Shedding light onto LED research

LEDs for the greenhouse industry: A perspective

By Cary A. Mitchell, Director, SCRI Project for Developing LED Lighting Technologies and Practices for Sustainable Specialty-Crop Production & Professor, Department of Horticulture & Landscape Architecture, Purdue University

Colorful possibilities
Growers of specialty crops have become intrigued with the possibility of using light-emitting diodes (LEDs) to meet their greenhouse lighting needs. LEDs have a mystique that appeals on many levels, but is enough known to risk replacing the tried-and-true of today with the spark and passion of tomorrow for ongoing commercial production?

Pros and cons
LEDs have a reputation for expending less electrical energy than traditional greenhouse lighting sources, as well as having a longer lifespan. And engineering continues to improve the efficacy of LEDs to convert the energy of electricity to the energy of light, and the cost of their manufacture continues to come down. The main obstacle that makes growers hesitate is the capital investment of equipping greenhouses with LED fixtures. For supplemental photosynthetic lighting, it takes a higher up-front investment to provide equivalent light intensity with LEDs than by staying with traditional lamps. So, where do these tradeoffs come out? It leaves growers asking, “Should I invest now, or should I wait?”

Unfortunately, there is no simple answer to these questions. It depends not only on capital investment vs. energy savings in a given region, but also on specific crop responses to narrow-spectrum vs. broad-spectrum light. Optimum “LED light recipes” combining spectrum, light intensity and photoperiod have not been worked out in detail yet for every cultivar of every specialty crop, and they do vary. Growers with contracts to meet cannot afford to guess about crop responses just to save energy, especially when the ante to get into the LED poker game is salty compared to traditional light sources. Comprehensive economic analyses must be conducted using experimental or pilot crop-response data.

Recommendations
Those of us conducting LED research applied to greenhouse specialty crops typically recommend that growers interested in trying LEDs, but worried about making costly mistakes, take it slowly and conduct some of their own trials on a small scale. Only after satisfying themselves that they can get as good or better crop response with LEDs should they consider the economic tradeoffs. If the projected break-even time works for them, then they can make a gradual transition as traditional lighting sources require replacement.
 

Promising Possibilities for LEDs in the Greenhouse
Control of flowering
The obvious application for LEDs in the greenhouse is photoperiodic control of flowering ornamentals (Figure 1). LED fixtures for photoperiod control cost more than (disappearing) incandescent light bulbs, but not nearly as much as high-intensity discharge (HID) lighting fixtures, and they fit right into standard screw-in light sockets. What’s more, the intensities of light required for night-break induction or inhibition of flowering are vanishingly small, so the energy bill is relatively low. That said, research is not finished regarding spectral optimization of photoperiod control for different crops, so even this likely application for LEDs in the greenhouse is not yet a turnkey deal for commercial application. Investigating effects of different spectral blends on floral control and accompanying plant development is still a work-in-progress. Growers will have to conduct some of their own trials, await the published results of public-sector research or trust manufacturer claims before jumping in with both feet.

Intracanopy photosynthetic lighting
High-light-requiring, high-wire trellised vegetable crops such as tomato, cucumber and pepper are self shading in the greenhouse, and production is particularly limited during low-light seasons and/or in low-light climates. The unique property of LEDs to be relatively cool because waste heat is rejected remotely from emitters allows LEDs to be placed close to plant tissues, including within the foliar canopy of vertically dense foliar canopies (Figure 2). Research is showing potential for intracanopy lighting to save considerable energy, stimulating yield without accompanying overhead HID lighting. But the technology and protocols are still under development.

Transplants in the greenhouse? Maybe.
End-of-day, daylength extension or supplemental lighting with LEDs can stimulate growth and development of young transplants during low-light seasons in the greenhouse (Figure 3). Manipulation of spectral composition with LEDs may alter morphology of seedling or grafted transplants as well as total growth in desired ways (Figure 4). Because dense LED arrays block considerable sunlight from reaching crops in the greenhouse when positioned close above crop surfaces, the question is, Is more sunlight blocked than supplemental light provided? If the LED light can be sole-source with even better crop-response outcomes (growth, morphology, color, phytonutrient content), why not propagate young plants in warehouses under LEDs rather than in greenhouses under partial solar and partial LED light? The tradeoffs appear to be complex, relating not only to energy utilized or saved, but whether LED light enhances desired effects of solar light, or whether solar light negates desired effects of LED light. There is growing evidence that all such scenarios are possible. The research jury is out on this one as well, and the choices likely will vary from species to species and cultivar to cultivar.

Large-scale overhead lighting? Maybe not.
Individual LEDs tend to be relatively low in wattage, even the high-output ones, compared to a high-powered HID lamp. The way LED arrays put out significant light intensity is by populating boards densely with individual LEDs so that their emission beams overlap, and by actively heat sinking them with cooling air or recirculating water. Heat-sinked LED arrays can put out significant light intensity at a distance from crop surfaces, but this does not take advantage of the unique advantage of LEDs. Because they lack the radiant-heat output of HIDs, LEDs can be placed much closer to plant surfaces. Therefore, closely placed LEDs can deliver as much photosynthetic light as more distant HIDs but with much less input power. Close positioning also is a current weakness of densely arrayed LEDs. They block solar light in the greenhouse, and of course the arrays are expensive. Thus, it is hard to envision large propagation or production greenhouses with massive arrays of LEDs delivering energy-efficient supplemental light at the intensity of HIDs, or for the fixture costs of HIDs.

Trends
As the shakeout continues, LEDs will find niches in the greenhouse industry, different niches in the warehouse-based plant-factory industry and HID lamps will continue to survive, mostly in greenhouses, for a while. LED applications for greenhouse propagation and production will continue to evolve as LED light-delivery technology advances, and should eventually become the dominant lighting technology for specialty-crop production, until something better comes along.
 

Can white LEDs control flowering?

By Qingwu Meng and Erik S. Runkle, Michigan State University

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 have been 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 nine-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).

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 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 (runkleer@msu.edu) is a professor and floriculture extension specialist in the Department of Horticulture at Michigan State University. 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.