Conference report: The growing pains of LED

Flowers being grown under high-pressure sodium lights in Aalsmeer, The Netherlands. Are LEDs ready to take over?

LEDs are widely touted as the next big thing for commercial horticulture, as trials suggest growing flowers and crops under LED lighting can improve colour and taste as well as boost production. So is it time for horticulture companies to be readying their cheque books to invest? The message from Lux’s recent horticulture conference is mixed. Yes, things are looking good – but life in the real world is never simple.

By Boys de Nijs


The Horticultural Lighting Conference, organised by Lux, took place at the High Tech Campus in Eindhoven on 23 May 2017. It gathered a mix of professionals concerned with horticultural lighting, ranging from manufacturers and developers of chips, semiconductors, lenses and other lighting technology to growers, researchers, breeders and horticultural consultants. 

Dr Phillip Davies of the Stockbridge Technology Centre warned that it is difficult to get lighting conditions just right. While LED spectra can modify the form, structure and chemical characteristics of plants in a way that enables growers to extract greater value, it is easy to get things wrong. 

Anja Dieleman of Wageningen University kicked off by explaining the role of photobiology in creating denser, healthier, greener crops of vegetables, fruit and flowers. High pressure sodium (HPS) has long been the source of choice for greenhouse growing. 

Osram’s Carolin Horst, having laid out the basic differences between HPS and LEDs, opined that this will not change in the short term, as a lot more research and development is required to improve the technical capacity and lifetime of LEDs. 

‘Single LEDs do not make a whole luminaire – you need the right power supply, drivers, cooler, optics, casing, PCB, etcetera,’ she noted, before acknowledging that LEDs do have many potential advantages over HPS that will make them a more efficient and flexible lighting source in the future.

Stefaan Fabri of the Belgian Proefstation (a research centre supporting Flemish vegetable production) presented the results of trials with a Merlice TOV tomato crop in different LED and HPS settings. The trials compared the production of tomatoes under Lemnis Oreon LEDs and HPS Son-T lighting, with varying light output levels. The LED settings increased production by at least 25 per cent, while the HPS crops showed more stress. 

Fabri emphasised, however, that the results do not justify immediate investment in LEDs. Other factors need to be considered. ‘LEDs might result in 30 per cent lower electricity costs, but at the same time extra heat is required,’ he said. ‘That is why each situation is different, as success also depends on the local cost of energy, gas, and crop type.’

He was backed by his colleague Isabel Vandevelde, who had run similar trials on lettuces. She found that three per cent more natural gas was required to maintain the same temperature inside the LED greenhouse as in the HPS setting. However, in four years of trials using Philips Greenpower LED toplights on Salatrio lettuce types, the crops were heavier, more colourful and compact, creating a quality advantage. Under LEDs Lollo Rossa lettuces grew faster and also showed better colour, while Red Oakleaf lettuce had less tipburn (discoloured leaf edges).

Phillip Davies of the Stockbridge Technology Centre warned that it is difficult to get lighting conditions just right. While LED spectra can modify the form, structure and chemical characteristics of plants in a way that enables growers to extract greater value, it is easy to get things wrong. ‘More light is not always better, for example. More light means more growth, but also means more cost, and does not always give a better result.’

Different crops require different lighting strategies, he explained. Bell pepper plants are hard to grow with supplemental lights, for example. Blue and red ratios affect plants, such as cucumber, differently. Translating knowledge into practice means looking at all the key factors, including CO2, media and temperature. ‘It’s about the complete picture, not just the light factor.’

Joris Van Lommel of the Belgian Proefstation looked at the effects of different LED spectra in hydroponic greenhouse lettuce. More light produces a heavier crop, he concluded, but also increases the risk of tipburn. More light also requires a higher investment without necessarily increasing production throughout the season. For example, using LED lighting in March is less beneficial than using it during the winter. Calculating the return on investment is therefore tricky and depends on factors such as time of year and variety of crop.

His colleague, Liesbet van Herck, noted that bell peppers are difficult to grow under LED lights, with variety playing a crucial role. In her trials with five varieties, four produced a reduced crop under LEDs. Only the Kaite variety did better, delivering a 27 per cent increase. This, she explained, was because many bell pepper varieties lack the vigour to thrive throughout the season.

Belgian tomato grower Den Boschkant has already implemented LEDs in its large-scale commercial greenhouse operations. The company’s Marja van Dessel said that in spite of initial doubts, a decision has been taken to install more Philips LEDs. She, too, warns of the pitfalls in translating knowledge into practice. ‘In research studies they have grown 100kg of tomatoes with LEDs, but those were very big, ugly, tasteless tomatoes of a very bad quality. This is not how it works in practice. We need to collaborate more with researchers to discuss more practical applications of new technology to prevent these kinds of situations.’

Towards the end of the day, Dr Jaimin Patel of the Lighting Research Center at Rensselaer Polytechnic Institute spoke about the role of light in controlling plant diseases and pathogens. Once again, his message can be summed up as one of wait and see. Commenting on the use of UV-B and UV-C lighting, and the effect of shorter and longer exposure to it, he noted that a UV-B LED does not offer the same output as a UV-B fluorescent light, and that current LED technology is not up to scratch when it comes to pathogen control. More research is needed.