Diffuse light is a hot item in greenhouse horticulture, not in the least thanks to Leo Marcelis. Professor Marcelis conducted research into the benefits of diffuse glazing as early as 1987. ‘Diffuse glazing may be more expensive, but with an increase in yield of 5% you can earn back your investment in no time at all. In many cases the increase in production will even exceed this 5%.’

‘When I conducted a study on diffuse light in 1987, this could only be performed through simulation in calculation models. Theoretically, we were able to conclude that diffuse glazing definitely had positive effects on the plant growth and crop production. Twenty-five years later we learned something, after all. The results achieved with diffuse glass were even better than those calculated at the time.

‘Around five years ago the production started of various types of diffuse glazing with good light transmission properties. Several tests were conducted on this, showing, above all, that diffuse glass works: it has a positive effect on production, resulting in production increases from 5 to 10% a year.’

The application of diffuse glass: Marcelis is convinced of its advantages. Whether the horticulture industry backs his conviction is not entirely clear, particularly because only few new greenhouses have been built in the past years.

‘Nevertheless, many of the greenhouses being built today are equipped with diffuse glazing.’ He continues, diplomatically: ‘Any grower not opting to use diffuse glass will probably have a good reason. Diffuse glass may be more expensive, but with an increase in yield of 5% you can earn back your investment in no time at all. In many cases the increase in production will even exceed this 5%.

‘The diffuse glass on the market today generally has poor light transmission properties, so I would not recommend it. What you gain on the one hand (light distribution), you lose on the other (light). Light is, of course, a basic requirement for plant growth. You have to use as much of it as you can, and as efficiently.’

Can diffuse glazing be applied in all countries of the world?

‘The positive effects produced by diffuse glazing are not restricted to the Netherlands; diffuse glazing will produce beneficial results in every other country in the world, and particularly in countries where there is a lot of light.

‘The more light, the greater the impact produced by diffuse glass. It is particularly beneficial when the direct light is strongest; the strong beams no longer focus as directly on the top of the plant. The intensity of the light is uniformly distributed throughout the greenhouse.’

Which crops benefit most from diffuse light?

‘With conventional glass, you will alternately have shady spots and spots with a high light intensity. This can cause damage to leaves, which is particularly undesirable if you are growing potted plants. This is why growers of potted plants frequently reduce the amount of light through screening or the application of chalk. This does, however, have a negative impact on the growth rate and the colour of the flowers.

‘Many growers of potted plants employ a low light intensity programme to prevent any chances of leaf scorch. Diffuse glazing will, however, allow for a higher light intensity. This could be of considerable advantage to growers.

‘When there is a lot of direct light, the temperature will rise while the humidity drops. Achieving a better balance in this will allow more light to be absorbed by the plant. Anthurium, for instance, is generally grown at a light intensity of 5 mol without humidifiers. Depending on the pot size and the season, a crop will take 22 weeks to grow. With diffuse glazing and a humidity that does not drop below 70% you could easily increase the light intensity to 10 mol. As a result, your crop will be ready for sale in 16 weeks and will be heavier, too. That is an astounding result. In the 25 years of my research, I have never conducted a study that produced such a big leap in production in comparison to what is being done in practice.

‘That you can achieve such leaps in production in 2014 is hardly believable. You would only expect this in developing countries, where all crops are heavily screened.’

Does this mean that you need to be able to control humidity to gain more light?

‘To fully enjoy the benefits of diffuse light you must be able to fully control the greenhouse climate. In those instances where screening is used during the day you would achieve better results with a diffuse screen.

‘Growers who already have their greenhouse set up won’t be eager to switch to diffuse glazing, because that would create a lot of havoc. In those cases, you could consider installing a diffuse screen; that will also produce positive results. Typically, new glass is only installed in newly built greenhouses; in that case diffuse glass should be used. All of this applies even more so to the cultivation of potted plants.’

What would happen if an outstanding replacement for glass were to be invented?

‘I assume that greenhouse covers will continue to be made of glass, but you cannot exclude the possibility of a good synthetic alternative being brought to the market in a few years. An alternative that is available today is an outstanding foil with excellent light transmission properties: F-clean. Apparently, there is also a diffuse variant. However, this foil is very expensive. If it were to drop in price, it may very well replace glass.’

Leo F.M. Marcelis (Elst Gld, 1963) studied horticulture at Wageningen University, where he obtained his PhD in 1994. He was a professor by special appointment of Crop Production in Low-Energy Greenhouses at Wageningen University until 2013 and team leader at Wageningen UR Greenhouse Horticulture. On 1 December 2013 Prof. Dr Leo Marcelis was appointed Professor of Horticulture and Product Physiology at Wageningen University.

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Source/photo: Tuinbouwteksten.nl/Theo Brakeboer.