Energy is a major expense for rose growers. At Arend Roses, in the Dutch Maasland, research is being carried out into the effect of a customised light spectrum, through means of LEDs, on yield and the possibilities of it saving energy. The findings are encouraging but the energy efficiency of LEDs needs to be improved further to make the purchase of a second lighting installation profitable.

Research at Arend Roses took place in the winter of 2014-2015 and was initiated by operations director, Richard van der Lans. “For several years we toyed with the idea of introducing hybrid lighting to become more energy efficient, and hopefully have positive effects on yield and quality,” he says. “When I spoke to LED manufacturer Valoya, I had the feeling they understood exactly what I wanted to achieve. Since then we have carried out small-scale trials using prototypes of new LED-lamps which last winter culminated in a larger project involving several partners.”
These partners are Plant Dynamics and Wageningen University & Research Greenhouse Horticulture, which leads the research project. Both have a lot of experience with complex crop measurements. Initially the rose grower wanted to manage the research himself but without the input from the Dutch knowledge centres he wouldn’t have received financial support from the Kas als Energiebron (Greenhouse as Energy Source) program. “With hindsight I am very glad we did have their input, quite apart from the financing,” says Van der Lans. “Thanks to their knowledge and expertise we carried out very thorough research with very accurate measurements. We wouldn’t have been able to achieve the same level of sophistication and quality ourselves. In addition their input and involvement led to interesting discussions which substantially deepened my knowledge on the subject.”

Less energy

Van der Lans had calculated that by adjusting the growth light he could over time use 25% less electricity during the cultivation cycle. He calculated this, his initial goal, based on the hypothesis that a specific, energy efficient LED-lamp that could supplement the PAR-spectrum of the SON-T-lights – in combination with daylight – would enable the plants to better use the available light for photosynthesis and growth. Even at a low light intensity this should lead to a modest increase in yield.
“The 25% saving using this type of lamp, which provides a much broader spectrum than normal LEDs, is currently a bridge too far,” he says. “The positive effects on early growth and crop production that we saw in our previous trials were confirmed here but they were less large than we hoped for. When more efficient LED-lamps become available – and that is only a question of time – more energy can be saved and an additional LED installation can be profitable. That is not the case yet.”

Higher light efficiency

What exactly did this research yield? Project leader Nieves García Victoria, of Wageningen University & Research explains: “We compared three different growth light concepts over three repetitions and measured the effects on production and quality, photosynthesis and morphological characteristics such as colour, leaf surface area and leaf position.”
The growth light variations were the currently used SON-T-installation of 191 μmol PAR/m2.s as the reference and hybrid setups of respectively 103 μmol SON-T + 57 μmol LED and 103 μmol SON-T + 103 μmol LED.
“The hypothesis that supplementary LED-light results in higher production was confirmed for both variations,” says Van der Lans. “Also, we harvested fewer A2-quality.” Sander Pot, of Plant Dynamics, adds: “Under 206 μmol of mixed light, the light utilisation efficiency of the crop, expressed in grams production per mol light received, was 9% higher compared with SON-T only. Even under 160 µmol mixed light the light utilisation efficiency was 7% higher than under SON-T. Therefore the plant is better able to convert the mixed light into growth.”

No good explanation

Van der Lans and the researchers say that the positive effect of the mixed light was visible in the crop development within just a few weeks. “We saw the effects and of course wanted to know exactly how it was caused,” says Pot. “However, despite all the measurements we saw no clear morphological differences. Also the photosynthetic activity was almost the same and that is strange; we measured higher production but couldn’t put our finger on exactly how and why.”

Break-even point not yet reached

Now that the positive effect of hybrid lighting using broad spectrum LEDs has been established and the trial has ended, Van der Lans says it is now a case of waiting for further development of a light that offers the desired level of efficiency (see box). “In order to earn back the cost of a second installation within a reasonable period of time it needs to be more efficient than the prototype that Valoya developed for this project,” he says.
The break-even point has not yet been reached but hopefully that will happen within a few years. “The requirements for light emissions are stricter and it would be good if we can use fewer or less heavy SON-T-lights under the screen and supplement that with LEDs. Then the heat surplus, that is difficult to get rid of under a closed screen, would be less. And the energy bill would of course be lower which was the reason for starting the project in the first place.”

‘Better crop development with broader LED-spectrum’

The trials used a new type of LED-light, the Valoya G1. These emit light in a wider range of the PAR-spectrum than the monochrome red and blue LEDs that have been in use for some time.
“The advantage of these, especially the monochrome red LEDs, is their highly efficient use of energy,” explains Gonçalo Moreira Neves, of light manufacturer Valoya. “However, such lights have a very narrow spectrum. The plant can use it but it is too one-sided. If the light balance is disturbed too many morphological abnormalities and other undesirable side effects can occur, as independent research has shown on several occasions.”
To reduce this danger and to enable more efficient hybrid lighting comprising a larger proportion of LED-light, the Finnish company developed LEDs that supply a broad spectrum to fully support crop development. The G1 is the resulting prototype.

Efficiency 60% improved

A limitation of the new type of light is the significantly lower energy efficiency compared with the monochrome red LEDs. Meanwhile the company’s technicians are working on a follow-up. Currently we are achieving 60% higher light output of 1.8 μmol PAR-light/watt, but it has to be and can be even better,” says the spokesperson. “In the course of next year we hope to be able to offer a marketable version that combines the desired qualitative properties with the desired efficiency.”


A trial using a new type of LED-light with a broader spectrum confirmed that when this is used in combination with SON-T, production of roses is higher than when using SON-T-lights only. The energy efficiency needs to be improved further to make an investment in such hybrid lighting cost-effective. This would be beneficial in terms of energy and cultivation because such installations under a closed screen generate a lower heat surplus than SON-T-installations with a similar light output.

Text and images: Jan van Staalduinen