When the natural days are short (October to March), lighting during the middle of the night (night interruption, NI) can create long days. The spectrum and intensity of the light source is critical for its efficacy. For instance, blue light does not influence flowering at a low intensity (e.g., 2 µmol·m−2·s−1), but does at a moderate intensity (e.g., 30 µmol·m−2·s−1). In addition, at a low intensity, a mixture of red and far-red light effectively promotes flowering of long-day plants, whereas only red light is needed to inhibit flowering of short-day plants.
Incandescent lamps are conventionally used for photoperiodic control, but they have been phased out of production. Replacing incandescent lamps with compact fluorescent lamps may delay flowering of some long-day plants. Light-emitting diodes (LEDs) have emerged as an alternative because of their energy efficiency, longevity, and spectral controllability. LEDs with a similar red-to-far-red ratio to incandescent lamps are effective at controlling flowering. However, some growers wondered whether more affordable white LEDs would also be effective.
White LEDs, which are really blue LEDs coated with a phosphor, cast a broad spectrum but emit little far-red light. They are categorized into cool, neutral, and warm types based on light appearance. We grew five long-day plants (calibrachoa ‘Callie Yellow Improved’, coreopsis ‘Early Sunrise’, petunia ‘Wave Purple Improved’, rudbeckia ‘Indian Summer’, and snapdragon ‘Liberty Classic Yellow’) and two short-day plants (chrysanthemum ‘Cheryl Golden Yellow’ and marigold ‘American Antigua Yellow’) at 68 °F under 9-hour short days with or without one of five NI lighting treatments from LEDs: red, blue+red, cool-white, warm-white, or red+white+far-red.
All NI lighting treatments similarly promoted flowering of calibrachoa and rudbeckia. In coreopsis, petunia, and snapdragon, the most rapid flowering occurred under the red+white+far-red LEDs (Fig. 1). Surprisingly, the white LEDs did not create long days for snapdragon at all. Therefore, they were sometimes not as effective as the LEDs that emitted both red and far-red light. In contrast, the white LEDs were very effective at inhibiting flowering of the two short-day plants. Chrysanthemum plants did not flower under the white LEDs. Also, flowering of marigold was delayed under the white LEDs. In addition, some plants (e.g., coreopsis and marigold) were shorter under the warm-white LEDs than under the red+white+far-red LEDs (Fig. 1).
Fig. 1. All night-interruption lighting treatments promoted flowering of coreopsis, but it occurred about 10 days earlier under the treatment that included far-red light (right). Means followed by different letters are significantly different by Tukey’s honest significant difference test at P ≤0.05.
In summary, the white LEDs were generally as effective as the red and blue+red LEDs as an NI. These lamps stimulated flowering in four of the five long-day plants. However, flowering was delayed in some long-day plants compared to those grown under the red+white+far-red LEDs. Therefore, lamps that emit both red and far-red light (e.g., the DR/W/FR flowering lamps from Philips and the Night & Day bulbs from TotalGrow) are recommended for the most rapid flowering of long-day plants. General white LEDs may be useful to inhibit flowering of short-day plants, but additional research is needed before large-scale implementation.
Qingwu (William) Meng is a Ph.D. student and Erik Runkle (firstname.lastname@example.org) is a professor and floriculture extension specialist in the Department of Horticulture at Michigan State University (MSU). They thank the USDA National Institute of Food and Agriculture’s Specialty Crop Research Initiative and MSU’s Project GREEEN for funding this research, C. Raker & Sons and Syngenta Flowers for donating plant material, and Nate DuRussel for technical assistance.