Abstract: Normalized difference vegetative index (NDVI) values are vegetative indices, calculated from active canopy sensor measurements using the reflectance values for red and near-infrared. Use of NDVI sensors offers the possibility of rapid, non-destructive readings that correlate with plant properties such as plant biomass and plant nutrition when plants are green, however, the affects of color on NDVI sensor readings has not be investigated. Poinsettia cultivars ‘Enduring Marble’, ‘Ice Punch’, ‘Winter Rose Early Red’, ‘Prestige Red’, ‘Prestige Maroon’, ‘Peterstar White’, ‘Maren’, and ‘Orange Spice’ were analyzed by an NDVI sensor starting four weeks after transplanting and through bract color development. The results indicated that cultivar effect, time effect and the interaction of cultivar and time on NDVI value were significant (P < 0.0001). An increase of NDVI value occurred from initial measuring date, and reached the greatest value (ranged from 0.718 to 0.837) between week 6 and week 7 for all cultivars except ‘Orange Spice’ and ‘Winter Rose Early Red’. From the peak readings, all NDVI values in tested cultivars declined significantly, especially after bract coloration. This result demonstrated an inverse relationship with increased bract coloration and reduced NDVI readings, so use of NDVI readings to detect N deficiencies should be before bract coloration in poinsettias. However, the NDVI sensor could be used to monitor bract colour development. Because the effects of cultivar, time, and the interaction between cultivar and time were significant, standards using a pocket NDVI sensor in specific cultivars during determined growth stages may need to be established at each growing facility.