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In search of the optimum light level for potted plants

Diffuse light benefits almost all potted plants that have been tested in trials. Often the grower can allow in a greater amount of light, which will make the plants grow faster.

Filip van Noort of Wageningen UR Greenhouse Horticulture led the well-known project, Grip on Light, and has continued further with similar trials in the Daylight Greenhouse in Bleiswijk, the Netherlands. The objective is to explore how much light the plant can convert into production. Pot plant growers are naturally conservative for fear of light damage, such as (flower) discolouration or dried leaf edges. But after Grip on Light the world has changed. “The growers in the group who were supervised now grow differently and colleagues are starting to follow. They are convinced of the benefits of diffuse light and cultivate faster. They dare to allow in more light. The questions are mostly about how you approach it and no longer about the principle,” says Van Noort.

Light damage can occur when there is a combination of lots of light, a high temperature and a low humidity. To prevent that from happening commercial growers shade severely. “Often this happens even at levels when photosynthesis is a long way from its maximum. Then you waste potential production,” says the researcher. Together with his colleagues he has succeeded in raising the usual daily light sum total from 3 - 5 mol/m² to 7.5 - 10 mol/m² for various crops, without damage and with a spectacular acceleration in speed of production or yield increase.

Keep control of climate
The condition is that the three climatic factors (light, temperature, humidity) are kept under control. The light distribution should be as even as possible to prevent any damage occurring in any spots of bright light. The best way to achieve such a uniform light distribution is with a diffuse roof.
“The second factor – the temperature – has a clear connection with the light level,” says van Noort. “If you admit more light, you can often maintain a higher 24-hour temperature. There is a wide variation between the crops. Sometimes you see a delay in flowering at a higher temperature, for example with Spathiphyllum or pot Chrysanthemum.”

A coating that reflects infrared, such as ReduHeat or ReduFuse IR, can certainly help to lower the temperature, without affecting the light level, he thinks. “A large number of crops are protected to limit the temperature, not because the light level is a problem. We haven’t done any research ourselves on IR reflection but I would certainly urge more independent scientific research into this area to get a better picture of the possibilities.”

The third factor that can contribute to crop damage – low humidity – can be overcome by fogging. “Then it is important to use foggers that don’t wet the crop, even when fogging has to be used a lot and often,” he says.

Allowing in more light means less playing for safety. Then it's handy if you can monitor the situation well. This is possible with existing plant sensors that measure the fluorescence, an indirect indication of photosynthesis. However, all the existing equipment has its drawbacks. New equipment is being developed that eventually should result in online measurements, possibly directly linked to the climate control.

For more information 
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