LED lighting technology has seen remarkable growth in the horticulture industry since the development of high-brightness blue LEDs. This article reviews the latest research, the first study of its kind to investigate LED technology as supplemental assimilation lighting for the production of cut flowers. By STEVEN CARRUTHERS
Light Emitting Diode (LED) technology is not a new development. In fact, it’s been around since 1927, when Russian inventor Oleg Losev created the first LED. His research was published in Russian, German and British scientific journals, but no practical use of the discovery was made for several decades, when it was developed for optical communication applications using infrared (IR) diodes.
The first visible-spectrum (red) LED was developed in 1962 by Nick Holonyak, Jr., while working at General Electric Company, and the first yellow LED was invented in 1972 by M. George Craford, an American electrical engineer who also improved the brightness of red and red-orange LEDs by a factor of 10.
The first commercial LEDs were commonly used as replacements for incandescent and neon indicator lamps, and in seven-segment displays, which are found in digital clocks, electronic meters, calculators, and other electronic devices that display numerical information. However, while these LEDs were bright enough for use as indicators, the light output was not enough to illuminate an area.
As LED materials technology grew more advanced, light output increased while maintaining efficiency and reliability. In fact, since the 1960s, the efficiency and light output of LEDs has grown exponentially, with a doubling occurring about every three years.
The development of the high-power white-light LED, led to the use of LEDs for illumination, replacing incandescent and fluorescent street lighting, architectural, automotive and aviation lighting, as well as being used as backlighting for LCD televisions and laptop displays, aquariums, flashlights, camera flash; the list goes on.
In 1994-95, came the development of the first high-brightness blue LED by Shuji Nakamura, Isamu Akasaki and Hiroshi Amano of Japan, which earned them the 2014 Nobel Prize for Physics. With this development, LEDs have been increasingly used as supplementary lighting in commercial greenhouses, especially in latitudes with short day-lengths, and ‘plant factories’ that rely entirely on artificial lighting sources.
Like all new technological developments, there is a lot of disinformation and it’s been no different for LED horticulture lighting where there has been little applied research. Although the technology dates back to the beginning of the last century, it’s only been in recent years that they have become suitable for horticulture applications. The biggest challenge for researchers is finding the right colour balance of LED lights for important commercial crops. There has been some research into this aspect by Wageningen University in the Netherlands, but we still know little about the relationship of colours and plant development.
Latest LED research
The latest research into LEDs and plant growth comes from a team of scientists at the University of Guelph in Ontario, Canada, who report better rates of photosynthesis for cut gerbera production compared to conventional HPS (High Pressure Sodium) lights. In addition to greater yields, the researchers also report the LED lights delivered 40% energy savings. This was a collaborative research project involving representatives from the horticultural lighting industry (Lumigrow Inc), greenhouse cut flower growers (Rosa Flora), and researchers from the University of Guelph.
Gerbera was chosen as the trial species of cut flowers because of its importance for North American growers and the reliance on supplemental lighting for production between November and March in northern climates, when day length is short. The trial focused on three different varieties of gerbera – Acapulco, Terra Saffier and Heatwave—using a drip irrigation system.
Of primary importance were to have statistically valid replication of lighting treatments and to provide identical canopy level photosynthetically active radiation (PAR, µmol.m-2.s-1) under each lighting treatment.
To maximise the usefulness of the results to the growers, the LED lighting was directly compared to industry standard HPS lighting systems and was done using carefully-designed experimental protocols to ensure a high level of statistical reliability of the results.
The project consisted of eight greenhouse benches (4.57 x 1.07 m each), with each bench equipped with blackout side curtains that were partially closed to eliminate direct spill-over of light from adjacent benches.
The trial had two main treatments: HPS and LED lighting, with each treatment replicated four times (i.e. four benches per treatment). The HPS fixtures were 400W PL2000 (PL Lights, Ontario, Canada) using the ‘DEEP’ reflectors and Lucalox lamps (LU400; GE Lighting Inc., Cleveland, OH). The LED fixtures were Pro 325 provided by Lumigrow (Novato, CA), which have a combination of red (R, peak 660 nm), blue (B, peak 440 nm), and white LEDs, each with an individual rheostat, enabling modification of overall intensity and spectral output to user-defined specifications.
The greenhouse environment parameters were set at similar levels to those at local cut gerbera producers. Supplemental lighting was turned on each day 11 hours before dusk and turned off at dusk (i.e. 11-hour photoperiod). Daytime and night-time temperatures were set at 21 and 16°C, respectively. Relative humidity was maintained at 70%. Temperature, humidity and PAR sensors were set up at canopy level in the centre of each bench and were logged continuously throughout the trial.
Each bench had three fixtures of either HPS or LED lights positioned above the benches to provide identical bench-level PAR between lighting treatments and as uniform distribution as possible. The target for canopy level PAR settings was 60 µmol.m-2.s-1, based on crop level lighting measurements that were made at night at a commercial cut gerbera greenhouse (Rosa Flora, Dunnville, Ontario, Canada). After extensive work to optimise the lighting distribution (all done at night) while maintaining the similar mean bench-level PAR for the two treatments, the final mean bench-level PAR ranged from 53.8 to 56.7 and 55.1 to 58.4 µmol.m-2.s-1 for the HPS and LED light treatments, respectively. To achieve comparable bench-level PAR, the LED fixtures were positioned roughly 20cm farther away from crop level and they were turned down to 80% of full power.
The research team notes that while the mean crop-level PAR for both lighting treatments was virtually the same, this is not a true indication of the actual production of PAR from each fixture as they have vastly different beam patterns and the bench-level side curtains effectively ‘chopped off’ unknown amounts of the light output from each fixture. Overall, the plants received about 5.5 µmol.m-2.s-1 with supplemental lighting providing about 2.2 µmol.m-2.s-1 (Fig. 1).
While LED lighting technology has seen remarkable growth in the horticulture industry, especially in the last decade, this is the first study of its kind that has investigated in detail the use of LED technology as supplemental assimilation lighting for the production of cut flowers. The results have shown that commercially available LED technologies have matured to a level where they can compete favourably with traditional HID lighting technologies for supplemental lighting applications in greenhouse floriculture.
However, this research cannot be extrapolated across all crops. A clear disadvantage of LEDs is that they are monochromatic; whereas, plants need more than one colour to optimise plant growth and development. This means a combination of LEDs with different colour wavelengths is needed to optimise plant growth. The difficulty for growers, researchers and lighting manufacturers is that plant species have different lighting needs, with no one LED lighting combination suitable for all plant species. It’s also not clear if other colours in the light spectrum play a role in plant growth and development.
Getting the LED colour balance right is recognised by LumiGrow and University of Guelph researchers who will soon embark on another study to determine which combinations of red, blue and white LEDs will maximise crop performance further. LumiGrow also plans to partner with the Greenhouse and Processing Crops Research Centre (GPCRC) in Harrow, Ontario, to identify optimal LED light schedules for tomato growers in Ontario and throughout North America. These are important research priorities to increase yields and drive down energy costs even further.
The full report, ‘Evaluation of the use of light emitting diodes (LEDs) in the production of cut gerbera’, by Dr Youbin Zheng, David Llewellyn and Katie Vinson, can be requested from the LumiGrow website: www.lumigrow.com Ω
PH&G December 2014 / Issue 150