Diffuse glass is becoming routine in new build greenhouses – and much more quickly than anticipated. But it is still difficult for growers to estimate precisely what added value it has for their crops. So researchers Silke Hemming and Tom Dueck are calling for growers to share much more of their experiences. Here they look at some of the issues faced by growers in practice.
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Over the past few years, Wageningen University & Research in the Netherlands has been scaling back its research into the advantages of diffuse light. After a number of years with very convincing trial results – a 5-12% increase in production under a diffuse roof – very little is being invested in fine tuning these days, as the greenhouse horticulture sector is expected to finance its own projects.

Bringing knowledge together

The general picture is clear. “There are already plenty of reasons to choose diffuse light to improve cultivation,” researcher Tom Dueck explains. “The crops we tested in all the trials produced bigger, better quality yields. But a lot of crops have hardly been tested at all, including some major ones like sweet pepper and chrysanthemum.” Dueck advises a group of sweet pepper growers who have a lot of questions. A great deal of knowledge has been accumulated on tomato and cucumber, but this can’t necessarily be applied to sweet pepper because it has some unique characteristics of its own, such as fruit setting in flushes, which causes highly fluctuating sink strengths.
Dueck adds: “It would definitely help if there were more data available. A lot of practical knowledge is being accumulated worldwide – a veritable treasure trove of data if it could all be brought together.”
His colleague Silke Hemming, who heads the research team, calls on growers to share the results of growing under a diffuse roof in the same way as in the energy monitoring programme. “If we have a lot of data, we can come up with new rules of thumb,” she says. “We have already been able to fine-tune the traditional rule of ‘1% light = 1% higher production’ using large volumes of data for a lot of crops. For chrysanthemum, for example, 1% more light equals 0.6% higher production. We would also like to produce similar rules of thumb for crops grown under a diffuse roof. This would help growers with their investment decisions. You can find this data in research papers, but the underlying database is limited. A lot of practical data would extend the database significantly.”

Ageing tests

For growers, the current shortage of sufficiently wide-ranging information leads to uncertainty and questions such as: Is the effect of diffuse glass not mainly due to the improved condensation distribution? Could you not get just as good results with AR-coated clear glass? And how does the glass behave over time?
At the WUR LightLab in Wageningen, they do in fact use older diffuse glass for their research, Hemming says. “But if we don’t know the history of the glass, we can’t draw any conclusions. Was it substandard to begin with? Has it been cleaned the wrong way, with fluoride, for example? In any event, what we can say is that there is no one single supplier whose glass often produces poor results.”
Dueck wants to see ageing tests carried out. “At IKEA they fatigue test their chairs millions of times to simulate ageing. Glass manufacturers should also have to test their glass with an accelerated ageing method relevant to greenhouses.”
In collaboration with the inspection agency TÜV Nederland, the WUR LightLab has developed an accelerated ageing test by adapting various solar panel ageing standards to suit the greenhouse setting. This has recently been made available to manufacturers.

Explanatory factor

Another question that arises in the commercial setting is whether the additional yield achieved under diffuse glass is not largely due to the fact that light transmission is better when the glass is wet. This would be the case if the condensation formed a water film on the roof. When the measurement protocol for wet greenhouse roofs was developed, it was discovered that this kind of water film could in fact increase light transmission on some types of diffuse glass by as much as 3% (0% was also obtained with the panes tested). “Condensation can therefore never be the explanatory factor for production increases of 5-12%,” says Dueck. “What’s more, a comparative test carried out in the winter with clear and diffuse glass produced an average of 5% higher production. That also points to a greater effect than could simply be ascribed to improved light transmission.”
In addition, Dueck’s colleague Tao Li carried out doctoral research in 2013 in which he investigated the reason why a diffuse roof resulted in better photosynthesis. He found that the improved horizontal light distribution was the most important factor, followed by better leaf photosynthesis and better vertical light distribution. None of these factors have anything to do with transmission.

Crops vital for longer

“We ruled out the effect of better light transmission many years ago, in our very first trial,” Hemming reports. “We used old greenhouses with clear glass and covered the outside of the glass with clear or diffuse film. Nothing was changed on the inside of the roof, where the condensation forms. Yields in cucumber and pot plants in the greenhouses with the diffuse film were much higher.”
Moreover, claims that the same results could be achieved with double-sided AR-coated glass can easily be dismissed with Tao Li’s research results and analysis. Dueck: “AR glass could have advantages in the winter due to its higher light transmittance, but the positive impact of diffuse light continues throughout the spring, summer and autumn. The crop suffers less stress, its photosynthesis capacity is higher and growers report crops remaining vital for longer in the autumn. Any additional yield under AR glass in the winter is completely outstripped by the benefits of diffuse light the rest of the year.”
Some growers report slightly higher energy consumption in the morning under diffuse glass. That is not caused by the diffuse properties but can occur if the glass has lower light transmittance, he says. “But 8% additional yield always offsets the cost of a little extra gas.”

Estimating added value

A central issue is that it is difficult for growers to estimate the added value for their crops. Manufacturers offer a whole range of glass types, from plain, low-iron and double-sided AR-coated to low diffusivity, medium diffusivity and high diffusivity. How are growers supposed to choose if they don’t have enough data on their own crops? Higher diffusivity is almost always better, the two researchers say. “We speak to growers who decided to go for just 20% haze and now regret their decision because their crop yields less than expected over the year and still suffers too much stress,” Dueck says.
They find diffuse coatings ideal for existing glass. “It’s true that they take away 4-5% light, but that’s not an issue in the months in which these kinds of coatings sit on the roof,” Hemming says. “And in southern countries, these coatings make a lot of sense,” her colleague adds.

Sharing experiences

A temporary coating offers flexibility, but the same definitely goes for a diffuse screen, which can be opened if the light loss continues for too long. “On the other hand, the screen intercepts more light than a coating,” Hemming says. “The trade-off will be different in every situation. The fact remains that growers need to do more to share their practical experiences to make it easier for growers to choose the best option.”


All trials with diffuse light lead to the same conclusion: higher yields and better quality. But many crops have barely been tested at all, which makes investment decisions tricky. Researchers are calling on growers to share operational data to help improve understanding. They take a look at some of the issues that crop up regularly in practice.

Text: Tijs Kierkels. Images: Wilma Slegers and Lé Giessen.