Growing flowers without fossil fuels?

Greenhouse horticulture is facing daunting challenges because emissions and leakages of CO₂, water, nutrients and plant protection products will eventually have to be eliminated entirely. With financial support from the Ministry of Agriculture, Wageningen University & Research (WUR) has built a demonstration greenhouse (KAS 2030). It is being used to investigate how growers can realistically achieve the climate objectives while remaining financially viable.

The demonstration greenhouse was built in 2020. It is located in Bleiswijk, next to WUR’s Greenhouse Horticulture site. The complex consists of four sections of 350 m² each. Every year, a decision is taken on what crops to grow in the four sections. In previous years, the researchers grew crops including strawberries, bell peppers and even small melons. In 2026, flowers and pot plants could be found growing in the greenhouse. A growers’ committee is brought in for each crop. The committee members regularly come along to examine the plants, give advice and learn in turn from the Wageningen experiments. 

Project manager Frank Kempkes and his colleagues keep a close eye on the emission flows — of energy, water and crop protection agents — in the greenhouse. They look at those flows as an integrated whole. ‘Researchers often study each aspect individually,’ says Kempkes. ‘But now we want to know what happens to the other emissions if we do something to alter one of those emission flows.’

A greenhouse full of gerberas, one of the most popular cut flowers in the Netherlands, guzzles a lot of natural gas. Dutch horticulture is currently navigating a transition from fossil fuels to sustainable energy sources. In Bleiswijk, Kempkes and his colleagues are looking at how flowers can be grown without using any fossil energy.

‘First, we did everything we could to save energy,’ says Kempkes. ‘We use only LED lighting, which is very energy efficient. We also have screen systems that retain the heat at the top of the greenhouse. Many gerbera growers have one or two of these screens, but we have three. That insulates the greenhouse better.’ Nearly all gerbera growers have switched to full LED lighting.

An electric heat pump can keep the greenhouse heated all year round. The lighting and air treatment also use electricity. ‘You can never reduce the energy requirements to zero because then it wouldn’t be possible to grow crops all year round. So it’s OK to use some energy, but it has to be sustainable energy.’

One conundrum the Wageningen researchers had to solve involved the humidity inside the greenhouse. Screen systems keep the greenhouse very well insulated, but they also make it more difficult for moisture to get out. That increases the risk of fungal growth. ‘That’s why we also have a dehumidification system,’ says Kempkes. ‘It lets us recover up to 100 litres of water per square metre. This is water that would be lost through the windows in a normal greenhouse. But that isn’t the end of the story. If the dehumidifier is running a lot, it produces heat that we can’t get rid of. In that situation, we might decide to grow the plants in slightly more humid conditions. You’re constantly trying to find the right balance.’

Another challenge concerns the CO₂ that the greenhouse plants need to grow. Rather than natural gas, KAS 2030 uses a non-fossil source for this: ‘We get our CO₂ through the OCAP network, a grid of pipelines in the west of the Netherlands. That is mainly CO₂ released during bioethanol production, for example from the digestion of maize.’ It is harder for growers who are not connected to the OCAP network to end their use of natural gas. ‘We are testing systems that capture CO₂, but they currently consume a lot of energy and may as a result end up emitting more CO₂ than they capture. We still have a long way to go here.’

Not all the water used in horticulture ends up with the thirsty plants; some is lost through leaks or has to be discharged because the quality is poor. When that happens, nutrients such as phosphate, potassium and calcium are lost too. In KAS 2030, all the drained water is collected and decontaminated using an ozone purifier to stop diseases from being spread via the water. This makes it possible to reuse all the water from most crops. 

Spathiphyllum (the peace lily) is currently being grown in one corner of the Bleiswijk greenhouse. This is a tropical plant that needs relatively large amounts of water. Most Spathiphyllum growers use systems that water the plant from above, but this method has several disadvantages, says Kempkes. ‘First, you end up with leakages — the water that falls next to the plant and isn’t collected. You also need to apply heat to get the droplets on the leaves to evaporate, and that costs energy in the winter that has to come from the heating system. Another problem is that the organic crop protection products get washed off the smooth leaves.’

For these pot plants, KAS 2030 uses an ebb-and-flow system. The plants are put on a watertight table that can be flooded, so that the plants take up water from below. The surplus water is then discharged for reuse later. 

However, the Wageningen researchers encountered another problem when using the ebb-and-flow system: root diseases. ‘These diseases spread faster with this system, which is why growers are reluctant to use it. This was a big problem in the past in particular, when water purification methods were less effective. But we still need to monitor the situation very closely, so we’re constantly carrying out measurements.’

The water quality is crucial, says Kempkes. ‘If your water composition isn’t right, for example because it has too much sodium or fluorine, you’re more likely to need to discharge it. We have demonstrated that you never have to discharge it if you start out with good quality water. That can be rainwater, or groundwater in many places. But you would need huge reservoirs to collect enough rainwater to keep going the entire year, and they take up a lot of room. In our greenhouse, we use osmosis water after we have run out of rainwater. That is water created in a process that removes the salts. Thanks to this, we’ve never had to discharge water because any element exceeded the permissible concentration. The quality has always been up to standard.’ 

A greenhouse will naturally have all kinds of insects crawling and flying around the plants. Growers always find it a challenge to control the insects that are deemed pests because they damage the plants or fruits. Pest control by deploying their natural enemies is already standard practice in most Dutch greenhouses, but chemical crop protection agents are still needed sometimes if a system is out of equilibrium.

‘Our ultimate goal was clear: to reduce the use of chemicals to zero. We are now nearly there after progressive reductions,’ says entomologist Ada Leman, who has been involved in KAS 2030 from the start. ‘Each time we had a new crop in our greenhouse, we started with the strategy growers use currently. Then we began making modifications, adding or removing something, introducing a different natural enemy or technique.’

In the case of the gerberas, where predatory bugs are used to combat various pests, the researchers introduced ‘banker plants’. These are plants that are placed between the crop plants to encourage useful insects, explains Leman. ‘We use great mullein, a plant in which predatory bugs can shelter and lay their eggs, and Lobularia, which produces a lot of nectar that benefits parasitic wasps. These plants sustain the populations of natural enemies so that they are ready to attack as soon as a pest appears.’

The same banker plants were tried out in the cultivation of Anthurium pot plants, but without success. A different approach was therefore investigated for this crop: using experimental natural enemies — spiders — to combat thrips. The researchers still needed to apply a lot of chemicals initially, but five years later Leman and her colleagues did not need to spray the plants at all.

The banker plants are generally a success, says Leman. ‘Gerbera growers were very sceptical about the idea to start with. But when we showed them what could be achieved, they were won over slowly but surely. Now a lot of gerbera growers are using this approach — as many as 80 per cent, I believe.’ There are downsides to the method, however, as the growers have discovered in practice. ‘Predatory bugs are omnivores and they find the crop tasty too. If you have too many predatory bugs and too few pest insects, the bugs may start eating the very crop they are supposed to be protecting. So it’s always a puzzle finding the right balance. It’s a never-ending challenge.’

Thus, the researchers and growers are constantly trying to solve the greenhouse puzzle. ‘The moment you pull on one lever, that leads to changes elsewhere,’ says Kempkes. ‘Some things don’t work at all. But it’s better if things go wrong for us than for growers in their practice.’ Ada Leman hopes to be able to find more and more strategies that work well, including ones for difficult crops. ‘Growers are also constantly having to deal with new pests, in part because of climate change and the international trade in plant material.’

‘Nobody will be building greenhouses like the one we have here,’ concludes Kempkes. ‘We designed it for maximum flexibility, a greenhouse where we can grow different crops at the same time and make as many technical alterations as possible. Growers don’t need all of that. But various elements of KAS 2030 can be adopted in practice, and that’s precisely why we are doing this.’