Effectiveness of Five Vision-Lighting Sources on Photo-Regulation of 22 Species of Ornamental Plants1

When compared at a level of 1 ft-c for 16 hours at a night temperature of 20°C, light from 5 types of sources delayed flowering of short-day plants ( Chrysanthemum , marigold, Rieger begonia), promoted vegetative growth of 2 species of Ulmus , 2 each of Acer , and 1 of Koelreuteria, Rhododendron, Rhus , and Zelkova , and promoted flowering of long-day plants (carnation, marguerite daisy, Petunia) in the order from most to least effective: incandescent (INC) > high-pressure sodium (HPS) > > metal halide (MH) = cool white fluorescent (F) > > clear mercury (Hg). Poinsettia, Betula, Catalpa, Platanus, and Tilia continued to grow vegetatively in response to all sources. Ilex and 2 species of Pinus did not respond. Foot candles of light from HPS lamps had to be increased at least 4- to 8-fold to regulate vegetative growth of long-day plants and delay flowering of short-day plants in comparison to INC lamps. High-pressure sodium lamps were ineffective in promoting early flowering of long-day plants, regardless of intensity or duration.