Effects of blue and red LED lights on growth and flowering of Chrysanthemum morifolium

C. morifolium are obligate short day plants that can be induced to flower throughout the year and their stem length can be controlled by maintaining artificial long days before the onset of flowering. The ability to strictly control the flowering of chrysanthemums enabled this crop to become one of the leading ornamentals grown for the cut flower industry. The most effective inhibition of chrysanthemum flowering was found to be red light, while blue light had no inhibitory effect, and in some studies was even suggested to induce flowering. Here the effect of light quality and duration of day length illumination on chrysanthemum growth and flowering was examined. In contrast to previous studies, the results presented here suggest that blue light does inhibit chrysanthemum flowering, but the efficiency of inhibition is dependent on the duration of the lighting. Overnight illumination with blue LED lights, resulting in inhibition of flowering of three chrysanthemum cultivars, and had no significant effect on their growth. When illumination time was shortened blue light was found to be less efficient than white or red light in preventing flowering. Red LED lights were also tested in a larger scale experiment in a greenhouse, in preventing flowering of two chrysanthemum cultivars. Plants were illuminated for six hours after sundown, with either constant or intermittent red LED lights and harvested two months after the lighting was turned off. Red intermittent light resulted in somewhat shorter stems and lower quality branches in comparison to constant light.