Home Posts Tagged "roses"


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Unlike roses, the protected vegetable sector in Kenya is still quite small-scale. On a recent trip there I set out to find some larger nurseries growing vegetables under plastic. Ultimately I found one of 0.5 hectares; most of the others I visited were between 1,500 and 3,000 m2. In Uganda and Kenya I am involved in a project in which we are using sensor technology to make growers more aware of the climate in their greenhouses.

The basic questions we ask are: how hot and humid is it in their greenhouses and what temperature and RH are they aiming for. When we introduce this sensor technology, I notice time and again that people don’t have the faintest idea what the temperature is during the day or night, let alone how humid it really is. If I ask how hot it gets during the day, the answer I usually get is 40°C and that it doesn’t drop below around 20°C at night. But after taking measurements for a while, we discover that the night-time temperature often drops to the ideal level of 17°C and that during the day it never really gets above 37°C and is more likely to be around the 30-32°C mark. So in fact it’s a pretty reasonable climate for growing tomatoes and other fruiting vegetables.

Humidity is much more important, of course. The sensor readings show that with a little wind during the day it soon gets too dry and the vents have to be closed. Many of these greenhouses have side walls that can be opened and closed. The trick is to keep the humidity inside without letting the temperature rise too high. An additional problem is that a lot of polytunnels here are much too low: they are often still built with gutter heights of as little as 2-2.5 metres. Luckily, as European influence spreads, heights of around 4-5 metres are catching on quite quickly. However, you also have to bear in mind that they don’t have cherry pickers, so everything is done by ladder.

Other physical laws come into play too, such as the fact that heat rises to the top of the greenhouse, so you need vents in the ridge as well as in the sides. Local growers and especially greenhouse builders often still need convincing of this. In the meantime, a number of smart new greenhouses are shooting up in various places in East Africa, in which they are growing excellent tomatoes. As a consultant, it makes you proud to see these success stories; passing on knowledge is so satisfying.

Herbert Stolker.
Senior Consultant Africa.

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As every grower knows, thrips are a huge problem in ornamentals grown under glass. The usual suspect is the Californian thrips, a species with a strong preference for flowers. But in recent years another polyphagous thrips has been increasingly raising its head: Echinothrips americanus. Without timely intervention, this typical leaf-dwelling thrips can cause considerable damage to ornamentals such as gerbera and rose. Scientists have been taking another close look at how to control this thrips with a range of species of predatory mites and bugs.

In this study, striking differences were found between the four species of predatory mite used and the controlling effect was boosted in some cases by providing pollen. Predatory bugs in the Miridae family have been found to be very effective predators of Echinothrips but the options for using them very much depend on the type of crop.

Switching methods

Unlike the Californian thrips, which is a typical flower thrips, Echinothrips americanus prefers leaves. It is easily identified by its black body with two distinct white spots on the wings. But because it often hides away low down in the crop, its presence in the greenhouse can be something of a surprise. Although the species has been present in the Dutch greenhouse horticulture sector for about 20 years (it was imported from North America, as its name suggests), we still know very little about this creature.
In recent years there has been a clear increase in this species at nurseries switching to integrated pest control. The reduction in the use of broad-spectrum insecticides has given this thrips more chance to establish itself in crops. To avoid disturbing biological control of other pests, it makes sense to also tackle this insect with natural predators. Predatory mites and bugs are good candidates for this.

Control with predatory mites

In the lab, the researchers investigated how susceptible the various stages are to predatory mites and how many individuals of each stage are eaten per day. This was tested with the predatory mites Amblyseius swirskii, Amblydromalus limonicus, Euseius ovalis and Euseius gallicus. The latter two Euseius species establish readily in rose and are therefore interesting candidates for this crop.
Striking differences were found between the species. A. limonicus ate the most Echinothrips larvae, followed by E. ovalis and A. swirskii (see figure). A. limonicus also laid the largest number of eggs out of these mites. Surprisingly, the thrips were more or less left untouched by E. gallicus. The exact reason for this is unclear.
Interestingly, the pupae were also susceptible to the mites. This immobile stage, which is also found on the leaf, cannot defend itself and is therefore suitable prey.
With the exception of E. gallicus, all species of predatory mite ate the pupal stage. However, as the pupae are much larger than the larvae, the number of individuals eaten per day was a lot lower than in the larval stage. None of the predatory mites ate adult Echinothrips. In addition to the number of thrips being eaten by each individual predator, an important indicator in pest control in a crop is how well a predatory mite establishes itself and in what densities: after all, a high density can easily make up for lower predation rates per individual.

Results in the crop

The predatory mites A. swirskii and A. limonicus were compared in a greenhouse gerbera crop, with the results matching those in the lab quite closely. The thrips were controlled better with A. limonicus than with A. swirskii. This difference has also been observed in other studies with roses and sweet peppers.
Curiously, though, Echinothrips often doesn’t disappear completely: it is somehow able to survive the pressure from the predatory mite. So in a subsequent study the scientists looked into whether control could be improved by offering pollen as an alternative food source. Pollen can massively boost the density of predatory mites, which could lead to better pest control. The downside is that the Californian thrips also feeds on pollen, potentially causing scenarios that are detrimental to thrips control.

Effects of pollen

An important question, therefore, is whether this also affects Echinothrips. With bulrush pollen, the answer was a resounding no. Unlike with the Californian thrips, there was no impact on development time, egg laying or population development whatsoever. This is helpful in that it means supplementary food can be provided for predatory mites selectively, as long as there are no Californian thrips present.
The effects of pollen on thrips control were tested on non-flowering pepper plants using the predatory mites A. swirskii, E. gallicus and E. ovalis. Providing pollen increased the densities of all these mites. Control of Echinothrips using this method was significantly better with A. swirskii but despite the higher densities there was no control effect with E. gallicus. E. ovalis responded extremely well to pollen but the control effect was just as good on plants without pollen.

Predatory bugs

It has been known for a long time that the bug Orius majusculus is an effective predator of Echinothrips on sweet peppers. Orius is used to control thrips in this crop with great success. But the bug doesn’t establish in many ornamental crops. In recent years, scientists have been looking at the effect of omnivorous predatory bugs in the Miridae family on gerbera. In addition to Macrolophus pygmaeus, various Dicyphus species such as D. maroccanus, D. tamaninii and D. errans have also been trialled. All these species controlled thrips extremely well. Plants were made completely clean and remained so.
The bugs seem to be excellent candidates for preventive use against a range of pests and they can maintain themselves well because they eat a variety of prey and plant sap. This can also be a disadvantage, however, because feeding on plants can cause flower damage. Further research will be carried out in the future to see whether and when that happens and whether it can be avoided. The use of these bugs in gerbera could be a breakthrough in biological control of Echinothrips as well as other pests. Future research in other crops, such as roses and pot plants, is planned to ascertain whether the use of predators with banker plants can be supported.


Echinothrips is appearing more and more frequently in ornamentals grown under glass. In trials with a number of species of predatory mite, A. limonicus provided the best control while E. gallicus had little if any effect. Control with A. swirskii was improved by offering pollen. Besides predatory mites, predatory bugs of the Miridae family were found to be excellent predators but further research is needed to prevent potential flower damage and to improve establishment in different crops.

Text and image: Gerben Messelink, Somayyeh Gasemzadeh and Ada Leman (Wageningen University & Research).

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Completely separately from one another, two Belgian rose growers with premises barely ten kilometres apart came to the conclusion that a combination of SON-T and LED top lighting is the best solution for their nurseries. This makes them the first rose growers to use hybrid top lighting. Since then, other growers from as far away as Russia have come to take a look.

Electricity is expensive in Belgium: it costs around € 35-40/MW more than in the Netherlands as a result of higher transmission and distribution prices. So it makes sense to generate as much as you can yourself and buy in as little as possible. This was a key factor in growers Rozen Scheers’ and Wimceco’s decisions to install combination lighting.
Both nurseries were looking to upgrade their partly outdated systems and were thinking about how to do so without having to start buying in a whole lot more electricity. At Scheers they were able to increase the light level from 100 to 150 µmol/m2/s using exactly the same amount of power. Wimceco went from 130 to 180 µmol/m2/s with a modest additional input of 150 kW.

Practical experience

Wim and Annelies Scheers’ nursery Rozen Scheers in Kontich in the province of Antwerp has 3.8 hectares of glass. They have 15 varieties in their product range: popular roses such as Avalanche and Red Naomi along with special varieties like Jumilia and Cupcake. Sales take place via Euroveiling in Brussels, but direct sales to consumers are also an important channel.
Wimceco in Boechout, also in Antwerp province, grows Avalanche and Sweet Avalanche on 1.75 hectares. They also grow a few lesser varieties to vary sales to consumers. Their roses are auctioned at Euroveiling in Brussels and FloraHolland. Co-owner Danny van Nuffelen: “Our lamps and reflectors needed replacing as their output was dropping. We wanted to increase the light level but we didn’t want to buy in too much electricity.”
That could be done by combining SON-T (HID) with LEDs, but no growers had ever tried this combination with cut roses before. “That’s why our suppliers Agrolux advised us to go and visit Leo van der Harg, a pot rose grower in Vierpolders in the Netherlands. He has been using a hybrid system since 2014. His experience went some way to removing the uncertainties – after all, he is also a rose grower. What’s more, our starting point was that a photon is a photon, no matter what kind of lamp it comes from,” he explains.
At around the same time, Wim Steeghs of Philips Horticulture LED Solutions suggested setting up a trial with hybrid lighting at Rozen Scheers. That was when Wim Scheers first heard that his colleague ten kilometres down the road had similar plans. He also went to visit Van der Harg.

Climate control

A complete switch to LEDs only was not on the cards. “That really is too new and would be much more expensive. Also, you would have to completely revisit your climate control because you wouldn’t have the extra heat from the HID lights,” Van Nuffelen says. “And anyway, LED light on its own is too red to work in. No research has been done yet into what effect that has on people. It looks very dark.” He and his brother Bart decided on a 50/50 split between HID and LED. The lights hang in the same line: three LED fixtures followed by one 1,000 W SON-T.
Fellow grower Scheers uses LEDs for 70% of the light and SON-T for 30%. “We have two CHP units of 1.6 and 1.2 MW. We wanted more light but without using any more electricity. That’s why we decided on this combination. We have two continuous lines of LEDs in each bay interspersed with one HID reflector every metre.” The system is fitted in a 1.5 ha greenhouse which makes up 40% of their total greenhouse area. It was installed by Dutch B-E De Lier. In both cases, the LED fixtures are Philips GreenPower LED top lighting.


The availability of a subsidy from the Flemish Agricultural Investment Fund (VLIF) made the decision on the high investment costs a little easier. Nominally, the VLIF contribution for LED systems is 30% of the investment costs, but it works out at less than that in practice on account of special conditions such as a cap on the investment amount per square metre. The ultimate support percentage is around 20-25%.
The systems were installed in November 2015, so the growers now have well over a year’s experience with hybrid lighting under their belts. They are reckoning on a payback time of 4-5 years.


The HID and LED lights at both nurseries can be switched on and off separately. Van Nuffelen: “This has made us more flexible. The LEDs are left on until there is around 250 W of daylight; the HID lights are switched off earlier. So the LEDs were on for 890 hours longer than the HIDs last year.” Fellow grower Scheers used his LEDs for 1,100 hours more than the HID lamps.
When switching to LEDs, a big question is whether or not the radiated heat from the HID reflectors needs to be compensated for. That can have advantages and disadvantages. In theory, the grower needs to ventilate less to get rid of surplus heat. This is less of an issue in Belgium than in the Netherlands because light emissions don’t yet have to be controlled with blackout screens, so Belgian growers are spared the situation in which the temperature under the closed screen rapidly gets too hot. Because the nurseries kept no records of how long the vents were open, it is difficult to comment on this. Moreover, the light level was higher in the new situation in both cases, making it difficult anyway to draw a comparison with the previous situation.

Heating up

On the other hand, the radiated heat can be a welcome way of preventing the head from cooling down compared with the rest of the crop. “We can use top heating to compensate for the heat from the HID reflectors. In theory the crop should cool down quite a bit in the new situation, but our measurements show that the temperature didn’t drop as much as we had anticipated,” Scheers says.
Advisor Steeghs has an explanation for this. “LEDs also provide some indirect heating. They are 55ºC and transfer their heat to the aluminium profile. That transmits the energy to the light and water in the greenhouse via longwave radiation. The crop isn’t warmed up directly but indirectly.”


A trial with 100% LED top lighting above Red Naomi has been running at the Delphy Improvement Centre in the Dutch town of Bleiswijk since April 2016. The light level is 200 µmol/m2/s and OPAC heat exchangers keep the temperature at the right level. Steeghs: “Before the trial the growers had said that they needed the radiated heat from SON-T and that 100% LEDs would not produce good results. But so far the plants have been growing very well.”
The results achieved in Bleiswijk in the summer and autumn were surprising: heavy roses with a long stem, a consequence of the lower bud temperature due to the reduction in radiated heat combined with the cooling properties of the OPAC heat exchangers. Outside the trial, growers were having trouble getting good stem lengths in the warm autumn. It remains to be seen what the results of the trial in the winter will be.


A year on from the investments, both growers are happy with their decisions. Van Nuffelen: “The system has definitely lived up to our expectations. We have an older crop but it is in better condition this year than last year and has survived the summer better. A big advantage of this system is that it makes you more flexible: you can go on lighting for longer on a hot day with LEDs without pumping in heat from the HIDs.” At the time of writing he was unable to say how much more production he was getting with the higher light level.
At Rozen Scheers the figure was 20%, with 30% for some varieties. “That is much as we expected, but it could be even more. We are trying to grow as organically as possible but we had a lot of trouble with thrips last year and we had to cut out unsaleable product. Once we have that under control, production will be up,” he says.
Convinced by the results, in early November he had exactly the same combination installed in another 1 ha greenhouse, again with a 70/30 split between LEDs and HID reflectors. This time the lights were supplied by Agrolux. The way the system is connected up means that his large CHP unit can power the LEDs throughout the whole nursery and the other unit can be switched off when only the LEDs are on.


Scheers plans to replace another two older low greenhouses in the future. As there is a stream running right behind the greenhouses, the authorities have made it a condition that he makes a contribution to solving the flooding problems in the area. His fellow grower in Boechout will be changing the old crop first. A new greenhouse may be added in the future.
In the meantime, there is a lot of interest from other growers: sometimes entire coachloads drop by. Philips regularly gets enquiries from Russian rose growers who want to see the impact of hybrid lighting on the crop in practice.


Belgian nurseries Rozen Scheers and Wimceco are the first cut rose growers to install hybrid top lighting. Their decisions were influenced by the high cost of electricity, subsidies and the ageing of their previous systems. Initial experience is good: production is up as a result of the higher light levels and the heat produced by the HPS lamps can be easily compensated for. Scheers has just installed the same system in a second greenhouse.

Text: Tijs Kierkels. Images: Wilma Slegers.

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The German cut rose sector mainly consists of lots of small companies that grow a wide range of cultivars and focus on private sales. This is also the case at Rosen Sabrowski, a company based in a residential area in the German town of Recklinghausen. Owner Rudi Attenberger has managed to turn this risk into an opportunity in recent years, and currently sells 70% of his roses to private individuals. Despite this, increasing costs – for example for power and personnel – present the grower with considerable challenges.

It may seem a rather unlikely location for a rose nursery, nestled between blocks of flats in Recklinghausen, a town with about 120,000 inhabitants in the northern Ruhr area. Nevertheless, Rosen Sabrowski has managed to survive here for over 60 years. “My father-in-law started growing various cut flowers here in 1955: chrysanthemums, daffodils and later roses,” says Rudi Attenberger (58).
Although this grower does not have a horticultural background, he and his wife decided to continue the business in 1977. To begin with, they also grew daffodils, but the focus soon turned to roses. “Rose cultivation appeals to me,” he explains, “on top of which the rose is the number one cut flower.”

Focus on non-business sales

When the grower took over the company, it had a growing area of 1.2 ha and most of the roses were sold through the Blumengrossmarkt in Dortmund. “A Grossmarkt is typically German; this is a kind of market where growers can sell their flowers or vegetables to market vendors and retailers. For many cut flower growers in this country this market is one of the main distribution channels.”
Today, Attenberger sells just 30% of his cut roses through the Grossmarkt, with the rest sold in his own shop. “We haven’t expanded the business since we took it over as there is simply no space to do so at our current site. Even so, starting somewhere else was never an option as I believed that our location was in fact full of opportunities. After all, our clients were right on our doorstep. So I decided not to invest in more glass, but in creating a new market.”
The grower has worked on professionalising private sales in the past decades, and has built a large, modern shop. Large investments were also made in advertising. “Because of this, and largely through word-of-mouth advertising, we were steadily able to increase our private sales. We now have eight employees for both the nursery and the shop. Our daughter Tina, who is a florist, is also involved in the company.”

Local for local

Attenberger’s customers come from an area in and around Recklinghausen, up to a distance of about ten kilometres from the company’s location. “Our advantage is that we offer a wider range of fresh products than flower shops. We also don’t have to worry about competition from other growers, as the nearest rose nursery is almost 100 kilometres away. It’s not for nothing that ‘Unique in the region’ has been our slogan for many years. To supplement our cut rose assortment, we also sell potted shrub roses in the spring and summer.”
Germans prefer to buy vegetables, fruit and flowers locally. “More than consumers in other countries, they want to know where a product comes from and how it was made; they believe in sustainability, reliability and quality. Plus, they are prepared to pay a little extra for this. So it is hardly surprising that many German rose growers focus on private sales; there are very few companies who only produce for auction.”

Wide assortment

The decision to serve the non-business sector also influences the product range. To provide his clients with choice, Attenberger grows 21 different cultivars in a wide range of colours. “This allows us to offer mixed bunches, which are very popular here. It also enables us to distinguish ourselves from flower shops, which usually stock a limited range. We do however focus mainly on orange, two-tone and large head roses, as these are the most popular among our consumers.”
A wide assortment brings with it some technical challenges. For example, he always needs to steer a middle course when it comes to climate, fertilisation and irrigation. “Partly because of this, our production is no higher than 140 to 150 stems per square metre. This is much lower than growers in the Netherlands. However, Dutch growers concentrate on high-production species, while we choose cultivars for which there is demand. And as our roses sell for an average of € 1.50 per stem, we don’t actually need to achieve the highest production.”

Optimising the greenhouses

Although Attenberger has not expanded the business since 1976, he has invested over the years in optimising and modernising his company. “My greenhouses are not ultra-modern, some of them are almost 40 years old. About a third of the greenhouses were built in the traditional way, with roofs that are 9 or 13.2 m wide. This is not ideal in terms of climate control, as ventilation is more limited. The rest are Venlo greenhouses, with 3.2 m wide roofs. We modernised the greenhouses about 10 years ago, when we increased their height from 2.2 to 4 m. I would have liked to have made them even higher, but it was difficult to obtain permits. In the summer, the temperature in our greenhouses is higher than in most modern greenhouses, and climate control is more limited.”
The rose grower was also one of the first German growers to invest in a UV system for recycling water. He also invested in new energy screens last year. “And we were one of the first growers in our country to start implementing integrated crop protection ten years ago. That was sometimes difficult to start with, but we are getting increasingly better at it. Only thrips are a problem these days.”

Increasing costs

The grower invested in assimilation lighting about 15 years ago so that he could supply his clients year-round. “Only a handful of growers in our country still switch on the lights during the winter months. This is because of the high electricity prices; we currently pay about 20 euro cents per kWh.” Of this, about 7% is for the electricity tax that the German government uses to pay for the transition from fossil to renewable energy. These tax goes up every year. As a result, many growers have been forced to stop using lighting in the winter. “We still manage because of our private sales, but I don’t know how long that will last.”
For heating, Attenberger uses residual heat from a nearby wood-burning power station. “We had a contract for the last ten years and paid just 2.2 euro cents per MW. However, this contract has now ended and the price went up to 3.0 euro cents per MW. So we are faced with rising costs in this area too.”
Labour costs are also on the rise. “Since here the minimum wage was introduced in 2015, we have had to pay more every year. We need to recover these higher costs in one way or another and, although our consumers are prepared to pay more for a high-quality rose grown locally, this cannot go on forever. This is the biggest challenge we face in the future.”


At 1.2 hectares, Rudi Attenberger’s company can be considered an average-sized German rose nursery. The company’s location, in the middle of a residential area, makes expansion impossible. Therefore he decided to concentrate on private sales, increasing them to about 70%. The grower is reasonably satisfied with his results, but he is concerned about the increasing costs, for example for power and personnel. The extent to which he can pass on these costs to his customers remains a question of concern.

Text: Ank van Lier. Image: Tina Attenberger en Ank van Lier