Home Posts Tagged "chrysanthemum"


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Dutch chrysanthemum growers are feeling quite optimistic about thrips control. After years of high pest pressure, growers are getting more adept at integrated pest control, thanks to meticulous scouting, a good predatory mite, supplementary feeding and biological crop protection products. Of course, there isn’t a one-size-fits-all approach that works for every nursery. Chemicals are still an indispensable part of the mix as a backup. Growers and suppliers outline the latest developments.

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Growers are, of course, keen to ensure that their crops grow as optimally and quickly as possible. An efficient – or more efficient – growing method will, of course, automatically produce a high – or higher – yield. To help growers gain greater insight into the growth process PhenoVation developed a device that makes photosynthesis visible.

The device that makes photosynthesis visible is called the CropObserver. The measuring device is suspended from the greenhouse ceiling and measures the values of the crops growing several metres beneath it.


PhenoVation’s device measures parameters that correlate with the maximum efficiency and effective efficiency of the crop. Based on this data, growers can gain insight into the effect of specific light and growth strategies on a crop’s growth processes, thus allowing them to determine the optimum growth conditions for their crops. Additionally, growers can optimise their Leaf Area Index (LAI) using the input provided by the CropObserver.


Chrysanthemum grower and chairman of LTO Glaskracht’s National Chrysanthemum Committee David van Tuijl is currently testing the Crop Observer in the greenhouse in Brakel where he grows his flowers. Van Tuijl is the first grower to use energy-efficient LED lights throughout an entire greenhouse, making it a prime example of what a conventional greenhouse would look like in the future.

Initial assessment

Van Tuijl’s experiment receives assistance from various experts at Wageningen University & Research, Philips Lighting, the Delphy knowledge centre and the Glastuinbouwpact greenhouse horticulture association. The pilot project will last one year, in principle, but an initial assessment will be take place after six months. “This assessment will decide if another six months will be worthwhile”, says Van Tuijl. “We have only been operating for one cycle, so it is too early to draw any definite conclusions about the CropObserver.”

A grower request

A wish communicated by growers for a different way to measure photosynthesis prompted the development of the CropObserver. “Before the CropObserver, measurements were taken with a system that recoded the values of only one leaf”, says Vincent Jalink, who developed the device. “Besides, this system was not wireless, which was rather inconvenient for some growers. Growers indicated wanting to measure the values of multiple leaves via a wireless system.”

Compatible with climate system

“Depending on how high it is suspended, the CropObserver can measure 4 to 6 square metres of crops growing beneath it, fully wireless”, explains Jalink. “What’s more, it is compatible with LetsGrow and Hogendoorn climate systems, which allows us to link certain actions to specific values. One of our customers is already doing this. As soon as the CropObserver measures a specific value the climate computer will open the screens. This has enabled the grower to shorten his crop cycle, thus allowing him to fit more cycles into a single year.”


According to Jalink the CropObserver can be used to measure photosynthesis and growth in all crops. “Of course, not all plants lend themselves equally well to using the CropObserver for climate control purposes”, adds PhenoVation’s developer. “Tomatoes, for example, flourish when exposed a lot of light and heat and will therefore not respond as strongly to changes in light ingress and temperature in the greenhouse. Therefore having your climate computer controlled on the basis of photosynthesis makes less sense when growing plants like these. However, with crops like tomatoes the system can be used to measure production by gaining insight into the ETR (Electron Transport Rate), considering that the ETR value correlates very well with the amount of carbon dioxide absorbed into the crop.”

Perfecting the growth strategy

According to Jalink sun-sensitive crops and potted plants perform much better under those conditions where a CropObserver is linked to a climate computer. “These crops respond more strongly to a change in light ingress or temperature, which allows growers to perceive the effect of their actions within a day, or even within a few hours – and to perfect their growth strategy accordingly. Various tests have shown that this will increase production yield by at least five per cent for the same surface area.”

For rent

Because Jalink does not yet know which crops respond well to the CropObserver-climate computer combination, he is also making the measuring equipment available for rent. “This will help growers independently decide if the CropObserver is an interesting device for them, without having to purchase it immediately.” Growers who are interested in this system can contact Jalink through the contact details on the PhenoVation website.

Text: Leo Hoekstra. Photo: Marleen Arkesteijn.

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The predatory bug Orius has been used to control thrips in sweet pepper for many years with great success, but the results have so far been disappointing in ornamentals. Researchers Marjolein Kruidhof and Gerben Messelink now think they have found a solution. With a new method of using the bugs that involves supplementary feeding, thrips can now be successfully controlled in chrysanthemums.

Thrips are the biggest threat to ornamental growers’ crops. Research into biological predators for this pest has been going on for many years. Good results have been achieved with predatory mites, but this has often failed to eliminate the problem because the predatory mites only attack the young larvae. The predatory bug Orius is a very effective weapon against thrips in both the larval and adult stages but it has trouble establishing in ornamental crops. Numerous ways of overcoming this problem have been investigated, ranging from banker plants to feeding stations, but there has been no real breakthrough. Until now, that is.
In the spring of 2017 the Wageningen University & Research Greenhouse Horticulture business unit in the Netherlands started experimenting with a new approach to thrips control in chrysanthemum cultivation. Instead of starting off with chemical crop protection products, the researchers are now introducing biological agents in the cuttings phase. The predators are given high-quality supplementary food so that they can form a strong population or a “standing army” to nip the outbreak in the bud.
“The results that have been achieved this time are due to good coordination between two projects: the PPS Thrips project, in which we are looking for a good alternative supplementary food source, and the Green Challenges project, in which we are optimising the role of biodiversity in crop protection and achieving paradigm shifts,” says researcher Marjolein Kruidhof.

Biological start

In chrysanthemum cultivation, there is usually only a short time window in which you can start using biological control, according to Kruidhof. “Also, the presence of chemical residues delays the growth of populations of natural predators,” she says.
The researchers experimented with a biological start using the predatory bug Orius. They ordered cuttings that were almost pesticide-free, rooted the cuttings themselves and added the bugs a few days before the plants went into the greenhouse. “A biological start is a real change in thinking,” says Kruidhof’s colleague Gerben Messelink. An important part of this strategy is the supplementary feeding, he stresses. “After a series of trials in which we compared different types of food, we ultimately went with Artemia, the cysts of the brine shrimp. This is a potentially good food source and has a long shelf life.”
Trials using Artemia as a feed supplement for predatory bugs had been carried out before but with only moderate results, he says. “The quality of the Artemia that is available on the market at present is good enough for feeding predators like Macrolophus in tomato but not for Orius.”

Significant effect

The researchers therefore got together with the University of Ghent to come up with a good quality food source. Meanwhile, the Israeli company Biobee had also started producing high-quality Artemia which the researchers were able to use in subsequent experiments.
The results exceeded expectations. The number of Orius rose substantially as a result of the supplementary feeding. Having started with fewer than one bug per cutting, by the end of the production phase the researchers were counting 40 bugs per plant. What’s more, the natural predator seemed to respond very well to the availability of food. “It turns out that they are highly mobile,” says Kruidhof. “This has potential because it allows you to manage your biological control better. Plus it means you will very likely be able to reuse the bugs. If you end up with 40 bugs per plant, it would be a shame to spray them dead. That’s destruction of capital. You might be able to lure the adult specimens to new cuttings with targeted supplementary feeding.”

More effective than predatory mites

The impact on thrips damage was significant. “In the control section, in which no Orius or Artemia were used, half the younger leaves were damaged by thrips,” says Kruidhof. “The figure for the plants with the bugs was less than two percent.” The predatory mites did less well than the predatory bugs in terms of thrips control, despite the fact that they had built up a good population with the chosen food source. Researchers still found about 20 to 25% thrips damage on plants following the use of these biological predators. “So Orius really are more effective than predatory mites because they also attack adult thrips,” says Messelink.
“We have proved that the system works,” says Kruidhof. “We can build up the population of bugs by using biological controls and good quality nutrition right from the start, and this population provides good thrips control even in the presence of another food source.” However. that doesn’t mean that this method can simply be replicated in the commercial greenhouse setting. “We still need to optimise certain aspects,” she says. “For example: when is the best time to introduce the bugs? Should they be used in the rooting phase or can they be brought in later? How many bugs should you use? What will your feeding strategy be? How much food should you provide?”

Excellent development

This method of control is based on one generalist. What do you do as a grower if you also have to deal with leaf miner or aphids? “Growers will have to control leaf miner with additional biological measures or selective chemicals. Aphid control can become a problem, but the expectation is that high densities of this predatory bug will also keep aphids under control. Other possibilities for controlling aphids are parasitic wasps, gall midges or perhaps other predatory bugs. We therefore want to investigate whether other types of bugs can be combined with Orius to deal with aphids.”
Crop protection specialist Helma Verberkt of the Dutch growers’ organisation LTO Glaskracht sees this as an excellent development. “It is a good addition to developments in the commercial greenhouse setting, where good results have been obtained in recent years using predatory mites,” she says. “For use in practice, there will need to be enough affordable, good quality Artemia available and it is important to ensure that Orius is compatible with other biological agents and pesticides used.”

Pesticide-free cuttings

The question is also whether cutting suppliers and producers will be willing to come on board. Cuttings with few or no crop protection product residues are currently hard to find. “It’s a bit of a chicken-and-egg situation, but I think we will manage,” says Messelink. “There’s also a real change in thinking going on among cutting suppliers. More and more growers want to start biological control earlier and are asking for cuttings with fewer or no chemical residues. Cutting suppliers are also looking for alternative options. I think biological control is the solution.”
“We have shown that it works now, and that is quite a breakthrough,” Kruidhof adds. “We plan to carry out another greenhouse trial this year and we expect growers themselves to start developing the strategy further as well. As a result, the market for pesticide-free cuttings will only get bigger and more demand-driven. So producers and suppliers will have to meet that demand.”
Both projects are funded through the Top Sector Horticulture & Propagating Materials and are being implemented within this sector with funding from the government, various crop cooperatives and Koppert. The projects are coordinated by LTO Glaskracht Nederland.


Researchers in the Netherlands have made a breakthrough in controlling thrips in chrysanthemums. By starting biological control early on and providing good quality nutrition, it is possible to build up a good population of the predatory bug Orius. This population controls infestations well, even in the presence of food.

Text and images: Marjolein van Woerkom.

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We know much more about photosynthesis than about the flowering of the plant. This sometimes leads to surprises, especially with new crops. The grower has to take into account the juvenile phase, effect of temperature, light, size of the plant, day length, and the interaction between hormones, sugars and other compounds in the plant.

Before a plant can flower it first has to become an adult. Many plants have a juvenile phase. Even under optimal conditions they are unable to flower during this stage.
This is logical because a plant flowers in order to reproduce. Therefore the flowers must be of sufficient quality to actually achieve this. They have to be developed to the extent that they can be pollinated, for example by insects. And after pollination all kinds of processes need to start for the fertilisation and development of seeds and fruits. All of this costs a lot of energy. So from the plant’s point of view it’s considered ‘wise’ to postpone these processes until sufficient assimilates are available in the plant.

From juvenile to adult phase

The duration of the juvenile period varies enormously, from a few days to a few decades in trees. Of course, for the grower this can be very unprofitable if you have to wait a very long time for it to become productive. That’s why it’s good that a cutting or graft taken from a plant that is already in the adult phase also remains an adult.
The switch from juvenile to adult phase happens quite abruptly. The moment at which this occurs can depend on the size of the plant, age, number of leaves and growth factors.

Hormonal factor

From a wide range of research it’s clear that a hormonal factor plays a part in the transition from vegetative to generative. Suddenly the apical bud changes in shape as a forerunner to flowering. For a long time researchers looked for a hormone that stimulated flowering. The unknown flowering hormone was even given a name, namely florigen. But, it is now clear that florigen does not exist.
Although gibberellins play a role in many plants – this group of hormones was for a long time the leading candidate for the role of florigen – the situation is still ambiguous. In some plants gibberellins actually slow down flowering. Bearing this in mind, it’s also remarkable that growth inhibitors, such as daminozide, that slow the activity of gibberellin, do prevent the long and thin development of flowering plants, but not the flowering itself.
Another hormone group, the cytokinins, plays an important role in the induction of flowering. But again no general rules apply.
It seems that an interaction between hormones, such as gibberellins, cytokinins and ethylene, as well as sugars and other substances, such as polyamines, causes the induction of flowering. It’s different for every crop. The limited knowledge about the mechanism of flowering makes it difficult to effectively influence flowering. This is especially the case for new ornamental crops. Usually, the practical research focuses on achieving the most appropriate cultivation measures, without knowing exactly what happens inside the plant.

Leaves under first truss

Fortunately, a lot of research has already been done on the major horticultural crops. One of the many crops examined is tomato. A grower would like the plant to start producing quickly, and in terms of the tomato this means: the number of leaves under the first truss has to be limited.
In theory, a certain amount of assimilates must first be present in the tomato plant before it can start to flower. Indeed, research shows that any procedure taken to increase the amount of assimilates speeds up flowering. More light means fewer leaves under the first truss. A higher temperature at a low light intensity also leads to more leaves under the truss because the plant consumes more energy at a higher temperature.
As well as having a minimum quantity of assimilates, distribution is also important. At a lower temperature the top of the plant – the apex – has an advantage as it competes with the leaves.
This knowledge is difficult to translate into other crops. In fact the influence on flowering should be examined separately for each crop.

Short day = long night

A special phenomenon is the sensitivity of a flower to day length. In this respect, the origin of the plant makes a big difference. At the equator the length of day and night are the same and tropical plants are not day length sensitive. Plants from higher altitudes, that flower in the spring or even in the autumn, do tend to be sensitive to day length.
Sensitivity to day length exists as a result of natural selection. Therefore it’s also possible to remove this sensitivity by selection. By consistently selecting and further propagating the most insensitive plants it’s possible to solve this inconvenience. This doesn’t work sufficiently well with all crops, so we do encounter short day plants such as poinsettia, chrysanthemum and kalanchoe and long day plants such as gypsophilia, trachelium and carnation.
The naming is actually wrong. A short day plant is actually a long night plant because it’s all about the length of the dark period. And if this is broken – even for just a very short period – the whole effect of the dark period is lost.

Length of dark period

The plant registers the length of the dark period in its leaf but flowering takes place elsewhere. Therefore there has to be some communication between the leaf and the point where flowering occurs. This is carried out by a hormone that is produced in the leaf and then travels to the point of flowering.
How does the plant measure the length of the dark period? Previously researchers thought that the pigment phytochrome slowly broke down during the night into another form and that this was a signal to the plant to start flowering. But it’s more complicated than that. There is an interaction between the endogenous rhythms in the plant (‘the biological clock’). As a result the same length of darkness can sometimes produce different effects, whereby temperature can also play a role.
Some short day plants need just one long night. One of the most well known short day plants in horticulture is the chrysanthemum and it actually needs several weeks of long nights. If a grower stops the dark period prematurely, abnormalities occur. Just after a few short days the growth point becomes generative and stops producing leaves. Yet a grower has to continue with the long night regime for several weeks. It’s likely that multiple genes are involved in the flowering of chrysanthemum and it’s not simply a transition from vegetative to generative based on one gene that can be turned ‘on’ or ‘off’.

Mutual competition

Once the plant has switched from vegetative to generative and then flower buds have actually formed, many things can still go wrong. The buds can dry out or fall off and the flower may not open properly. This is mostly a question of how well the flower bud and the flower have been supplied with water, minerals and assimilates.
The flower has to compete with other parts of the plant and sometimes loses the fight. Optimal climate conditions, providing enough light and water, reducing the competition with the young leaves (by picking leaves) are all ways to ensure that flowering is successfully achieved.


A plant can only flower when it is mature. In horticulture, we bypass the juvenile phase by using cuttings and grafting. The transition from vegetative to generative appears to be controlled by a hormones. Flowering is the result of an interaction between several substances, for example, gibberellins. We still know too little about flowering which is sometimes difficult when working with new crops. A lot of research has been carried out on the major horticultural crops such as tomato and chrysanthemum. The latter is the best-known short day plant, although we should call it a long night plant.

Text: Ep Heuvelink (Wageningen University) and Tijs Kierkels. Images: Theo Blom (University of Guelph, Ontario, Canada).

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The new diffuse screen from Svensson provides even better light distribution than the previous version. But growers are also still very keen on the first generation of these climate screens. “I would choose this screen again straight away in any new build,” says chrysanthemum grower Wilco Hofman from Bleiswijk in the Netherlands.

The Harmony climate screen slides out slowly below the glass. The young plants underneath are enveloped in light shade. There is a marked difference in the places in the greenhouse that are not screened. The chrysanthemums are still in full sunlight there. “We often use this open structured climate screen, particularly in the bays housing the young plants,” says chrysanthemum grower Wilco Hofman of De Landscheiding in Bleiswijk. “Young plants are delicate. Diffuse light is therefore just what we want in those bays.”

Positive effect

And yet it wasn’t so much for light distribution that he bought the screen back in 2009, but rather to improve the climate in the greenhouse. “It’s an interplay between various factors. Our Santinis almost never get enough light. You don’t actually need to use a screen for plant growth; I mainly do it for cooling and to keep the humidity in the greenhouse stable. If it gets too hot, the plant’s stomata close and the plant doesn’t cool itself. Closing the screen allows the plant to cool down.”
This also has a positive impact on the young rooted chrysanthemum cuttings. The climate screen prevents the roots from drying out unevenly. “I also have an older greenhouse in ‘s-Gravenzande. We use whitewash there. The direct radiation from the open windows dries out the pots more quickly in some places than in others. That means we have to water flexibly there, which makes the crop less reliable.”

By chance

Hofman came across the Harmony screen quite by chance. He used to use a blackout screen in his greenhouse and whitewash on the glass. When he was building his new greenhouse in 2009, he temporarily transferred production to a rented greenhouse. This was equipped with a blackout screen with silver-coloured shading above it, which could be closed separately. “It struck me how pleasant it was to work under. The plants are not in full sun, but nor are you yourself. I took the idea back with me to my own greenhouse, and after taking advice we decided to have a new climate screen installed while we were building the new greenhouse.”
He has had a 45% screen ever since. The grower decided to install this open structure screen and the blackout screen together on the same wire frame. “This does mean that we can’t close the screens at the same time, but we never need to do that anyway. Installing them in this way keeps the costs down.”

Less delayed growth

Hofman was the first chrysanthemum grower to have this climate screen installed. “It is certainly not common in chrysanthemum growing,” says Ton Habraken of Svensson, the screen manufacturer. “Santini and disbud chrysanthemum growers were the first to use it. Now it’s gradually catching on among spray chrysanthemum growers as well. More and more growers are attracted by the diffuse light it provides.”
“I think its effectiveness is underestimated,” Hofman adds. “We grow chrysanthemums in lots of colours and I get the feeling that the intense colour is preserved better under this screen. But most chrysanthemum growers only grow white and yellow. Those colours are less sensitive to solar radiation.”
Whether the screen makes for faster growth, a more uniform crop or a heavier chrysanthemum, the two find it hard to say. Habraken: “Compared to the Solaro aluminium summer screen, the white Harmony screen does a better job. The research shows that Solaro absorbs some of the light, which makes the screen hot. The new screen absorbs virtually nothing at all, and that keeps the greenhouse one or two degrees cooler.”

Saving energy

“In our nursery the effect is difficult to measure because we don’t have anything to compare against,” the grower says. “But we do have less delayed growth in summer. We used to notice delayed growth in plants on extremely hot days, but since we have been closing the screen on those days, it has been happening less. That’s money in the bank.”
The screen also yields results in winter. “If it’s snowing or if it’s a dark day, I close the screen to keep the temperature in the greenhouse up. So I can reduce my pipe temperature by as much as ten percent. That saves energy.” Habraken agrees: “Our research has definitely shown that the screen can deliver energy savings of between 15 and 20 percent.”

Pleasant working environment

The only thing the chrysanthemum grower did not take into account in his choice is soiling. “The screen has not been as white as it used to be for a while now. Soil preparation work and fork lift trucks generate a lot of dust which settles on the screen and reduces diffuse light radiation. Next time I would opt for a slightly lighter screen to compensate for this.”
But they will definitely be using this kind of screen again next time they build. “Apart from all the benefits for the plant, one of the most important aspects is that it makes the working environment more pleasant. When we are working in the greenhouse, we can now close part of the screen and stand in the shade without impacting too much on the quality of the crop. It’s wonderful working in the summer with the screen closed and a light breeze blowing below it.”


A new diffuse climate screen gives 32% more light to parts of the crop shaded by the greenhouse structure compared with the classic variant. What’s more, the light is more evenly distributed so there are fewer variations in the light level. Although it’s difficult to attribute all the positive effects on the plant to the screen without a comparison, growers are seeing improvements in aspects such as colour intensity and growth. But most importantly, perhaps, it makes the greenhouse more pleasant to work in.

Text and images: Marjolein van Woerkom.

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Plant bugs like the European tarnished plant bug and the common nettle bug are a serious problem in crops such as aubergine, cucumber and chrysanthemum. Even in small numbers they can do considerable damage: abortion of the flower in aubergines, stem and fruit damage in cucumbers and splits in chrysanthemums. As soon as growers spot bugs or bug damage, they feel they need to intervene fast with products that are harmful to the biological predators they are using for other infestations, marking the beginning of the end of their biological pest control.

Bugs usually enter the greenhouse from outside. They can arrive early in the season but most sightings of bugs, particularly the most harmful species, the European tarnished plant bug, are reported in the summer months. A good method of spotting and monitoring the presence of bugs can help growers decide when to use pest control products. It would be even better if bugs could be effectively eliminated from the plants with traps.

Pheromones and plant aromatics

A trap with a pheromone attractant for the European tarnished plant bug (Lygus rugulipennis) was originally only available for outdoor use, mainly in strawberry crops. Producers of biological pest control Entocare biocontrol C.V. and Wageningen University & Research have been working with a number of growers in the Netherlands to study and optimise the use of the trap and pheromone in the greenhouse. This trap is now available for detecting the presence of the European tarnished plant bug in various crops (aubergine, cucumber).
The traps were tested in a season-long trial and show peaks in the occurrence of bugs (Figure 1). The relationship between the numbers of bugs caught and the damage they cause is currently being investigated more closely. The level of the peaks and the increase or decrease in the numbers captured in weekly counts help the grower decide which crop protection measures to take.
Trapping bugs certainly helps control them but as yet it is unclear what proportion of the bugs already present can be eliminated with traps. The distribution of bugs across the greenhouse is very irregular: catches in traps show no evidence of bug hotspots.

Concentration critical

During the trial it was discovered that the luring effect of the pheromone works best in the presence of plant or crop aromatics. A simple pheromone trap catches fewer bugs than a trap with the smell of the crop in the background. But it is mainly the males that are attracted by the smell of the pheromone, so the researchers have set about finding attractive alternatives to lure females as well. Their main focus is on plant aromatics.
Several plant substances that seem to attract the females have already been identified. Lab trials indicate that the concentration at which the aromatic is offered is critical, however (Figures 2 and 3). Too little fails to attract them while too much scares them off. Some substances are attractive to both males and females (substance B) while others only attract males (substance D) or only females (substance A). The research is now focusing on finding the right substances, or combination of substances, and on finding a formulation that will go on producing the right intensity of aromatics over a long period of time in practice.

Trap colour and shape

The funnel trap with pheromone which researchers are currently using needs to be further optimised for catching bugs. Video recordings of males landing on these traps showed that less than five per cent of landings on the trap actually resulted in capture. The colour and the shape of the trap will be studied in more depth in future research, along with ways of further optimising the trap. If a combination of pheromone, plant aromatics and better traps proves successful in trapping both males and females, this will open up new opportunities for tackling the bug problem.

Biological control

Besides observations, the aim is also to improve biological control with an effective combination of attractants and biological agents. To begin with, the researchers are looking at a biological agent based on an insecticidal (entomopathogenic) fungus. They are investigating whether it is possible to use attractants to target fungal spores better in the crop in order to increase their effectiveness in controlling bugs. With a modified formulation of the fungal spores and an effective method of transferring them to the bugs, it has been shown that it is possible to get at least three to four times more spores onto a bug.
The next question is whether this combination of methods (luring, infecting and transferring) actually helps combat the infestation. Further research on this will be taking place during the coming year.


For some years now, scientists and growers have been working together to come up with a better monitoring and control plan for the European tarnished plant bug. A pheromone trap that captures males has already been successfully tested. Research into attractants for females has yielded some new substances that are effective, offering new options for controlling this bug.

Text and image: Rob van Tol (Wageningen UR), Maedeli Hennekam and Daowei Yang (Entocare biocontrol).

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Chrysanthemum growers Arcadia and Van Uffelen are definitively going to use 'The Next Generation Growing' at their nurseries in De Lier and Maasland. Technokas was asked to equip the companies with the air conditioning systems needed.

Van Uffelen’s greenhouse was designed and built by Technokas two years ago. The chrysanthemum nursery was also prepared to some extent for the installation of the climate system required by The Next Generation Grwoing. The Arcadia installation is part of a new project involving replacement and expansion at an existing site. The installation will suck dry outside air via the gable ends, possibly mix this with greenhouse air, and if necessary heat this locally until it reaches the indoor temperature. It will then be distributed among the cultivation departments with transparent hoses.

Portals in gable ends

Air handling units (AHUs) will be installed in the purpose-made portals in the gable ends. The greenhouses will be provided with manifolds and mix groups for the heating elements, where these do not already exist. In addition, the installations for sprinkling and lighting are designed in such a way that the air distribution tubes can easily be hung over the crops, and do not negatively affect the reach of the treated air from the holes in the hose. These greenhouses are also provided with a second energy screen.

Long-term testing

Arcadia and Van Uffelen decided to use this climate system as a result of the good results achieved with them in one of Arcadia’s other sites. Wageningen UR, Kas als Energiebron, DLV, Deliflor and other chrysanthemum growers have been testing this system for a long time. During prolonged tests with other crops, positive results in terms of controllability of climate requirements and energy efficiency have also been achieved using Technokas’ climate system.

Energy savings and a better climate

Growers expect the Technokas installation to help further improve the regulation of temperature distribution and relative humidity of the climate in a cultivation department. In addition to a more uniform climate, growers expect that this, in combination with the energy screens, will result in 15 to 30 percent savings in energy consumption. The technology is also expected to lead to even better quality chrysanthemums.

Source: Technokas. Photos: Fotostudio GJ Vlekke.

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Chrysanthemum grower Maurice van Os, like many Dutch colleagues, has a problem with thrips in the nursery. He solves it mainly by using Amblyseius swirskii, supplemented with Amblyseius cucumeris in the cold season. He steams to control primarily Fusarium, nematodes and Rhizoctonia. Good hygiene practice during steaming and the stimulation of soil life after steaming are also necessary, he says.

Maurice van Os has a nursery of 2.5 ha in the southwest of the Netherlands and grows the varieties ‘Feeling Green Dark’ and ‘Alana’. The grower comes from a true chrysanthemum family. He started in 1997 as the third generation. His grandfather, father and uncle also grew chrysanthemums at a different location. When the family built a new greenhouse in 1998 his uncle left the company and his father followed later.
The location of the company was deliberately chosen for its 'good' ground: a sufficiently airy soil, with a sandy subsoil containing humus. The good soil structure makes watering easy.
He steams the nursery annually just like his father, uncle and grandfather. They used to hire a steam boiler but in 1998 Van Os purchased a high-pressure steam boiler. In those days heating and steaming with one boiler was the trend but now he wouldn’t purchase such a high-pressure boiler. “The mandatory annual inspection is an extra cost. The advantage is that the soil becomes less wet and the sheet cover over the soil gains a sphere shape faster.”

Steam after the summer

Opinions about the best time to steam vary. The chrysanthemum grower steams annually between week 30 and 39. “Some of my colleagues steam before the holiday. I chose afterwards. After steaming the plants grow better.”
Combined with the greenhouse temperature, which during the summer can rise to more than 32ºC, it can lead to too many buds in the bunch; ten instead of six. “We call that a ‘wild’ stem. In addition, the varieties that I grow are already quite heavy. If we steamed before the summer they would become too heavy. Then you also have more chance of yellow leaves. If the autumn is disappointing then you are very pleased with the heavier stems.”

Steam via the drainage

The chrysanthemum grower laid a steam drainage system 55 cm deep. Before steaming, Van Os loosens the ground to a depth of 50 cm using pens. Then the ground is nice and airy and the steam can spread optimally through the ground. He rolls out the steam sheet with an automatic roller and anchors it at the front and rear with steam chains and the sides with heat proof water pipes. Then the mesh is lowered down and the steam boiler and fan are turned on. The small pipes belonging to the steam drainage system stick out above the cover. The fan sucks the steam through the pipes, pulling it from under the cover deep into the ground.
“I steam for about 6.5 hours. Usually I start steaming as soon as the staff go home at the end of the day. I leave the cover in place and keep the fan running until the next morning.” He uses 3.5 m3 gas per m2 for steaming. Contrary to advice he does not use an extra energy cloth over the steam sheet. “As far as I am concerned, the mesh and supports can also get hot. The steam scorches everything it touches."


Only steaming is not enough according to the grower. Steaming is carried out in one area while the rest of the nursery is in full production. Therefore it is important to ensure that the clean ground is not immediately contaminated again. “Before steaming we thoroughly sweep the path and we ensure that the tilling machine and tractor are clean.”
He purposefully doesn’t partition the greenhouse by lowering the internal walls around the steamed area because he doesn’t believe this helps combat the spread of thrips.

Thrips the biggest problem

Thrips is the biggest problem at the moment. “No one really has the answer.” Every week since the spring he has released Amblyseius swirskii: 200 units per m2. “Every two weeks I take samples to see if there are still plenty of predatory mites in the crop. Many colleagues release Amblyseius cucumeris. I only do that in the colder period from week 48 to week 3. The idea behind the early release of swirskii is that you create a biological balance early in the crop. That tends to work better with swirskii than with cucumeris. So far it doesn't help enough but I don’t need to correct too much in between.”
Before harvesting he sprays the crop clean with Vertimec and Actara.

Quickly bring soil in balance

According to the grower healthy soil leads to more resistant plants, which are essential for reducing the risk of pests and diseases. “After steaming the soil is sterile and the plants, which grow faster have a softer leaf that is extra attractive to thrips. That is not an ideal situation,” says the grower. Therefore once or twice in the winter, between week 40 and week 10, he scatters poultry fertiliser and lime granules to redress the soil balance as quickly as possible. “The fertiliser also helps create an airy soil at the bottom. That’s good because as autumn approaches the ground is wetter.”


Chrysanthemum grower Maurice van Os uses a high-pressure steam boiler and special steam drainage at a depth of 50 cm to combat Fusarium. He uses Amblyseius swirskii, supplemented with Amblyseius cucumeris in the cold months, to combat thrips. In addition he maintains a healthy soil by ensuring an airy soil structure and applying poultry fertiliser and lime granules after steaming.

Points of attention for steaming

René Corsten of the Delphy chrysanthemum team has some points to remember when steaming the greenhouse:
- Make sure that the ground to be steamed is as dry as possible. Trying to steam wet ground is a waste of energy so stop the watering early enough.
- Use insulation material over the steam sheet. Insulation saves up to 0.5 m3/m2 and there is less condensation under the cover so the ground shuts itself off less quickly especially at the outer edges.
- With a little effort the steam transportation pipes can also be insulated.
- Preferably use rainwater.
- Flushing on time and good water treatment are important for effective steaming.
- Measure the temperature to check what you are achieving. These days good thermometers are available.
- If there is a problem with nematodes or soil fungi aim for 60-60-60: 60 centimetre deep, 60ºC and 60 minutes. The aim is not to use as little gas as possible but to get the best result. Vacuum (suction) steaming is a must. After steaming allow the fan to run for around 12 hours. Then there should be a continuous heating effect to a greater depth.

Text and images: Marleen Arkesteijn

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The first symptoms of chrysanthemum white rust (Puccinia horiana) are visible on the upper side of the leaf. These are light green to yellow coloured spots of 2 to 5 mm in size. The centres turn brown and necrotic. The characteristic pustules or teliospores occur on the underside of the leaf. When there is a heavy infection the leaves wilt.

High humidity and a period of wet leaves are conditions during which this fungus is able to spread. Under optimal conditions new infections of white rust are possible within five hours. Climate changes in the greenhouse and watering at an unfavourable time increase the risk of contamination.
The fungus is mainly present on the leaves but stems can also be affected. Spores are spread via water, air, plant material, tools, hands and pets. White rust only occurs on chrysanthemums and has a quarantine status in several countries.

Text/photo: Syngenta

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Light is more than just the big mechanism behind photosynthesis. Parts of the light spectrum, or simply more or less light, influence the development of plants: germination, flowering, cell division and cell elongation. Light induction also affects the formation of compounds that are useful to humans. It’s a new area with much to discover.

According to researcher Tom Dueck, of Wageningen UR Greenhouse Horticulture, the Netherlands, a plant ‘sees’ its environment via light. It’s not just the fact that light is present but the amount, direction, length (day length) and colour of the light spectrum that play an important role. Plants use light with a wavelength between 400 and 700 nm for photosynthesis. Growers can use UV, blue, red and far-red light to steer the plant. In particular, the ratio of red to far-red determines a number of physiological processes.

Red and far red

Plants absorb light of certain wavelengths via pigments, also known as photoreceptors. There are different types. UVR8 photoreceptors respond to UV-light and are involved in the stress response. Phototropins are involved in the way plants grow towards the light. Cryptochromes ‘notice’ the difference in day length. Phytochromes are sensitive to the ratio between red (circa 660 nm) and far-red (730 nm) light. They cause changes in the hormonal balance and stimulate the production of some compounds.
The researcher focuses mostly on the phytochromes. How these work exactly is complicated. According to Dueck the ratio of red to far red light affects the three dimensional shape and function of the pigments. This ratio is also called the ‘phytochrome stationary state’ (PSS). At a higher PSS there is more red light in the ratio than at a lower PSS. A higher PSS leads to, for example, cell elongation and it influences the flowering processes. A low PSS affects germination and sensitivity to day length.
The ratio red:far-red is very subtle. In the summer, sunlight with 0.7 has a low PSS (red:far-red ratio). The PSS of red LEDs, without sunlight, is 0.87. This is high, but the difference in ratio is just 0.17.

Flower bud induction of phalaenopsis

Dueck gives a few examples of steering plant processes. Normally phalaenopsis plants have to remain cool at 19ºC for six to nine weeks. The cooling breaks bud dormancy, stimulates growth of the stems and causes flower bud induction. According to Dueck a whole mechanism of plant hormones are behind this and are active during the various stages of development.
The cooling period activates phytochrome B. This phytochrome stimulates the production of cytokinin, inhibits auxin production and stimulates the formation of gibberellin. Cytokinin breaks the bud dormancy and stimulates bud development and branching. Auxin works against apical dominance and ensures that several stems develop at the same time. Gibberellin stimulates flower bud development.
The question is does lighting also work in this way on phalaenopsis. It appears likely, but if the light spectrum influences the hormone balance in the same way as cooling is not clear. During the research Dueck considered if flower bud production is also possible during this period by giving the plants on two occasions four weeks of light of a high PSS via artificial lighting with SON-T lamps that contain a relatively high amount of red. The aim is to activate the phytochrome B.
“A lot of red light, especially in the second phase of the induction, appears to do approximately the same as the cooling. Red induction light can partly replace cooling. We want to start a follow-up project to research this further,” says the researcher.

Chrysanthemum cuttings

A second example mentioned by Dueck is the research into the effects of far red light on the development of chrysanthemum cuttings. The cuttings arrive from Africa and rooting takes seven to ten days before the grower pots them on. During the trial, cuttings from the cultivars ‘Baltica’ and ‘Feeling Green Dark’ received different colours via LED-lighting: 40 µm red (660 nm); 40 µm red plus 8 µm far red (730 nm); 40 µm blue (450 nm) and 30 µm red plus10 µm blue and 8 µm far red.
“The roots increase in number and become longer with more far red light. This shows that with just a little light you can steer the plant so that it can be planted earlier. This is a gain for the grower.”

Effect on compounds

Induction light influences the formation of different secondary metabolites, which can be used for numerous applications, such as artificial colourings, medicine and cosmetics or in the food industry. Dueck gives a few examples.
Since 2010 colleague Silke Hemming has been working on the production of high quality ingredients from algae grown in Dutch greenhouses at Wageningen UR, Bleiswijk. Part of this involves stimulating the production of the red colouring astaxanthin in the algae, Haematococcus pluvialis, by using induction light. This red algae can be used as a food ingredient during salmon and shrimp production to influence their colour.

Greenhouse as pharmacy

A new projects aims to stimulate the production of the dark indigo colour by the ordinary plant, Polygonum (knotweed). Dueck: “We want to stimulate production through a combination of red induction light, more light, a longer day and more CO2.”
Research has also been carried out into the possibilites of stimulating plants to produce the protective substances anthocyanins. These could help plants to be more resistant to high radiation when they are grown in space. Light induction can stimulate the production of these protective anthocyanins.
Dueck sees good opportunities for using light induction to stimulate the production of expensive ingredients. In this respect Wageningen UR is running the project Kas als Apotheek (Greenhouse as Pharmacy). Its a new path along which there is much to discover.


It is possible to use light to steer certain plant processes or to encourage the production of substances. The ratio of red to far red light appears to influence plant physiological processes such as elongation, flowering and germination. Practical examples are the use of induction light to replace chilling of phalaenopsis to encourage bud formation and increasing the amount of far red light during the rooting of chrysanthemum cuttings for more and longer roots. By using induction light it is also possible to stimulate the production of certain substances, such as colourings, protective substances and products that can be used in medicines or cosmetics.

Text/photos: Marleen Arkesteijn