Diffused light is formed when the direct rays from the sun are scattered by clouds, smog or additives to a greenhouse’s covering or glazing materials. The greatest effect occurs in regions that have a lot of sunny days. Observing shadows on plants during a sunny day will give greenhouse growers an indication of the amount of diffusion they have. Sharp shadows indicate direct rays from the sun, while not having shadows indicates diffused light.
Light diffusion varies depending on the covering used in different greenhouses. Fine-tuning the level of diffusion can improve plant growth.
Diffusion properties
Diffused light can be beneficial for growing plants. It penetrates deeper and more uniformly into the surfaces of plant leaves, allowing more of the leaves to be productive photosynthesis generators. It is especially effective on tall crops such as tomatoes, cucumbers and roses, and also in facilities that grow vertically.
Getting light to the lower leaves has shown to increase growth and yield of vegetables by 5% or more, and it can also increase flower production. Meanwhile, diffused light can reduce scorching, lower container temperature, reduce fungal spores and decrease insect propagation. Growers can take advantage of these principles by selecting the correct greenhouse covering or glazing designed to diffuse the light coming from the sun.
Using diffused light is most beneficial in sunny climates, where direct light will overheat the top leaves of plants, creating hot spots. In climates with cloudy weather, clouds already diffuse the sunlight. For example, research at Wageningen University in Holland has shown that in winter weather with cloudy conditions, diffused light may not make up for a reduction in light transmission of 4% or more caused by the glazing.
Moisture that forms on glazing also provides some diffusion, but is usually not present during the daytime when sunlight levels are high. Moisture also increases heat loss.
Scattered light can change the balance between red and far-red light. If more of the red light is absorbed by the top leaves, lower leaves may stretch as they also look for more light from that end of the spectrum. With some crops, this could require more growth regulator.
Mike Kwesell, co-owner and founder of Strawberry fields in Colorado, says the company built its 90,000-square-foot greenhouse for both access and control over natural light.
Materials
Most glazing materials are now available with treatments or additives that diffuse light. Different materials can be compared using percent diffusion measurements.
– Glass: Stippling, roughening or tinting the surfaces will diffuse light transmission and increase light transmissivity at lower sun angles, such as morning and afternoon. An anti-reflective coating can also be applied to increase light transmission.
– Acrylic and polycarbonate: Available in single-thickness, corrugated and double-wall flat sheets, most manufacturers make products that have diffusing properties. However, this also reduces the light transmission level some, so careful selection needs to be made based on climate and crops grown.
– Polyethylene film: A regular, four-year grade of polyethylene has limited diffusion properties and a photosynthetically active radiation (PAR) transmission of about 90%. With co-extrusion commonly used in the manufacture of poly, additional properties can be incorporated. Poly with an energy-saving, infrared inhibitor should be used as the inner layer of most double-poly installations. This additive also diffuses light as much as 50% without decreasing light transmission more than a percent or two. White or bronze poly provides diffusion but reduces light transmission too much for most crops.
Existing greenhouses without a diffusing cover can also be enhanced some. A light exterior shade cloth during the late spring and summer — when light levels are high — will provide some diffusion and cut heat gain. In gutter-connected greenhouses, installing a shade screen above the energy screen will give the option of using diffused light on sunny days, while increasing direct light on cloudy days with an open screen. This also adds a second energy barrier that can be used on cold nights.
The benefits of diffused light can be measured in greater production and more uniform growing conditions in the greenhouse. It is produced naturally in cloudy weather or artificially with modifications to the greenhouse glazing.
Either way, growers should consider if it can help their operations reach their full potential.
John W. Bartok Jr. is an agricultural engineer, an emeritus extension professor at the University of Connecticut and a regular contributor to greenhouse publications. He is an author, consultant and certified technical service provider conducting greenhouse energy audits for U.S. Department of Agriculture grant programs in New England.
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