The application of supplemental blue light in greenhouse chrysanthemum production is part of a biological control strategy to enhance reproduction of Orius insidiosus Say, a natural predator of the western flower thrips, Frankliniella occidentalis Pergande. Two greenhouse experiments were conducted to determine the influence of a blue light—supplemented long day on flowering and vegetative growth in three cultivars of the short-day plant Dendranthema ×grandiflora (Ramat.) Kitamura. In Expt. 1, two cut chrysanthemum cultivars (`Manatee Iceberg' and `Naples') were exposed to: a) 9-hour ambient light and 15-hour artificial blue-biased (400-500 nm) light at two blue light intensities (3.6 or 7.0 μmol·m-2·s-1); b) 9-hour ambient light and 15-hour artificial broad spectrum light at a broad spectrum intensity of 3.6 μmol·m-2·s-1; c) 9-hour ambient light maintained with black cloth; or d) an ambient short day. Under a continuous photoperiod, flower initiation in both cultivars in the lower intensity blue light was not significantly different from that in short-day regimes. However, in both blue light intensities, flower size and dry mass were significantly less than in the short-day regimes. Increasing the dose of blue light decreased flower dry mass in `Naples' by 60% and in `Manatee Iceberg' by 72%. Plants were shorter with less vegetative mass in the short-day regimes. In Expt. 2, `Naples' and the pot chrysanthemum `Boaldi' were exposed to a) 9-hour ambient light and 6-hour artificial blue-biased (400-500 nm) light at four blue light intensities (0.4, 0.7, 1.6, or 3.5 μmol·m-2·s-1); b) 9-h ambient light maintained with black cloth; or c) an ambient long day. For both cultivars, in all blue light regimes, neither flower dry mass nor vegetative dry mass differed significantly from those in the short-day regime. The results indicate that exposing D. grandiflora to a blue light—supplemented long day at blue light intensities <3.5 μmol·m-2·s-1 does not adversely affect flower initiation and development.