“It’s all about the right balance between yield and fruit weight”

“It’s all about the right balance between yield and fruit weight”

Grow light is gaining ground among high-wire cucumber growers. Reijm & Zn in Berkel en Rodenrijs in the west of the Netherlands are among the many who have recently ventured into lit cultivation. “Lots of colleagues are exploring grow light in cucumbers,” explains Reijm & Zn partner Jan Reijm. “Most people know by now what SON-T can do. We want to learn from a new concept with vertical LED lamps. It has lots of advantages, but there are limitations as well.”

The system that Reijm has been testing on a small scale since the end of 2016 was supplied by the Dutch company Lohuis Lighting & Energy in Naaldwijk, which has been specialising in LED lighting for many years. What catches your eye straight away is that the Saturnus Veg LED lamps are not suspended above the crop in horizontal fittings or strips but in vertical tubes hanging between the plants.

Representative and lighting advisor René Grootscholte: “A huge amount of research has been done into assimilation lighting with LED lamps over the past fifteen years. Reports from research institutions in the Netherlands and Belgium clearly show that vertical lighting concepts in climbing crops such as fruiting vegetables produce the highest lighting yield. And that’s quite logical: the PAR light emitted by lamps above the crop is intercepted by the top leaves, so the leaves half way down the canopy get very little benefit from the higher light levels. The middle of the plant also gets much less daylight for the same reason. Our lights transmit their light over a height of 1.15 metres, which is where it really benefits the crop most: not above but just below the top parts of the crop.”

More flexible

Reijm & Zn in Berkel en Rodenrijs have been growing two high-wire crops per year on 4.5 hectares for four years now and were keen to test the new concept. “Many colleagues are exploring grow light in cucumbers,” Reijm says. “SON-T has been widely used for some time and we all know by now what it can do. The heat radiated from these lights prevents a number of potential problems in the crop, including Mycosphaerella in the tops. You don’t get that heat with LED lamps. On the other hand, LEDs are more flexible and can be used over a longer period in the year. There are often times when you want a bit of extra light without the extra heat.”

Another striking feature of these vertical LEDs is that they come with two settings with different light spectra. Besides the setting with the usual red/blue spectrum, there is a separate setting that only provides far-red light which has a similar effect to natural twilight, Grootscholte says. “This setting is mainly designed to light the crop with far-red light for an hour longer in the evening after you’ve switched off the standard lights – just as you’d get in nature, really. This activates the phytochrome and gets extra assimilates going to the fruits. In essence, you’re inducing the transition to the generative state.”

Attractive increase in yields

“We’re keen to learn from this vertical lamp with two settings,” Reijm says. “I’m interested to find out how far you can get with this kind of system. For us, growing under grow light is vital if we are to continue to compete against Spanish growers. The gain – or rather a better return per square metre – needs to come from better quality and better fruit colour in the darker months, as well as an attractive increase in yields. After all, it involves a considerable investment which you want to recoup within a reasonable period of time.”

Before planting their first artificially lit crop (28 December 2016), they installed lamps giving around 85 μmol/m2/s of PAR light at 55 cm intervals in each plant row in the 540 m2 trial section. That worked out at more than one lamp per square metre and about 95 μmol extra grow light, Grootscholte says. “We have just released an improved version that gives eight percent more light using the same amount of power.”

Higher light levels and more effective use of light by the crop deliver higher yields in various ways. “You can get more plants or stems per square metre or more fruits per plant,” Reijm says. “It’s all about getting the right balance between higher yields and fruit weight. We aim for regular production and an average fruit weight of around 420 grams.”

Results of the first crop

In their first cucumber crop under grow light, Reijm kept the stem density the same as in the unlit crop. This was originally 1.5 stems per square metre and was doubled later to three per square metre.

The plant load was different, however, as the grower explains: “We thin out every second fruit in the unlit crop. Because the plants can cope with more under grow light, we left a set of two fruits on the plant each time before removing the next one. So we ended up with roughly 25 percent more cucumbers on the plant, but in retrospect that was too many.”

The fruit weight was lower and because the plant load was too high, production was less regular. “We noticed a drop in growth rate quite early on, so we waited a while before leaving a second stem to grow. That meant that production lagged slightly behind the reference crop in the first phase, which wasn’t what we wanted, of course. Ultimately it more than made up for that, but it was obvious that there was room for improvement at the start of the crop. We were very happy with the colour of the fruits.”

Second crop

The second crop was planted in mid-June. This time the young entrepreneur opted for a slightly higher stem density of three stems per square metre, compared with 2.5 in the unlit crop. “We thinned the fruits in the same way – alternately, in other words,” he says.

Because the crop was planted just around the longest day, in a period in which no artificial lighting was used, the fruit weight in the trial section was also slightly lower than usual to begin with. Reijm: “By mid-August we had cut 58 cucumbers of 408 grams per square metre in the unlit crop, compared with 67 of 375 grams in the lit crop. The LED system was on regularly from mid-June onwards. It clocked up quite a few hours, particularly in the first half of August, and you could see that quite clearly from the crop. It was healthy and strong and the fruits were nicer as well.”

Early days

With a few more months to go at the time of writing, the grower didn’t want to draw any firm conclusions just yet. “I know roughly what I can expect from a grow light system,” he says. Depending on the specifications and price and with two crops a year, you should be able to cut around 90 fruits more per square metre. “We’re not there yet, but that’s no bad thing. This is a year for learning and we have certainly learnt a lot already. We will also be keeping a close eye on how other growers are getting on. I’m still convinced that we can make headway with this system, and we’re certainly open to that.”


A new LED lamp hanging vertically in the crop enables climbing crops such as fruiting vegetables to get more out of the radiated light over a larger height than systems in which the lamps are positioned horizontally. The lamp has two settings with different light spectra. In addition to the usual setting with red and blue light, there is a setting for extra lighting with far-red light only.

Text and images: Jan van Staalduinen.


Advance of LEDs seems unstoppable despite critical noises

Advance of LEDs seems unstoppable despite critical noises

What does it say about a technical development in greenhouse horticulture when there is a marked increase in both research projects and practical trials? What can you conclude from the surge in the number of specialist suppliers in the market? How revealing is it that growers are willing to make significant investments based on a practical trial at a colleague’s nursery?

All this tells us that the development in question is clearly very special – one that promises to boost yields and quality while at the same time cutting costs. What other explanation could there be for such a thirst for yet another exciting advance in crop cultivation?

Besides energy, plant health and water, light is the fourth hot topic for innovative, pioneering greenhouse growers. Recent research and practical trials in cut flowers, greenhouse vegetables and pot plants demonstrate the added value that diffuse light with glass, coatings or screens delivers. Artificial lighting is the other aspect to this topic, and LEDs are the most attention-grabbing innovation in this area. That’s why light takes centre stage in this issue of In Greenhouses, with the focus on innovations, research and growers’ experiences of LEDs.

It has been many years since this latest type of lighting was introduced into greenhouse horticulture. Following its much-vaunted launch, a few leading Dutch growers took the plunge, keen to make their mark as early adopters. But sometimes it’s no bad thing to wait a while. This particular development bided its time and has only made real advances in recent years. And this despite some critical noises from researchers and consultants: Have we found the right spectrum for the various crop groups? Do we have enough understanding of what light colours actually do and how they impact on plants and plant processes?
The research portfolios in several horticultural countries are crammed with LED projects in various crops. Add in the practical trials that new suppliers have initiated in the sector and you can’t help but conclude that LEDs are the future – even if there are still some key questions to be answered.

Text: Roger Abbenhuis.

Direct Current highly promising alternative in horticulture

Direct Current highly promising alternative in horticulture

The use of Direct Current in greenhouse horticulture appears to be a very promising alternative. A pilot in the greenhouse horticulture sector demonstrated a positive business case for the use of Direct Current (DC) for greater durability of components, as well as cost and material savings. DC also supports the idea of climate-neutral greenhouse horticulture, as demonstrated in the Direct Current Roadmap.

The DC Roadmap, presented last Friday, is a report compiled by Berenschot at the order of RVO.nl for the Energy Top Sector and TKI Urban Energy. This DC Roadmap focuses on ‘DC microgrids’ and seven specific areas of application. A microgrid is defined as follows: ‘a system of interconnected sources and users that can operate, either independently or linked, on a higher-level grid and can exchange energy’.

Greenhouse horticulture comprises a DC microgrid

The various DC microgrids are, with respect to the innovation phase, at the beginning of the S curve: there is a great deal of uncertainty and there are numerous, divergent opinions and ideas about the value (social or otherwise) of DC microgrids. The report, however, revealed that DC is highly promising in greenhouse horticulture; only second to the market for public lighting. The reporters visited greenhouses whose entire indoor electrical system is set to DC. In this, a single, centralised AC to DC transformer is used, to which a lighting system with DC light fixtures (SON-T or LED) and in some cases a CHP unit is connected.

Advantages of DC in comparison to AC

The use of DC in greenhouses extends the life of the light fixtures. Using thin film condensers instead of electrolytic condensers allows greenhouse growers to opt for components with a longer useful life. In addition to this, material savings can be achieved because a DC system uses cables that are smaller in diameter, which therefore require less copper. Researchers also reported that DC makes the integration and control of systems easier. It enables light fixtures to be dimmed individually because the DC cabling simultaneously allows for the control of lighting (powerline communication). Lastly, the centralised conversion of AC to DC will ensure that less energy is lost in comparison to local conversion per lamp (2 – 3%) at the start of operations.

Rounding off the pilot phase

The Roadmap predicts that the pilot phase for using DC in greenhouse horticulture will be rounded off soon. Sustained growth is possible due to the increasing demand for sensors and PV systems. The first successful pilot was completed in the Netherlands and demonstrated a positive business case. This pilot is being conducted at the Jaap Vreeken bouvardia nursery. The pilot is currently being continued at a larger scale.

Conducive to LED systems

Newly built or renovated greenhouses can now also be fitted with DC electrical systems. This applies primarily to nurseries with DC-fed SON-T or LED (in the near future) light fixtures. It is anticipated that using DC will also decrease the costs of LED systems. In the future, priority will be attached to the use of PV panels and the integration of smart innovations (such as controllable light fixtures and smart sensors) in greenhouse horticulture. The integration of these technologies can strengthen the benefits of a DC microgrid.


New LED lamp rates highly in hybrid lighting trial

New LED lamp rates highly in hybrid lighting trial

A trial with hybrid lighting (SON-T + LED) at Dutch tomato nursery Gebroeders Koot has yielded good results. The LED lamp used in the trial, which was developed on British soil with Dutch input, offers several advantages. One stand-out benefit is its clever design which makes it easy to integrate into existing SON-T installations.

Yields up by more than nine percent after seven months (weeks 48-26). That was the auspicious outcome of a greenhouse trial at Prominent growers Gebroeders Koot in Poeldijk, the Netherlands, where a tomato crop grown under 150 μmol/m2/sec SON-T grow light was compared with an identical crop supplemented with 58 μmol deep red with a little blue LED light. Geert Koot, who had had no previous experience in growing under grow light, was very impressed. “I hadn’t expected the higher light level to make such a difference,” he says. “That will appeal to a lot of growers. The same goes for the lamp itself, which has a surprisingly simple design. It’s fully interchangeable with SON-T, so it fits seamlessly into an existing system.”
“A lot of thought has gone into the functional design,” cultivation specialist Maarten Klein adds. He and his assistant, Tim Valstar, oversaw the trial, which was run on behalf of the British LED manufacturer Plessey. Klein, who has had a lot of experience with grow light, developed this lamp in collaboration with the technology company.

Smarter design

“Most LED systems are difficult if not impossible to integrate into existing lighting installations,” Klein continues. “Growers looking to switch to hybrid lighting currently have to install a whole new system alongside their existing one, often with extra C profiles. That pushes up the cost and results in more light interception, which causes problems all year round. Plessey Semiconductors in Plymouth wanted to eliminate these problems.”
To test the practical value of the lamp in the greenhouse setting, Klein approached several Dutch nurseries. In addition to Gebroeders Koot, trial setups were installed at nearby alstroemeria and gerbera growers and a pot plant nursery.

Trial setup

Although Gebroeders Koot were not growing tomatoes under artificial lighting, they did have a SON-T system in place in a section that had previously been let to another grower. These 1000W lamps supplied 151 μmol/m2/s extra grow light and, of course, the usual radiated heat. LED lamps were added in one bay, ramping up the artificial light level to 209 μmol.
Tim Valstar assisted with the trial and, together with Geert Koot, took measurements in the trial and reference sections. All the relevant crop and fruit features of the variety grown, Brioso, were recorded, varying from growth rate and stem thickness to leaf size, leaf colour, fruit weight and Brix value.


The plants arrived in the greenhouse in week 46. “That’s later than the usual for an artificially lit Brioso crop – they would usually go in in mid-October – but the lighting period was long enough to get a reliable impression of any differences,” Koot says. “The plants developed well in both light environments. But the plants under the higher light level were that little bit stronger with slightly thicker stems and more dark green leaves.”
Due to the extra vigour, the plants under the hybrid lighting regime held the first trusses for longer and they were harvested a few days later than those in the reference sections. The higher yield potential quickly expressed itself in a higher average fruit weight. To maintain the desired fineness, one fruit more was kept on the truss (11 instead of 10) from the tenth truss onwards, without the plants forfeiting vigour.
Valstar: “After week 26 we stopped taking measurements and were able to take stock.” The harvest under the hybrid lighting regime was 38.32 kg per m2 compared with 35.04 kg under SON-T. That represents an increase in yield of 9.35%. The average fruit weight was also slightly higher than under SON-T, at 39.2 grams compared with 38.8 grams.

Flexible use

The attractive increase in yield can’t be ascribed solely to the higher light levels in the periods when both systems were in use. The SON-T system was switched off and the CHP unit shut down for maintenance at the beginning of week 19, whereas the LED system was used from 4 am to 7 am for a further three weeks.
“The option to only use the LED lamps either end of the lighting season would be an extra benefit,” Klein says. “Those are often the times when you don’t need the radiated heat produced by the SON-T lamps. LEDs have virtually no impact on the climate. You can always switch them on if you need more grow light. And because they are much more energy-efficient than SON-T lamps, you also have more flexibility when it comes to deciding whether to generate the energy yourself with CHP.”

375 and 600W

Klein is keen to point out that the prototype trialled at Gebroeders Koot was developed exclusively for research purposes. But the lamp has since undergone further development and a commercial 375W version was launched at IPM 2017. All the LEDs are now in one bay and the fitting, which has integrated cooling ribs, can be attached directly to the trellis.
The lamp is called Hyperion 1000 because it has a photon flux of 1000 μmol/s. “Because of the higher uptake of deep red light, it’s the equivalent of a 600W SON-T lamp but it uses 40 percent less electricity,” the cultivation specialist says. “The producer has also recently brought out a more powerful 600W version which is the equivalent of a 1000W SON-T lamp.”

Ten years ago

There is a lot of added value in the new lamp, Koot believes. “It’s efficient, it has a broad spectrum, and its clever design makes it easy to incorporate into an existing system. That will appeal to a lot of growers. I’m also quite impressed. But because of my age and the fact that I have no successor in place, I have decided not to invest in any more grow lights now. If this trial had taken place ten years ago, I would almost certainly have gone for them. But we very much enjoyed taking part in the trial.”


A new type of LED lamp produced in the UK is achieving interesting results. The clever design makes the lamp particularly attractive. It can be attached to the trellis without the use of C profiles and can be integrated into existing 600W SON-T systems with standard connectors. A more powerful version equivalent to a 1000W SON-T lamp was brought out earlier this year.

Text and images: Jan van Staalduinen.


Thinking about light in new project

Thinking about light in new project

Plants, insects, fungi and people perceive light colour and intensity via different organs and pigments.
The human eye is particularly sensitive to green light, while plants have various pigments that absorb light and control different processes. Insects are sensitive to light in a different way again. The advent of LED technology, with a wide range of light colours to choose from, opens up new opportunities for use in greenhouse horticulture.
But which combination of light colours is needed for optimum plant growth and development, and what effect does adding LED to the sunlight and high-pressure sodium spectrum have? Does using LED lighting on its own produce other reactions in the crop? And what does this mean in terms of plant cultivation cells in urban farming? Does growing plants using only LED lighting enable you to produce vegetables and flowers without using gas (i.e. fully electric)? These are just some of the questions that arise when considering the ways in which LED lighting could be used. The Denkkader Licht (Thinking about Light) project looks at these opportunities and uses.