Gerbera grower Batist, of Maasdijk, the Netherlands, fitted a new, moveable climate screen with improved light diffusing properties into one of its greenhouses this spring. Due to less direct radiation, the crop is less quickly exposed to stress and ultimately on sunny days more light can be allowed to enter the greenhouse. The combination of better light distribution and less shadow results in a more generative crop that yields more.
Thanks to the new screen installed by Ruud Batist this spring, he no longer needs to whitewash the greenhouse. He was quite pleased with the diffuse coating that he applied each spring but the permanent layer of chalk also blocked out light and heat when it was not necessary. In addition the radiation entering through open windows was sometimes too high in places, causing sensitive varieties to visibly suffer.
“I knew it could be better,” says the mini gerbera grower. “Therefore last year at another location we installed a Harmony screen with a shading percentage of 35 per cent, which also has light diffusing properties. This meant I could be more responsive to the circumstances. This spring we installed a variant on the further developed New Harmony line in an entire greenhouse. I am convinced that this will perform even better.”
Although Batist can’t make an objective comparison between the new screen, the classical type and the diffuse coating that he previously used, he says he is very satisfied with its performance. “The new fabric lets more light through and we have a more generative crop. Several colleagues within our growers’ organisation, Colours of Nature, work with the classic screens and we regularly meet in the greenhouse. They agree with us.”
The difference in generative growth is so large that the gerbera grower is at odds with the usual day length regimes. “Normally, under cooler, fertile growing conditions, such as those we experienced up to mid July, you need to maintain shorter days to keep the crop in a sufficiently generative state,” he says. “However, this screen lets through so much light without passing the critical value of 1,000 μmol, that the plants receive a generative boost.”
Batist can reduce the hours of darkness and the resulting higher light sum is, as a rule, converted into extra growth. “Many of my colleagues start to darken the greenhouse around 1 July but we can maintain a longer day. It can only translate into higher productivity.”
Paul Arkesteijn, of Svensson, nods affirmingly. “The first generation of Harmony screens was introduced eight years ago and developments have been ongoing,” he says. “At the time it was the first moveable climate screen with diffuse properties. The big difference with previous fabrics was that the usual aluminium strips in the open, knitted cloth were replaced with white plastic strips.”
According to Arkesteijn these screens allow more light through which is subsequently distributed wider. By varying the number of white strips in the cloth, gerbera growers can chose a screen that shades out 25, 35 or 45% of the light.
“Three years ago we picked up the thread again,” says Arkesteijn. “In the meantime, a lot of new, independent research had been carried out into the effects of diffuse light in crops. Based on that we wanted to research which aspects of our screens could be further improved. Growers are always raising the bar higher, both for themselves and for their suppliers. Our R&D department tested several new materials and the result of that exercise is the new series.”
According to the screen specialist the diffuse property of the fabric has above all improved in the screens with a lower shading level. “That makes these versions very interesting for gerbera and rose growers for example,” he adds. “The 25 and 35 per cent versions of Classic Harmony are already used a lot in these crops. We know that when the radiation is more than 1,000 μmol PAR a gerbera crop can experience stress. Measurements show that in Batist’s greenhouse, the light level remains well below this figure under the new screen. It has a shading percentage of 23 per cent.”
Less stress, more light
Better diffusion offers several advantages. Firstly you can allow more light to enter the greenhouse before the crop suffers stress from the high direct radiation. Secondly, better light distribution reduces the amount of shadow from greenhouse parts such as gutters, columns, air mechanisms and (trellis) rafters.
Arkesteijn: “The results of a trial under practical circumstances show that under the new climate screen, the crop in the shadow receives 32 per cent more light than under the first generation screens. The sunlight is more evenly spread over all plants and all parts of the plant. The result is a more uniform and faster growing crop.”
Another step further
Batist notes that the new climate screen offers him even more opportunities to optimise the growth factor, light. “The plant balance and bud formation are influenced by many factors,” he says. “You have to use screens during the lighter months to protect the crop from excessive radiation. Whitewash offers protection but also prevents light entering when it’s not necessary. In this respect the classic screen was a clear improvement. This new screen goes a step further and enables even more light to be converted into growth and production.”
The grower says that he hasn’t been using the diffuse climate screen for long enough to confirm his high expectations with hard figures but he expects to have them within a few months. “When comparing nurseries or different locations with varying systems and varieties even within one company you need to be careful,” he says “However, I’d be surprised if over the long term this greenhouse doesn’t stick out above the rest.”
Paul Arkesteijn also believes this new screen will be an asset to gerbera growers, among others, even though it wasn’t tested on gerberas during the development phase. “We can’t attach any crop related figures. However, we know the transmission properties inside-out and they are excellent. They are the result of three years intensive research.”
A new type of climate screen with improved diffuse properties makes it possible to admit higher light levels during gerbera cultivation. This produces a generative crop response. The reduced shadow from light intercepting greenhouse parts and equipment also results in more uniform growth. On balance, this should lead to higher production.
Text and images: Jan van Staalduinen