substituted for visual color ratings, as they are well correlated but more objective. This study examines the utility of reflectancebased vegetation indices for evaluating turfgrass treated with iron or plant colorants. In a study in Madison, WI, creeping bentgrass (Agrostis stolonifera L.) was treated with ammonium sulfate (2.4 or 9.8 kg nitrogen [N] ha(-1)), Turf Screen (untreated or 4.0 L ha(-1)), or ferrous sulfate (untreated or 12.2 kg Fe ha(-1)). Visual color ratings, chlorophyll index (CI), normalized difference vegetation index, normalized difference red edge, red, and nearinfrared (NIR) reflectance were able to detect differences in N rate. When colorant or iron was applied, however, reflectance methods could no longer detect these differences in N rate. In a study in Ithaca, NY, the application of four colorants increased visual color ratings. The pigments Harmonizer and Foursome also increased CI, whereas Turf Screen and Green Lawnger did not. Three of the four pigments decreased the chlorophyll content in leaf tissue from 1 July to 22 August, despite having greater or equal CI values. Spectral analysis of six colorants revealed that these products reflect light in the red and NIR regions and likely interfere with reflectance-based vegetation indices, which measure leaf reflectance in these same regions of the spectrum. Overall, these findings suggest that reflectance-based vegetation indices should not be used to make inferences about plant health in studies where a colorant or iron is applied.