Home Posts Tagged "LED-lighting"


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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.

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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.

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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.

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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.

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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.

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Light plays an important role in demanding crop science research applications. A lot of research requires accurate replication of real-time outdoor light conditions to achieve different goals.

But it is impossible to replicate those conditions to the extent to which data collected indoors would be relevant. LightDNA was created to solve this problem. The system consists of Valoya’s latest technology: the 8-Channel Light, a high-power LED fixture with eight channels of light, an internet connected microcomputer and both local and cloud-based software for processing the light data.

Optimised LED configuration

The configuration of LEDs is optimised to meet outdoor light conditions with 90% or higher accuracy (380-780 nm range).
Other advantages are: light-weight, compact design suitable for various applications; strong light intensity up to 2000 µmol/m2/s with uniform light output; low heat emission with active cooling system and energy saving.
Stand number: 12.424

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Lohuis Lighting vertical interlighting is a unique design that emits red, blue and far red light. Light is used very efficiently. Trials with Apollo interlighting showed a 100% increase in yields in the first three months of the year.

The Apollo can be combined with the Venus I top lighting strip. Since it radiates little heat, this strip light can be used near the plants so there is no loss of light into the aisles. In tomato and cucumber crops, LED is now not only more effective but also cheaper than conventional HPS lights.

Vertical interlighting

With this new LED interlighting, nurseries can reduce their electricity consumption by approximately 50% compared to HPS lighting. Since LED produces less heat, less CO2 is lost through ventilation to get rid of excessive heat. Water consumption is also slightly lower because the plants transpire less in a cooler environment.

Stand number: 10.114

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The LED lamps in the light fittings underneath the top growing layer shine brightly on the plants in the cultivation greenhouse at phalaenopsis growers De Vreede in Bleiswijk in the west of the Netherlands. The light may look white but actually it’s the right combination of colours. It’s one of the innovations that brothers Herman and John de Vreede are working on as part of their drive to supply large volumes of uniform quality orchids more sustainably. They did most of the preliminary research into the right light spectrum themselves.

The phalaenopsis nursery moved to Bleiswijk in 1995. The brothers soon bought the nursery next door and then another two sites 300 metres and 2 kilometres away, making a total of 12.5 hectares of growing space. Each of the sites is equipped for a specific purpose.
The cultivation greenhouse, where the plants spend their first 35 weeks, is heated to a temperature of 28ºC. Then they move to the spike induction site, where they stay until about week 55. Here the plants start off warm and after a few weeks the temperature is reduced to 19ºC to induce flowering. In this phase, the plants are spaced wider apart, staked and sorted by flower size, colour and number of buds. Finally, they are transferred to the finishing site for three to four weeks. Orders are packed and shipped from there.

Large volumes

De Vreede produces 12 million plants per year. Even Herman de Vreede finds it hard to get his head around those numbers. A massive 200,000 young tissue culture plants arrive from various locations every week and leave the nursery again as adult plants more than a year later.
De Vreede specialises in eight outstanding orchids – exclusive varieties with a long life span and offering great value for money. They come from two breeders, with most of their stock supplied by Anthura. “We test about 30 varieties a year, including from other breeders. We want to keep up with the latest innovations.”
The brothers work with large volumes. “We are equipped to fulfil orders of 500,000 units at a time. The biggest challenge for us is getting all the plants to the same stage at the right time. Much of what we do is automated now. Soon we plan to install industrial Fanuc robots which will enable us to respond even more efficiently to market demand.”

Sustainable lighting solution

Orders arrive in peaks. “We supply more than half of our annual production in the first five months of the year,” de Vreede says. “There are a lot of special occasions like Women’s Day and Mother’s Day at that time of year. To accommodate peak production we decided to install a second growing layer above part of the cultivation greenhouse. We now have four hectares of growing space there instead of three. That helps make the crop more sustainable to grow because we’re maximising our space.”
It wasn’t practical to install a second growing layer directly above the original one, either in terms of climate or air circulation. So the brothers decided to put in a second layer along the sides of the three cultivation areas. It is relatively low, just 1.5 metres above the bottom layer. Lighting is needed to make up for the lack of daylight. The standard lighting with SON-T lamps used elsewhere in the nursery can’t be used here.
“There are SON-T lights above this part, but with 600W output, slightly less than the 1000W from the other lamps we use,” Herman de Vreede says. “We went with LED grow lights for the bottom layer. Not only because they generate less heat, but also because they are a sustainable solution. They use less energy and you can choose a particular combination of light colours.”

Three years of tests

At the time there was no such thing as a standard solution. So before they started building in October 2016, they ran tests over a three-year period to see which light spectrum produced the best results. “We tested the effect of different light spectra on properties such as development rate, root development and the hardiness of the plant, both inside and outside the nursery. A lot of knowledge is needed for that, as you have to see what the best result is for each situation. The light spectrum that is most suitable for the vegetative phase of phalaenopsis is not necessarily the right one for the spike induction phase, for example.”
The tests in the nursery were overseen by Simone de Vreede, who had gained a lot of experience in this area and carried out research at her parents’ nursery while still at university. Once they had decided on the light spectrum they wanted, the next step was to find out where to source the lights from. Ultimately they chose Philips GreenPower LED top lighting, which fitted the bill nicely. The lights give out light that looks white. The advantage of this is that it makes it easier to visually inspect the plants being grown in the greenhouse.

More stable climate

“Installing a second growing layer blocked out the daylight from the bottom layer,” says Stefan Hendriks of Philips. “They couldn’t use SON-T because of the short distance between the crop and the lamps: they would generate too much heat. With LED you can create a controllable climate in which phalaenopsis can be grown very efficiently with relatively little light.”
Since the second growing layer was installed in October 2016, the plant specialist has been visiting the nursery every two weeks to carry out analyses and take crop measurements, including length, leaf splitting and dry matter concentrations. In addition, the climate is intensively monitored by means of PAR, temperature and humidity sensors. These observations are linked to the climate data from the computer. “Based on this data, we want to fine-tune the use of the lamps and optimise our cultivation even further. Experience and knowledge are essential when using LEDs. That’s why we carry out a lot of in-depth analyses here,” says Hendriks.


The phalaenopsis grower is also considering buying in LED lights for the other sections when the time comes to replace the SON-T lamps there. Hendriks adds: “Besides being more energy-efficient, LEDs last longer. The life span of the models we use is given as L90. That means that after 25,000 hours of operation, the light output is still 90% of the original level. But the module will still go on working fine after that and will have many burning hours left in it.”
At De Vreede the lamps will probably wear out sooner than that, due to the number of hours they operate. With 14 hours of lighting a day, they are in use for 5,110 hours a year. But that also means that the LED lighting in the new no-daylight situation will pay for itself more quickly.


Dutch phalaenopsis growers De Vreede have 12.5 hectares divided into cultivation, spike induction and finishing sites. In order to have enough growing space available at peak times, they invested in a second growing layer above part of their cultivation area. To light the bottom layer, now in shade, they installed LED lighting with the right light spectrum for the vegetative phase, having first done their own in-situ research into which spectrum to use.

Text and images: Marleen Arkesteijn.

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Hoogenboom Alstroemeria in Nieuwe Wetering in the Netherlands expanded its grow light system last year. In one section of the greenhouse they have installed LED lights in between the rows of controllable SON-T lights, ramping up the light output there by 50% to 150 μmol/m2/s. “You don’t always need the radiated heat that SON-T lights produce. We can light our crops more intensively and more flexibly with this hybrid system,” partner Dick Hoogenboom explains. “It produces visible results.”

Brothers André and Dick Hoogenboom have been growing alstroemeria in their 2.9 hectare greenhouse in Nieuwe Wetering since 1985. Their aim is to produce the most beautiful, strongest and healthiest cut flowers. “It takes a long time to establish a good reputation, but once you have it, you can lose it again very quickly,” grower Dick says. “We have to work hard to stay ahead of the field because the competition never stands still.”
Grow light plays a key role in the battle for quality. The brothers installed their first lighting system in the late 1990s, with 600 W SON-T lights producing 4000 lux or 50 μmol/m2/s of PAR light.
“We increased the output every time we replaced or renovated something,” the grower explains. “We have had around 100 μmol for eight years now. Five years ago we upgraded our entire power system and we replaced the 600 W bulbs with 1000 W ones, which we can either have running on full or half power.”


The growers wanted to up their lighting capacity yet again last year. First they had to decide whether to go for extra SON-T lights or install a second, LED-based system.
Hoogenboom: “The easiest and cheapest option would have been to put in extra SON-T lights. But we didn’t need even more radiated heat so we opted for the more expensive LED system. We deliberately went for Phillips lights; they weren’t the cheapest but we have the most confidence in the quality, reliability and after-sales service you get from this supplier.”

Lighting strategy

LED lights don’t radiate heat onto the crop and can be used whenever additional grow light is needed. Hoogenboom aims for a day length of 16 hours. Between 1 August and 1 April the daylight is always extended with artificial light (and radiated heat) from the SON-T system. At the beginning and end of this period the days are usually long and light enough for them to manage with a light intensity of 50 μmol.
“We switch the lights on full power on around 1 October,” the grower adds. “Now that we also have LEDs in the greenhouse, I can decide which system to switch on first. If the temperature is still warm enough, we will use the LED system. I have often used the LED lights at times when the lighting in the section that only has SON-T was left switched off because it would otherwise have got too hot there. You can get rid of excess heat by opening the vents, of course, but that costs you CO2 as well, which impacts on growth.”
During the winter and on chilly spring days, the grower switches the SON-T system on first because the radiated heat is very useful. At that time of year the LEDs are added in to ramp up the light level to 150 μmol when necessary.

Noticeably higher yields

The grower is convinced that there is scope to get even more out of his hybrid lighting system by working with crop adviser Marco de Groot and tweaking the climate. Although the new LED system has only been in use since November last year and at the time of writing he has yet to experience using it between 1 August and 1 November, he is happy with the results so far.
“Under the higher light level we used from 1 November onwards, the crop has been producing noticeably thicker, firmer and stronger stems,” Hoogenboom says. “What’s more, yields are quite a lot higher. Depending on the variety, by 1 April we were cutting between 30 and 50 stems more per square metre than usual.”

More watering

Hoogenboom also noticed less bud dehydration and deformation in the greenhouse with the higher light levels. On the other hand, he has noticed that more light means more watering. “The crop is more active so it transpires more. I didn’t really take that on board to begin with. But there was a leaking tap in one bay so it was wetter there than in the other irrigation sections for some time, and in the spring we noticed that that crop was fuller and was producing more flower spikes. So I’m convinced there is even more room for improvement. That makes us even more confident of a positive return on our investment.”


Alstroemeria can get off to a good start with higher light levels than are generally used at present. SON-T lights also emit radiated heat and meet the plant’s basic needs. If you don’t need extra heat at the top of the plant, LED lighting can boost light levels in the winter months, which translates into more and stronger flower spikes and less bud dehydration and deformation.

“You still need to do the maths”

Crop adviser Marco de Groot is keeping a close eye on the developments with grow light in alstroemeria. He helped the Hoogenboom brothers do the maths to underpin their investment decisions.
“Lighting levels in many ornamental crops continue to rise,” he says. “Alstroemeria is no exception. And that has to happen if our growers are to compete with Africa, where alstroemeria cultivation is expanding rapidly. More light is good for the quality of the crop, yields and more regular workforce deployment between October and June. But you do need to take a critical look at where you’re coming from, where you want to go and the best way to get there.”

The adviser continues: “For growers currently working with low light levels, a heavier-duty SON-T system would be the best choice. If you are already using slightly more light and don’t need extra heat, you should rather opt for LED. Don’t forget that unlike rose and gerbera, photosynthesis in alstroemeria has a response time of around 30 minutes. LED light doesn’t increase the leaf temperature, unlike SON-T light. If the leaf temperature is too low, that can delay the response time even more. Extra LED light is only effective when the basic level is covered by SON-T light.”
In addition, the light response varies from variety to variety because of the strong genetic variations. Virginia can use extra light efficiently whereas other varieties often respond more slowly. “It’s important to take these aspects into account,” De Groot concludes.

More growers gaining experience

According to plant specialist Stefan Hendriks of Philips, a good number of ornamental growers are gaining experience with hybrid lighting or 100% LED. “We are seeing a growing trend towards LED in the market as growers become more and more aware of the advantages. But it is important to bear in mind that it’s not simply a question of installing LEDs and carrying on as usual. You also have to control the climate differently. LED lighting provides you with an extra tool because you can control heat and light separately, which you can’t do with SON-T.” In addition to research in vegetable crops, Phillips is also working with ornamental crops such as rose, gerbera, alstroemeria, chrysanthemum and phalaenopsis.
More and more growers are realising that LEDs deliver benefits for the crop, quite apart from the fact that you can’t always increase light levels with SON-T in some crops because of the excess heat they produce, Hendriks says. In practice, you can get more lighting hours out of LED than with the same crop under SON-T. “This boosts yields and quality. The use of LED is growing, and with increasing success. This trend looks set to continue and we expect that LED systems will most likely take over from SON-T completely in the longer term.”

Text and images: Jan van Staalduinen.

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There’s a lot of work going on into LED lighting at the moment. We already know from past research that light colour impacts on plant processes. The next step is to develop dynamic light recipes with the light colour adjusted to meet the needs of the plant at different times of the day or the growth phase. Last year saw researchers conduct the first trials with a dynamic light recipe in a semi-practical setting. Tomato plants were given blue or green light for part of the day and red light for the rest of the time.

This research was conducted in the context of a wide-ranging, innovative EU research project, the HI-LED project, which focused on three areas of application for LEDs: the use of light and light colour in the workplace, in museums and in greenhouse horticulture. The project saw the development of new lighting systems which can be controlled to provide the required light colour at any time. The four-year research project was launched on 1 December 2012 and has since ended.
Anja Dieleman was the project manager for greenhouse horticulture. Wageningen University & Research in the Netherlands worked on the project alongside Hortilux Schréder and the Spanish research institute IRTA. “IRTA looked at the effects of light colour on fruit quality, we studied the effect on the whole crop, and Hortilux supplied the specially produced lamps. When you use LEDs in museums or in the workplace, you only need one or two lamps. Cost and effectiveness also play an important role in greenhouse horticulture.”

Exploratory phase

The process started with the question: what does light colour do to plants? Dieleman explains: “Most of the research took place in climate chambers. The effect of red light there is also the effect of the absence of blue light, for example. In the greenhouse, the background is just sunlight.”
The exploratory trials with young sweet pepper and tomato plants, which have been reported on previously, looked at the effect of red, blue, amber, green and a combination of red and blue on the crop compared with white light as a reference. All six were against a background of natural daylight.
“The plants with red and amber light produced the same picture as the plants under the white reference light. The plants under blue light had shorter, smaller leaves, were darker green in colour and had higher chlorophyll levels. The measurements showed that photosynthesis increased in these plants once they were no longer under blue light. The upshot of this could be that plants that have been lit with blue light for a certain length of time process light more efficiently for the rest of the day. The plants that were under green light were more elongated and had a more open leaf structure. They looked similar to the plants that were under far-red light. You could make use of this in some way to improve light interception, at the start of the crop, for example,” Dieleman sums up.

Blue and green steering light

The basic trial with young plants and other trials with LEDs only give us a glimpse of the future. There are dozens of potential permutations for trials with different combinations of light colour, light intensity and times, and it was up to the scientists to make the right choices for the rest of the project.
In a preliminary trial, therefore, they first ran small-scale tests looking at the effects of blue light, which impacts on photosynthesis, and green light, which impacts on the shape and light interception of the crop. “In one series of plants we looked at the effects of green light at different times of the day. In terms of elongation it made no difference when we gave them a period of green light. Another series of plants was given different intensities of blue light (20, 100 and 200 μmol) at the same time of day. Even the lowest intensity seemed to have an effect. This means you can use blue light as a steering light to increase photosynthesis.”

Choice of lamp

There was no more time for preliminary research, as the start of the semi-practical greenhouse trial coincided with the normal time for artificially lit tomato plants and the researchers had to get their requirements to the lighting supplier beforehand. The supplier made the light fittings specially for the trial. It was decided to use LED fittings that gave green, blue or red light and were dimmable so that they could be fine-tuned for the trial. “We had the 0 series of these fittings; the first commercial series is available on the market in the meantime,” the project manager reflects.
The greenhouse trial with Komeett tomatoes ran from November 2015 to May 2016. “We had four 70 m2 greenhouse compartments at our disposal.” In the first compartment, the plants received 85 μmol/m2/s blue light for the first three hours in the morning, followed by 220 μmol/m2/s red light. The same was done in the second compartment but starting with green light instead of blue. The plants in the two reference compartments only received red light. The total light in the four compartments was the same. This meant that the reference plants were lit for a slightly shorter period of time in total. During the trial, large numbers of measurements were taken to monitor plant development, flowering, fruit development and quality and the effect on photosynthesis. “The expectation was that green light would mainly affect plant shape and blue light would impact on photosynthesis,” says crop researcher Kees Weerheim.

Different plant responses

Weerheim summarises the results. The plants that received blue light for the first three hours of the day increased production by 8% – a combination of the greater number of fruits produced and the heavier weight of the fruits. In addition, the plants were 10% shorter, at about 600 cm compared with 660 cm. The leaves contained slightly more chlorophyll but photosynthesis was not measurably higher.
The results of the plants that received green light for the first three hours were a little more difficult to explain. These were found to have lower photosynthesis, even though the leaves also contained slightly more chlorophyll which should allow them to absorb more light. The plants under green light were no different in length from the reference plants, although the crop was more open, allowing the light to penetrate through to the second leaf layer more easily. This could be beneficial for light interception, similarly to diffuse light through the greenhouse roof.

Sum total of little things

“The plants with blue light showed increased production, whereas the plants under green light did not. But the differences are small. We can’t pinpoint one specific factor as being responsible for the improvement in production. It’s the sum total of lots of little things: differences in photosynthesis, leaf position, chlorophyll content. What we do know is that pursuing this offers potential for the future. We don’t know or understand everything yet, but it is definitely the way to go,” project manager Dieleman confirms.
LED lighting in general is becoming more and more popular. Its potential lies in the use of light colours. “We need to generate knowledge and make growers and propagators aware of the opportunities.” Dieleman sees this as a big jigsaw puzzle to which more and more pieces can be added.


The EU’s four-year HI-LED project has ended. In the latest greenhouse trial with Komeett tomatoes, the plants received 85 μmol/m(sup>2/s blue or green light, supplemented with 220 μmol/m2/s red light. This treatment was compared with reference compartments in which the plants were only given red light. The plants under blue light increased production by 8% and were 10% shorter. Although the plants under green light had a more open structure which made for better light penetration, their production was more or less comparable to the plants in the reference section.

Text and images: Marleen Arkesteijn.