A lot of research has been underway to find the optimal conditions for plants´ growth in greenhouses, be it the construction of the greenhouse itself, the materials that are used or the impacts on the climate. One key aspect is the usage of diffuse light. If certain factors are considered, diffuse light brings a lot of benefits as opposed to direct light.
Direct Vs. Diffuse Radiation — The Benefits And Drawbacks
Direct radiation originates from one direction while diffuse radiation comes from many. Just before light enters the atmosphere, it is referred to as direct radiation, as no particles, greenhouse gases or water droplets interfere and scatter the light (diffuse radiation). The extent to which light is then scattered in the atmosphere, if at all, varies and will be higher during the winter months, as there are more cloudy periods than in the summer. Figure 1 (see slideshow) highlights the possible paths that light can take when it comes to greenhouse roofs.
Researchers have shown that using greenhouse covering materials such as plastic films or temporal coatings that convert direct (perpendicular) light into a diffuse one at varying degrees, a more homogenous light distribution is reached.
This benefits plants with a high leaf area index. With direct light, the plants and leaves in the upper part receive the majority of the light and have photosynthesis and growth going on there, while the leaves in the middle and the bottom show much lower rates, respectively. Diffuse light, now, has the ability to penetrate deeper into a plant canopy and enhances productivity and yield significantly.
The Wageningen University in the Netherlands showed that by using diffuse greenhouse covering materials, production of sweet pepper can increase 5 to 6 percent during the summer. Another experiment with cucumbers raised the number of crops by 7.8 percent, and showed a weight increase of 4.3 percent. If a material had been developed with the same haze but with 4 percent more light, then the production values would have raised to 11 percent and 7.8 percent, respectively. The list of trials conducted by Wageningen UR can easily be expanded with tomatoes on average being 8.5 grams heavier. With regard to potted plants, the cultivation period for chrysanthemum could be reduced by 25 percent.
Besides the positive effect of a greater exposure of the leaf surface to light, a more uniform temperature is also achieved in the greenhouse, along with decreased transpiration and less stress for the crops. Direct light causes high temperatures on the top of the crops at times of high irradiance, which in the worst case might result in burning the crop. At the same time, shadows caused by the construction elements lead to a more non-uniform growth.
Commonly Available Greenhouse Covering Materials
The light diffusing property (haze) of greenhouse covering materials encompasses one of two important factors, with the other being the transmission of photosynthetically active radiation (PAR, 400 to 700 nm), the plant’s energy source for photosynthesis. Hemispherical light transmission is the appropriate value to be used. It is vitally important to take into account that there is a connection between light diffusion and PAR transmission. Depending on the material type, lower and higher tradeoffs are observed and further material developments are necessary to achieve both high haze and high light transmission. Research is also carried out in the direction of surface structures and nano-coatings.
Table 1 (See slideshow) shows commonly available materials on the market and the respective diffusion (haze) and PAR transmittance values. The transmission data is given for direct (perpendicular) light and for diffuse light (e.g., especially on cloudy days). The data was assembled from literature, personal measurements and experience from the market. This is not a complete list but rather a general overview. The number of different materials on the market and their properties changes constantly. Even for one single type of cover, there is a variety of different light transmission properties available.
While glass exhibits one of the highest light transmissions, and lasts a long time, it is also one of the most expensive materials. Solutions with higher haze values include double-walled panels made from polycarbonate or two layers of polyethylene films that are often inflated with air bubbles, and thus act as good insulators, as well.
Additionally, available materials on the market include whitewash, a semi-permanent solution, which is normally applied somewhere during spring and remains until it is removed in autumn, as well as temporal diffusing coatings. Here, the coating exhibits a high haze when dry, as opposed to a low haze when wet. If it is not raining and the grower wants to make the covering transparent, there is the possibility to turn on the roof-spraying.
Researchers from the Wageningen UR also found that especially in the winter periods, when light is the limiting factor, diffuse materials do not compensate for the loss in light transmission, which was 4 percent in this case. A further analysis came to the conclusion that an increase in light transmission from 82 to 85 percent impacts crop growth more than increasing the haze factor from 62 to 71 percent. Thus, diffuse materials prove useful in hotter and semi-arid regions, while in moderate climates most benefits occur in periods with high direct sunlight.
Researchers in Central Greece arrived at similar results for tomato crops, namely that diffuse polyethylene films significantly enhance plant height and crop yield, in terms of number and weight of fruits per square meter, during the summer. This is caused by the aforementioned reasons of plant stress and the observed high temperatures with clear films. However, they suggest using clear films throughout the year — with the application of shading in hotter periods — as, surprisingly, during the winter better results can be achieved due to greater solar radiation.