Tulip forcers can reduce transpiration without affecting quality

Tulip forcers can reduce transpiration without affecting quality

Stem topple is a serious problem in the flower bulb sector. Bent stems are unsaleable and are therefore something to avoid if at all possible. It has long been known that this problem is associated with transpiration from the plant, and for many years the advice has therefore been to ventilate well during the forcing phase so as to prevent the RH from rising above 70%. Researchers have tested whether this is really necessary throughout the whole forcing period, since less transpiration means much lower energy usage.

The main energy gobblers in tulip forcing in two- or three-tier greenhouses are light, transpiration and, in particular, heat loss to the outside. Researchers Jeroen Wildschut and Martin van Dam of Wageningen University & Research in the Netherlands investigated the potential for light reduction a while back. A tulip doesn’t need photosynthesis light or grow light because the bulb itself is a plentiful source of carbohydrates. Only steering light is needed to keep the plant standing upright and to enable it to develop good colour. In 2005 these findings paved the way for the current system of multi-tier cultivation, a method that enables forcers to expand their production area vertically. This system leads to energy savings of as much as 40-50% per crop, not least thanks to the high utilisation rate.

In Next Generation multi-tier cultivation, bulbs are forced in six or more tiers in well insulated cells. The facility is no longer a greenhouse but more like a warehouse lit with low-energy LED lights. The different climate requirements needed in each growth phase are met by compartmentalising the forcing area.

Ventilating costs energy

Following on from their research into light reduction, Wildschut and Van Dam set their sights on transpiration and ventilation. Wildschut: “Moving from a single tier to a multi-tier system cut energy consumption by 50%. In this new situation, ventilating to remove moisture uses the most energy as the system that warms up the outdoor air and then removes the moisture runs day and night. Our research shows that you can save roughly another 50% by ventilating less. By reducing transpiration, in other words by removing as little moisture as possible, you quickly start saving energy.”

In multi-tier systems with two or three tiers, the researchers have observed that energy is consumed at a rate of 300 MJ per 1,000 bulbs. The figure for rooms with six tiers and balanced ventilation is just 180 MJ per 1,000 bulbs – an encouraging outcome. However, as the researchers emphasise, savings are all well and good but they should never come at the expense of quality. So the project started off by looking into the transpiration needs of the plant in each growth phase.

Susceptible phase

Van Dam: “In the first phase of the project, we explored the susceptibility of some of the main cultivars to topple. Leaf and stem topple, along with fading and too lightweight or too short plants, are the result of poor transpiration and calcium deficiency in the cells. Calcium deficiency causes the cell membranes to become more permeable, so the cells burst and moisture leaks out, forming the point at which the stem or leaf topples over. We know that the growing parts of the flower absorb calcium as the stems elongate rapidly in the growth phase. During this phase, transpiration draws calcium from sources including water to wherever it is needed in the plant via the sap flow. Sufficient transpiration is therefore crucial in this phase.”

Also noteworthy is the fact that toppling doesn’t actually happen in this phase per se. The flowers can be standing proudly upright at the time of harvesting but the symptom can rear its head at the auction or even in the vase. “Our research shows that the degree of susceptibility to topple is clearly cultivar-specific. For example, excessive RH was found to have no effect at all in the cultivar Barcelona. Strong Gold and Seadov turned out to be much more susceptible: the number of plants affected by topple at 83% RH was very high. The middle growth phase was found to be the most susceptible to high RH.”

Low RH in middle week only

The researchers established which phase was most susceptible to topple by dividing twenty “pin trays” of tulips between two compartments. The RH in the first compartment was kept at about 100%, with below 70% in the second. The researchers switched the trays round every two days. As soon as some plants started to display signs of toppling, they were logged and removed. This trial was carried out in three forcings. The same picture emerged in each cycle. Out of the roughly 20 days the bulbs spend in the greenhouse, it is the middle week that determines whether the plant will be susceptible to topple later on.

In their report, the researchers state that the tulips need to be able to transpire for between 30 and 65% of the forcing period. Therefore, during this short period (which can be between three and nine days, depending on the cultivar used) the grower can influence calcium uptake and should keep the RH sufficiently low. The tulip is not affected by high RH during the first and last weeks of forcing. Wildschut: “By keeping the RH below 70% in the middle week only, when the tulips are at their most susceptible to topple, you can therefore save a lot of energy without making the problem worse.”

Adjusting the cultivation strategy

Last January, Van Dam shared the findings of the research with attendees at the Day of the Tulip industry event. “I could see that there was a lot of interest among the audience. Obviously growers aren’t going to change tack straight away, but it is important for them to think about it. They will only start making adjustments to their own cultivation strategy if they believe that it works. That’s fair enough – and it’s also why it will be important to follow up on this research. We want to be able to offer practical guidance. That’s the only way our efforts up to now will actually lead to a reduction in energy consumption and therefore in CO2 emissions in the flower bulb sector.”

Once the researchers secure financing for the follow-up research, they plan to investigate transpiration in the phase most susceptible to topple in more depth. The latest research revealed that the RH doesn’t have to be low 24 hours a day at this time. They expect that it should be possible to switch the ventilation system off for between three and 10 hours a day in this phase. If that is the case, growers could save up to as much as 47% of the energy they use on dehumidification by allowing the plants to transpire for between 18 and 21 hours a day during the susceptible phase. Now that’s definitely worth investigating.


Ventilating less can save energy. However, if the RH is allowed to get too high, this can result in leaves bursting open or stems collapsing (toppling). Researchers looked into what effect reducing transpiration in different growth phases during forcing would have. They discovered a distinct period in which tulips are more susceptible to topple and in which adequate transpiration is essential in order to maintain quality. In the days before and after this period, growers can quite safely allow the RH to increase slightly without making the problem worse.

Text: Jojanneke Rodenburg.
Images: Studio G.J. Vlekke.


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.


Limits of dimming lights in phalaenopsis reached

Limits of dimming lights in phalaenopsis reached

Intensive lighting in winter is common in Phalaenopsis growing. A long-running Dutch research project is seeking to answer the question of whether less lighting could be used at that time of year. The trials in the first three years revealed that turning the lights up or down could cut electricity bills by as much as 30% without loss of quality or production. The results from the fourth trial year, with practical trials at Ter Laak Orchids, show that the limit has been reached in terms of quality – at least in the top segment.

The trials run in 2014-2016 by the specialist Dutch research companies Plant Lighting and Plant Dynamics, with support from growers, delivered some surprising insights. One of these was that timing is more important than the light sum in the vegetative and generative phases. They also discovered that a long day of 16 hours produces more CO2 uptake than a day of 11½ hours. Dimming the lights at the beginning and end of the lighting period seems to be possible without loss of production or quality. That generates electricity savings of more than 30%.

Dimming in cooling phase

Based on these results, two follow-on studies were run in the winter of 2016/2017. The first took place in climate chambers equipped with daylight simulators and SON-T lights from Plant Lighting in Bunnik, the Netherlands. In these chambers, the dimming treatment, which can cut electricity usage by up to 30%, was also applied in the cooling phase for the first time. In another room, the plants were lit in line with the biorhythm, starting at 05:00 instead of 01:00. This can save as much as 43% in electricity because it makes better use of free daylight.

Researcher Sander Hogewoning explains: “The CO2 uptake was the same in all three treatments. Yet something caught our eye: CO2 uptake ended relatively late: it didn’t stop until 2½ hours after the lights went on. So the plant rhythm is not always the same, and we have no idea why that is. This indicates that it is important to keep on taking measurements with sensors, otherwise you are taking a risk. We also noticed that the plants were a week behind in the treatments with dimmed light. That can be explained by the fact that the plant temperature was 0.5°C lower on average because the SON-T lamps were used less. In practice, you would compensate for that by turning up the heat. Although the percentage of double spiked plants was just as high, the percentage of branched ones was lower. With these treatments, you’re reaching the limits in terms of quality.”

Practical trials

The limits were explored and found in the second trial as well. This study took place in the Ter Laak Orchids trial greenhouses in Wateringen in the west of the Netherlands, in both the vegetative and cooling stages. The generative stage took place in the production greenhouse. The researchers and growers chose four varieties for the trials: Sacramento, Donau, Jewel and Las Palmas. Martin van Dijk of Ter Laak: “We grow more than 100 varieties here, so we wanted to know what effect a different lighting regime would have on different varieties. The quality and the number of double spikes must remain the same. That’s essential for us.”

In the one 80 m² trial greenhouse, the plants were lit for the usual 16 hours. The plants in the other trial greenhouse were also lit for 16 hours but with the dimming treatment used in the previous trial. The only difference was that the start was delayed until 03:00 in order to make better use of the daylight. To check the quality, the root weight and above-ground weight were measured three times. At the end of the generative stage, the researchers counted the number of double spikes and the number of flowers.

The same or marginally lower

The results? The quality of the plants was the same or marginally lower. With the dimming treatment, the roots in three of the four varieties were lighter than in the control treatment, although the weight of the leaves and flowers was comparable. The number of flower buds was also the same.
Another indicator of quality is the percentage of multiple spikes. With the dimming treatment, only Jewel showed significantly lower results by the end of the generative stage. The percentage was slightly lower in the other two varieties but not to a statistically significant extent. The differences are not massive, but they do show that the limits of dimming were reached in this trial as well.

Hogewoning: “My conclusion after these trials is that dimming saves a lot of electricity and produces the same or slightly lower quality. We advise growers in the top segment not to push the boundaries when looking for savings but to stop a little way from the limit. However, the quality differences are small. Growers with fewer lamps will find that switching the lights on later saves them money. By making better use of free daylight, they will reach their light sum more easily at the time of day that is most important for the plants. And finally, bear in mind that any differences in quality are very much magnified because we are simulating winter for 30 weeks of cultivation. In reality it’s not always December.”

Different choices

With the benefit of hindsight, the growers involved are making various choices depending on the capacity of lamps, but also depending on whether they have a CHP plant or have to buy in electricity, which is expensive. At Ter Laak Orchids they are biding their time. Van Dijk: “With the results of the study and the experience we gained last year, we are waiting to see what the results of next year’s sensor study will be. This winter we plan to turn the lights up and down incrementally, but starting at 01:00 as normal.”

Honselersdijk-based Levoplant has been switching the lights on later since last year. Cultivation manager Erwin van Vliet, a long-standing member of the supervisory committee: “We used to start at 01:00 and we would stop suddenly at 16:00, sometimes even earlier. Now we start at 04:00 in October and finish at 19:00. In November and December we start at 03:00 in order to achieve our light sum. We also turn the lights up and dim them incrementally. That works very well for us because it makes the climate in the greenhouse more uniform. An additional advantage last year was that we had less of an issue with premature spiking. The quality is every bit as good as before.”

Developing knowledge

Are these insights resulting in energy savings? Van Vliet: “We are not saving as much as in the study. We are lighting for longer, although we are saving energy by dimming. Before the study, the trend was heading towards 100% lighting for 16 hours. But we now know that really isn’t necessary. So we need to continue to develop our knowledge – by working together.”

The Pot Orchid Growers Cooperative this year invested in the development of robust, affordable sensors to provide ongoing information on the plants’ light usage. Van Dijk and Ter Laak are certainly convinced of the benefits. “We are installing a wireless network in the new Daylight Greenhouse we are building to allow for the use of wireless sensors in the future.”


The fourth year of the research into lighting Phalaenopsis in winter has confirmed the previous years’ results. The orchid needs a long day, but that can be achieved by gradually turning up and dimming the lights in the vegetative, cooling and generative stages. Switching on the lights later saves more electricity. The quality of the plants in the dimming treatments is the same or marginally lower. The limits of the savings thus seem to have been reached.

Text: Karin van Hoogstraten. Images: LD Photography.


Online software for energy-efficient greenhouses

Online software for energy-efficient greenhouses

Hortinergy is an online software package for designing energy-efficient greenhouses by simulating energy consumption and comparing technical solutions.

Energy is a major expense in greenhouse horticulture. There are currently several solutions on the market that can help reduce your energy bill. The dilemma is how to choose the best configuration adapted to the climate outside and inside the greenhouse and the crops grown in it. This is the first online software solution to simulate the energy consumption of an existing or planned greenhouse anywhere in the world.


Suitable for a wide range of users, from growers to consultants and greenhouse equipment manufacturers, it is user-friendly and it takes less than 15 minutes to enter your parameters. To simplify the user experience, equipment manufacturers can spotlight their branded products for selected pre-set parameters. Hortinergy is a decision-making tool for sizing equipment and optimising investments: users can compare energy efficiency and technical scenarios with a simple online interface.
Stand number: 12.132


Right light spectrum on bottom layer with half the energy

Right light spectrum on bottom layer with half the energy

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.