LEDs are increasingly being used by commercial greenhouse growers, but their spectra vary and sometimes there is confusion about their flowering application.

By Qingwu (William) Meng and Erik Runkle

Michigan State University Extension originally published this article in July, 2016.

Many ornamental plants flower when their biological clock is in sync with daylength. Short-day and long-day plants, as their names indicate, flower most rapidly under short and long days, respectively. During natural short days, lighting at night can delay flowering of short-day plants to stimulate vegetative growth, or promote flowering of long-day plants to speed up production. Most incandescent light bulbs have been phased out because of their energy inefficiency. Compact fluorescent lamps have higher luminous efficacy, but are not as effective as incandescent lamps for some long-day plants. What’s a suitable alternative, then?

Numerous LED lamps have been developed for commercial plant production during the past few years. However, not all LEDs are created equal; for example, some are better than others at regulating flowering. Research performed at Michigan State University has shown that the light spectrum is key to regulating flowering. A spectrum rich in red (600 to 700 nm) is capable of inhibiting flowering of short-day plants. The inclusion of far red (700 to 800 nm) can sometimes accelerate flowering of long-day plants. At the low light intensity typically delivered (≈2 µmol·m−2·s−1), blue (400 to 500 nm) and green (500 to 600 nm) light are less effective at regulating flowering.

We measured the light emitted from four types of LED lamps that greenhouse growers have used to regulate flowering of ornamental crops. These are the Feit Electric Plant Grow Light (BR30/11K/GROW/LED), Total Grow Night & Day Management Light (TG1A-1004) and two Philips GreenPower LED Flowering Lamps (DR/W and DR/W/FR) (Photo 1). We measured light output 8 inches (20 cm) directly below the lamps using a portable spectroradiometer. As expected, they had distinctly different spectra and intensities (Photo 2). The Feit lamp emitted predominantly red light (87%) while the Total Grow lamp had both red (60%) and far red (27%). The Philips R/W lamp produced primarily red (78%), and the Philips R/W/FR lamp generated both red (35%) and far red (46%).

Flowering LED lamps with names 1000 px

Photo 1. Photos of several screw-in LEDs developed for horticultural applications showing the lamps and the color of light emitted. Photo: Qingwu Meng, MSU.

Flowering LED lamps spectra

Photo 2. Four commercial light-emitting diode (LED) lamps developed to regulate flowering of day length-sensitive crops differ in their emission spectrum and intensity.

Which of these LEDs to choose depends on several factors including what crops are grown and when. For short-day plants like chrysanthemum, all of these lamps are generally effective since they all emit red light. While the presence of red light is sufficient to promote flowering of long-day plants like rudbeckia, some long-day plants (such as snapdragon and petunia) need far red for the most rapid flowering, especially when grown during periods of low light. Therefore, the Total Grow and Philips R/W/FR lamps, which emit both red and far red, are most suitable for long-day plants. Aside from the spectrum, growers should also consider durability, cost, light output and uniformity, life time and warranty.

Finally, don’t confuse these “flowering” lamps with supplemental lighting. To regulate flowering, only a very low intensity (1 to 2 µmol·m−2·s−1) is needed. To increase growth (faster rooting, thicker stems, etc.), a much higher intensity is needed, and typically 40 to 60 µmol·m−2·s−1 is appropriate for ornamentals. These flowering lamps are suitable to extend the day or interrupt the night but provide no benefit when turned on during the day. Visit the MSU Floriculture webpage for more information on lighting.

PhD student Qingwu (William) Meng and MSU professor and Extension specialist Erik Runkle work in MSU’s Department of Horticulture. Statements in this article do not constitute endorsement of any of the products tested.

Choosing the right LEDs to regulate flowering