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LEDs have green fingers

Research has long shown that plants respond better to particular wavelengths of light, specifically in the 400 to 500nm (blue) and 600 to 700nm (red) spectra. Now, the advent of high output, low power, more affordable LED modules configured at specific wavelengths is beginning to revolutionise the horticulture sector.

Field trials of selected spectrum LED lighting schemes in large scale commercial greenhouses, as well as in smaller research establishments, have demonstrated considerable improvements in plant growth when compared to conventional lighting. Measurable results include faster and more reliable seed germination, faster root growth, earlier and more abundant flowering and shorter plant cycles, leading to improved quality and higher yields.

LEDs consume less power, are controlled more easily and last longer. They generate less heat, which means less cooling and less watering. Because they are more compact and cooler, they can be located closer to plants, allowing multitier production to be evaluated. Meanwhile, new lighting form factors increase greenhouse capacity significantly and less obvious advantages potentially include reduced fertiliser requirements and improved resistance to pests and diseases.

LED lighting is making greenhouse and hydroponics cultivation more attractive commercially for new producers, both small and large, and has the potential to reduce costs and increase yields/profits for existing growers prepared to invest in installing or replacing lighting systems. In fact, there could be a more far reaching impact.

Urban farming
Stockbridge Technology Centre (STC), a broad based horticultural research centre in the UK, has set up the LED4crops project in conjunction with Philips Lighting and Cambridge-HOK to conduct trials on growing crops under LED lighting. STC is convinced that LEDs are a crucial step towards being able to produce food wherever and whenever it is needed in a more environmentally friendly way.

For example, in countries that lack agricultural land, such as Japan, and regions where populations are concentrated in cities, LED lighting is creating new opportunities for indoor farms or plant factories that operate year round.

"LED technology opens the door to the concept of urban farming. You can grow crops in multistorey warehouses, close to the point of consumption," commented. Dr Martin McPherson, STC's science director.

The ability to produce more food locally, economically and reliably, not only helps meet increasing demand due to urbanisation and increasing populations, but also for specialist and out of season produce.
Further, it can reduce transportation requirements, such as air and road freighting. This could be a significant contribution towards reducing CO2 emissions.

In countries with long winters, horticultural lighting is essential to extend the growing season. Scandinavia, not surprisingly, has been quick to embrace LED based plant lighting systems. But even in more temperate zones, research has shown that supplemental lighting, to extend 'day time' to at least 16hr per day, can have a significant impact on plant growth.

Researchers conclude that some plants can be brought from seedling to flower in half the time when using supplemental lighting. STC claims that while a normal lettuce grower can produce five crops a year in the soil, it can produce 15 with the LED system it has installed.

Research and technology
Not surprisingly, horticultural lighting research and trials are being extensively carried out in the world's largest producers and exporters of vegetables and fruit: US and Netherlands. Dutch greenhouses, for example, are among the most innovative adopters. Some farms are aiming to be energy self sufficient, generating wind or geothermal energy, not only to power the LED lighting, but also to run heating and cooling systems throughout the year. Photovoltaic cells are another obvious means of producing the energy required to run greenhouses.

Initial studies into LED lighting for plants commenced in the US in the mid 1980s, as part of a long term programme to develop plant growth systems for use in space stations. This was in the early days of LEDs, when only simple, but expensive, red devices were available. By the early 1990s, Philips had started working on LEDs in conjunction with research institutes and growers in the Netherlands, US and Japan, building on its existing knowledge using HPS lighting systems.

Around 2000, LED manufacturers started to resolve some of the high volume manufacturing issues to ensure that consistent and reliable, high output devices were produced that could be assembled into large arrays. Since then, the technology has advanced rapidly, with higher outputs and a broader range of specific wavelengths more appropriate for optimising plant growth.
Only recently has it been possible to deliver compact high lumen output LED modules containing specific wavelength (red and blue) LEDs at prices that allow a reasonable return on investment.

The biology bit
Academic studies have shown that plants demonstrate increased chlorophyll absorption and photosynthesis when exposed to red (640 to 660nm) and blue (400 to 500nm) light (see fig 1). Plant growth is strongly determined by the number of photons absorbed in the photosynthetically active radiation (PAR) region, which represents only 45% of the total light spectrum.

Photosynthetic photon flux density (PPFD) is a measure of the light energy, within the appropriate spectrum, which reaches the surface of the plants in a given area over a given time. PPFD is measured in mol/m2/day or µmol/m2/s. Using Lux or lumens as a measure of light output is regarded as inappropriate in horticultural applications, as these apply a human perception of light, which is heavily biased towards 555nm (green).

Similarly, lamp wattage merely indicates energy consumption and not how much useful light is generated. But this has not stopped vendors from promoting the advantages of 1W, 3W or 5W LEDs. Making energy efficiency comparisons, however, may require lamp wattage data to calculate light output for the same watt of input power on an equivalent area basis.

Light saturation point
Not all plants require the same amount or quality of light. The PPFD of sunlight on a clear day, reaching plants at sea level, is at around 2000µmol/m2/s – more than most plants can absorb – and research has shown that some plants fare worse with such high levels. Philips says bedding plants require at least 10 to 12mol/m2/day, while 4 to 6mol/m2/day is recommended for the propagation of cuttings.
Some plants, such as orchids, grow just as well with comparatively low daily light levels, while others are boosted with higher daily light levels, producing flowers earlier. Philips specifies the light output of its modular lighting solutions in µmol/m2/s, allowing growers to select the optimum size and combination of units for their greenhouse area and crop requirements.

US vendor Hydrogrow, working with the University of Washington, claims that 800 to 1500µmol/m2/s is optimal for large tomatoes, cucumbers and melons, while 100 to 300µmol/m2/s is enough for strawberries, small tomatoes and peppers.

Setting up a lighting system for a particular crop is complicated. Simply knowing the optimum PAR range of the crop is only one aspect. The optimum spectrum requirements differ across plant types and, in some cases, at different stages during a plant's life. The key advantage of LEDs is the ability to control spectral composition. In general terms, researchers say plants benefit from three times as much red light as blue, with red and far red critical for flowering. However, white LEDs are often incorporated into installations in which people work regularly and where it is important for crops to be seen from the outside.

New research is indicating that some plants benefit from additional wavelengths. Far red, for example, can help develop certain plant characteristics, such as longer stems. Responding to this mix of requirements, some vendors promote LED systems with six or seven wavelengths.

In addition, the length of the day can be varied; for some plants, researchers have found that extending the day to a constant 14hr has improved plant growth and stimulated earlier flowering. Chrysanthemums, conversely, need long nights before they start to flower. Recent trials have been experimenting with interrupted night time lighting, with varying results.

Light recipes
According to Philips, the ability to adjust the 'recipe' of wavelengths and light intensities to match particular plant types and growing phases is critical. It has developed 'recipes' dedicated to specific horticultural applications; systems designed to provide a specific light level and spectrum, dependent on positioning, timing (day length) and climatic conditions.

Other vendors promote the ability to tune and adjust the spectral composition of the modules they sell. Philips (among others) has also been experimenting with mixed lighting (HID or CFL and LED), with favourable results, thereby encouraging growers to transition to LEDs as older units need replacing.

Legislation is phasing out less energy efficient high intensity discharge (HID) models and there are problems of disposing of spent units containing mercury and other restricted elements. In any case, analysis of high pressure sodium (HPS) lamps has shown that much of the light output generated falls outside the peak absorption rates of chlorophyll. Osram OptoSemiconductor, a major supplier of LEDs for horticultural lighting, says only around 7% of the light created by HPS lights is being absorbed by plants.

Digital control
A major point in favour of LED based lighting systems is their solid state nature: LEDs are easily integrated into digital control systems, allowing both programmability and reconfigurability. Lights can be dimmed and brightened to better simulate dusk and dawn and the length of day can be adjusted. Some systems include daylight sensors, allowing supplemental lighting to be turned on or off when natural light levels change, saving more energy. Greenhouses can be zoned, each with separate lighting controls.

Another advantage of LEDs is that they can be positioned much closer to the plants. The ability to package LEDs in compact formats makes them suited to shelf lighting, allowing multilayer cultivation and better use of greenhouse space. Different shelves can have different light 'recipes' to suit subsequent plant phases in successional sowing, or for mixed crops. Bidirectional LED modules can be used as interlighting for tall plants, such as tomatoes, encouraging better photosynthesis of otherwise shaded foliage low down in the crop canopy.

In many cases, significant effort and energy is required to deal with the heat generated by traditional HID lighting, with fans or air conditioning systems, and careful monitoring of water requirements and humidity levels. While cool running LEDs simplify environmental control, some further consideration of heating requirements in winter may be needed.

But the combination of improved plant growth, reduced cost of ownership and a substantial body of evidence from research and trials in the academic and commercial sectors, is convincing growers worldwide that the case for LED based plant lighting systems has become compelling.

What LEDs bring
Advantages
• Lower power/energy efficient
• Longer bulb life
• Less heat generated, so less cooling required
• Less watering, easier humidity control
• Solid state, so easy to program and control
• Tunable to the optimum wavelength combination
• Compact fixtures mean better use of space
• Placement flexibility

Benefits
• Lower cost of ownership
• Increased yields, improved growth
• Longer growing season/more crops per year
• Better consistency and uniformity of crops, especially in winter
• Less chemical fertilisers required
• Colour balance triggers plant's defences against pests & diseases
• Less light pollution

Author
Louise Joselyn

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