Tomorrow’s greenhouse

New techniques are making greenhouse horticulture more productive and greener

There’s blue and red light shining, an ingenious climate system is calculating the temperature of each individual plant and branch, and a computer is giving the horticulturalist advice about trimming the leaves. Welcome to the greenhouse of the future.

Text: Dennis Rijnvis

Why does one rose in a greenhouse grow quicker than the other? And why does one bell pepper get mouldy but the other one not? Horticulturalists struggle with these quandaries all the time. Some plants remain much too small for no apparent reason. Or they get sick. This happens despite the fact that the growers regulate the climate to the best of their ability and other plants do manage to develop well.

Ideally, in the future a computer will detect climate variations in a greenhouse on time and simultaneously solve the issue.

‘This can be really frustrating,’ says Leo Marcelis, professor of horticulture at Wageningen University & Research. ‘Growers are losing part of their harvest and can’t figure out why.’ But that will change soon if Marcelis has anything to say about it.

It’s warm at the top

Marcelis discovered that the climate can vary considerably in a greenhouse. A leaf growing on a rose or tomato plant in the top part of a greenhouse is sometimes as much as two or three degrees warmer than a leaf in the lower part of a greenhouse. ‘That’s because the leaves in the top part receive direct light from the sun or a lamp.’ The plants in the lower part grow in the shade of the canopy. As a result, they’re cooler. And that can have unfortunate consequences. ‘Sometimes it’s not warm or light enough for plants in the lower part of a greenhouse. Then they won’t grow well.’ Growers aren’t always aware of this. For example, right now they only monitor the temperature with sensors above the plants. The climate variations also regularly cause diseases. The water that’s been absorbed by the roots evaporates in the leaves. Sometimes the air is so humid in places with a lot of foliage that it’s a paradise for moulds.

Marcelis and his colleagues are now working on a computer model that can display the climate three-dimensionally in greenhouses, so that horticulturalists can immediately see where in their greenhouse plants are too cold, too warm or too moist. Once you’re aware of the climate around each plant, flower or fruit, says Marcelis, then you’ll have a better understanding of why certain roses or tomatoes aren’t growing as well as they should. And how you can change that. ‘It means throwing away far fewer fruits and flowers. You’ll also save energy by measuring more accurately.’

Find the problem

Iedere plant zijn eigen sensor geven voor temperatuur- en vochtmetingen? Dat is volgens Marcelis te duur en te omslachtig. Hij hoopt computers te leren hoe ze het  klimaat tussen verschillende planten zo nauwkeurig mogelijk kunnen voorspellen met behulp van slechts een paar strategisch geplaatste sensoren. ‘De computer zou een simulatie kunnen maken op basis van  bijvoorbeeld de dikte van het bladerdak, diverse temperatuur- en vocht metingen  en het aanwezige zonlicht op bepaalde plekken. Je hoeft dan als tuinder niet zelf constant al je planten te checken. Daar is geen beginnen aan.’

How about giving each plant its own sensor to measure the temperature and moisture levels? According to Marcelis that’s too expensive and cumbersome. He hopes to teach computers how to predict, as accurately as possible, the climate around the plants with the aid of only a few strategically placed sensors. ‘The computer could make a simulation based on the thickness of the canopy, various temperature and moisture measurements, and the sunlight that’s present in certain places. As a horticulturalist, you wouldn’t have to constantly check your plants then, which is an impossible task.’

Perhaps this system will be able to send instructions to the horticulturalist of the future. ‘The computer could advise you to cut leaves in certain places to control mould or encourage growth.’ In some cases, the system could even intervene. Marcelis imagines a computer cooling, warming or giving plants more light. ‘The ultimate aim is to be able to individually manage every factor involved in the growth of plants.’ That’s still difficult at the moment. When horticulturalists turn up the lights in their greenhouses, they automatically raise the temperature too. That’s far from ideal when growing flowers and fruits. ‘Take a rose. It grows better when there’s lots of light. But if you give this kind of a flower more light in a traditional greenhouse, it will get warmer too.’ And that, in turn, is not always good. Because roses remain smaller at high temperatures.

Give the plant far red light and it shoots up vertically, which is useful as it enables plants to make long stems

Light yes, but no warmth

The greenhouses of the future will therefore have separate systems for heating and light, according to Marcelis. Pipes with warm or cold water will probably regulate the temperature. LED lighting will provide the light. ‘It emits much less heat than traditional lighting. It enables you to give your plants more light without overheating them.’ Playing with light gives you more control over the taste and shelf life of products. ‘For example, if you give a head of lettuce more light in the greenhouse while it’s growing, then the leaves will be especially high in vitamin C. These vitamins also extend the shelf life of the lettuce.’ But the greatest benefit of LED lighting is the ‘disco effect’. With the push of a button you can adjust the colour: from yellow to red or even blue. Why is that useful? Different kinds of light-sensitive cells, the photoreceptors, manage the growth of plants. Some of these cells react especially strongly to lights of certain colour. ‘For example, if you stimulate plants with far red light, then they mainly shoot up vertically. That’s useful with certain plants, because it enables to make long flowers stems.’ What’s more, you can also save a lot of energy with LED lighting, up to fifty per cent. ‘So in the long term LED lighting is better than old methods of lighting for both the climate and the horticulturalist’s wallet.’

Screen it off

There’s another benefit: LED lighting can substantially reduce light pollution. When German researchers did light measurements last year in different parts of the world, they concluded that the Dutch community of Schipluiden was the most lit up place on Earth at night, because of the many greenhouses there. At the moment, horticulturalists can’t block the light from their greenhouse lamps by sliding screens in front of the glass. The traditional lamps emit too much heat for that.

‘If you screen off the windows, the heat lingers. The plants get too hot then.’ Because LED lighting emits little heat, you would be able to use screens in the future. How long will it take for Marcelis’ greenhouse of the future – with all its smart computers, LED lighting, coloured light and light screening – to become reality? Not so long, in his opinion. ‘The techniques have been thoroughly tested already. It’s a question of years, not decades.’

Growing on Mars

If people ever travel to Mars, then they won’t get by without knowing how to grow in greenhouses. In addition to being a researcher at Wageningen University & Research, Leo Marcelis is also an advisor for growing in space. He advised Mars One, for example, a club that wants to start a human colony on the planet.

Vrouw in astronautenpak bij kweekopstelling

‘If you want to cultivate plant crops there, there’s no way you can do it outside,’ he explains. ‘It’s much too cold. So you would have to do your growing in a closed environment. Probably not in a greenhouse on the planet’s surface because there are too many cosmic rays there. You would need to dig up a bunker or use a cave.’ Many techniques from greenhouse horticulture would be useful in this kind of an environment. ‘You need lighting, a system that regulates temperature and humidity, and also computers to analyse the climate. These are all things that we’re working on as well in our study of horticulture on Earth.’

The history of the greenhouse

(photo credits: Spaarnestad Photo/HH)

  • The first greenhouses were possibly built around 14 AD in the Roman Empire. It’s said that Emperor Tiberius ate so-called Armenian cucumbers every day. It was too cold in Rome to grow them in the winter. Tiberius’ gardeners therefore grew them in small houses with a transparent roof, through which the sun could shine.
  • The first descriptions of heated greenhouses can be found in the books of a royal gardener from Korea. In about 1450, members of the Joseon dynasty grew small orange trees in greenhouses, among other things. An oven under the floor served as heating.
  • Greenhouses were first introduced in the Netherlands in the seventeenth century. Greenhouses were built in the botanical gardens of Leiden to cultivate tropical plants. In the nineteenth century, greenhouse horticulture started to take off in Westland. Initially grapes were grown to be exported to England.
  • After the Second World War, grape breeders faced stiff competition from Italy, France and Spain. At that point, many horticulturalists switched over to flowers and other fruits, such as tomatoes and cucumbers.

(Photo credits: Gerard-Jan Vlekke, George Frey/Getty Images, Wageningen University & Research, Spaarnestad Photo/HH)