"Diffuse glass: not a myth but clear as glass!"

Previously, we have published an article that contained a column by Sjaak van Dijk, a Dutch cucumber grower who has his doubts about the glass. The article caused quite a stir in the Netherlands. In this article researchers from Wageningen University's horticultural extension would like to reply to Van Dijk's article to ensure that a balanced view is presented.

In construction, more and more growers cover their greenhouses with diffused glass. And rightly so, say researchers at Wageningen UR Greenhouse Horticulture. "Despite some misconceptions that occasionally pop up, diffuse glass has already proven itself in numerous comparative studies by achieving a higher production of 5-10%."

What does diffuse glass do?

What matters is scattered light: Diffused light is scattered light that penetrates deeper into the crop. This way a larger leaf surface is exposed and can take in more CO2 for increased growth, production and quality.

What effects of diffuse glass, and therefore diffuse light, can we see?

• Light is distributed more evenly among the plants with no shadow or light spots, which leads to a more uniform greenhouse climate and higher sugar production (all crops).
• Light penetrates deeper into the crop (all crops).
• Lower plant temperature at the top of the crop at high irradiance under diffused glass. This reduces the high peaks in sheet temperature, resulting in less stress for plant (all crops).
• The fruits are on average 8.5 g (tomato) and 10-15 g (cucumber) heavier.
• Especially with cucumber, more fruits are harvested.
• Starting at the beginning of the harvest, production is higher under diffuse glass, with ultimately 5-11% more production from (tomato and cucumber).
• Potted plants are heavier, the cultivation period is shortened to 25% on chrysanthemum or even to 40% with anthurium

Diffuse light has a positive effect on the crop.

The crop is able to convert more light into growth (photosynthesis). This is due to a more uniform greenhouse climate, better horizontal light distribution, vertical light transmission and light absorption of the lower leaf layers. This increases the photosynthetic capacity of the plant.

The crop undergoes less stress, a lower temperature in the upper leaf crop layers and a higher temperature in the lower sheet layers.

The morphology and development of the crop change. The crop is more generative and the fruits develop faster. The fruits are heavier and there are often more fruits. Potted plants develop faster resulting in a shorter growing time.

What happens if diffused glass is wet?

Some glass (eg etched glass) becomes transparent when wet. As this often happens when it’s already cloudy, this can have a desirable effect. Other glass has increased light transmission (1-2% compared to dry) which may also yield a desired effect. If there is a coating on the glass, the wetness makes it redundant. The light permeability is 2-4% lower depending on the coating.

Are there any disadvantages to diffuse glass?

There are a number of effects that have not been fully investigated. It is likely that the risk of pollution in some diffuse structures is higher than in others, although there is little experience with cleaning diffuse glass in the long run. If the structure is on the outside, the risk of contamination is somewhat higher, but proper washing can clean this up. If the structure is located on the inside, the possibility of contamination is slightly lower. Glass with AR treatment can only be cleaned with (hot) water, not with fluorine, which affects the coating.

By Tom Dueck, Silke Hemming & Jan Janse, Wageningen UR Horticulture