Poinsettias need a minimum amount of light energy to maintain their living processes. When the light intensity falls below this level the plants deteriorate, the leaves fall off and there is a reduction in consumer satisfaction. The minimum interior light intensity for plant maintenance differs among poinsettia cultivars. Placing poinsettias near windows during daylight hours for additional light exposure sustains them for longer periods of time. Avoid direct sunlight, however, which may over-heat and burn the plants.
Light quality refers to the balance of colors emitted from a light source.
Sunlight emits all colors of the rainbow plus invisible ultraviolet and far-red energy. Electric lamps emit most of these colors, but in different proportions than sunlight. Incandescent lamps, such as the common light bulb, emit a higher ratio of red to blue light compared to cool white fluorescent tubes that emit higher ratios of blue light. Lamps that emit higher ratios of red light enhance the poinsettia bract color. Under blue light, red bracts have a dull bluish-red appearance and look old.