Comparison of clear and stippled glass
Photo: John W. Bartok, Jr.

Diffused light is formed when the direct rays from the sun are scattered by clouds, smog or additives to the greenhouse glazing materials. The greatest effect occurs in growing areas with many sunny days. Observing the shadows on the plants on a sunny day will give an indication of the amount of diffusion you have. Sharp shadows indicate direct rays from the sun. No shadows indicate diffused light.

Diffusion properties

Diffused light can be beneficial to plants. It will penetrate deeper and more uniformly into the plant leaf surfaces. This allows more of the leaves to be productive photosynthesis generators. It is most effective on tall crops such as tomatoes, cucumbers, roses and for vertical growing. Getting light to the lower leaves has shown to increase growth and yield of vegetables by 5 percent or more. It can also increase flower production. Diffused light can also reduce scorching, lower container temperature, reduce fungal spores and decrease insect propagation.

Diffused light provides the most benefits in climates with many sunny days. Direct light will overheat the top leaves on plants, creating hot spots. In climates with cloudy weather, diffusion has already taken place from the clouds. Research at Wageningen University in Holland has shown that in winter weather with considerable cloudy conditions, having diffused light may not make up for the reduction in light transmission of 4 percent or more through 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.

Dark shadows formed by structural members in direct sunlight
Photo: John W. Bartok, Jr.

Scattered light can also 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 looking for more red light. This could result in more growth regulator being needed.

Most glazing materials are now available with treatments or additives that diffuse light. Percent diffusion measurements are usually available and are useful in comparing materials.

Glass: Stippling, roughening or tinting the surfaces will diffuse light transmission and increase light transmissivity at lower morning or afternoon sun angles. 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. The light transmission level is reduced some, so careful selection needs to be made based on climate and crops grown.

Uniform light on plants from glazing with diffusion properties
Photo: John W. Bartok, Jr.

Polyethylene film: The regular 4-year grade of poly has limited diffusion properties and a PAR light transmission of about 90 percent. 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 provides diffusion of light as much as 50 percent without decreasing light transmission more than a percent or two. White or bronze poly also provides diffusion but reduces light transmission too much for most crops.

Existing greenhouses without a diffusing cover can 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 houses, installing a shade screen above the energy screen will give the option of having diffusion on sunny days and when open, increased direct light on cloudy ones. 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.

John is an agricultural engineer, an emeritus extension professor at the University of Connecticut and a regular contributor to Greenhouse Management. He is an author, consultant and certified technical service provider doing greenhouse energy audits for USDA grant programs in New England. jbartok@rcn.com