Dimming down

Sometimes, it’s necessary to reduce light intensity in the greenhouse, especially during seasons when light levels are very high and temperature control (cooling) is difficult. Two types of materials are available to reduce light intensity: shading fabrics and greenhouse shading compounds.

A shading fabric can be applied over the crop when needed and is easily removed during low-light conditions. Shading fabric can also be applied to just a few benches instead of the entire greenhouse.

Spraying a shading compound on the greenhouse is a less expensive method and is used when repeated removal and reapplication is not necessary or practical.

Shading fabrics
Shading fabrics can be used in greenhouses to reduce the crop solar load, but they are more commonly applied for temperature control. Shading fabrics can also be used as thermal blankets to provide additional insulation for the greenhouse on cold days and nights by trapping a layer of air between the crop and the greenhouse roof.

Synthetic fabric choices include polypropylene, spun-bonded polyester and saran, all of which are available in various levels of shading. Shade curtain systems can be automated with photocells so that the greenhouse is shaded on bright days and light is maximized during low-light conditions by retracting the shade curtains.

Shading fabric can be used in propagation and in the production of certain crops where it is necessary to increase humidity. This is desirable when light intensity and temperature may be too high to prevent water stress even when mist is used. When used in propagation, there is less likelihood of conditions becoming too hot as compared to polyethylene propagation tents, which do not provide air exchange.
 

Ed Van Wingerden inspects greenhouse anthuriums at Ever-Bloom Nursery in Carpinteria, Calif. To meet the demanding environmental requirements of this crop, he uses a computer-controlled automatic shade system with two curtains: one for shade control (40%) and one for humidity control (20%). The humidity curtain helps to maintain 60% humidity, which offsets the 5- to 8-degree temperature rise as long as a crack is open for ventilation. Ed also uses both curtains as thermal blankets for energy management. The humidity curtain is used year-round, but the shade curtain is only used in the winter. In the summer, Ed uses an exterior greenhouse white wash.
 

Shade curtains can be installed inside or outside of the greenhouse. When installed inside, curtains are suspended from the overhead structure and installed in sections.

Installation of a shade curtain over a greenhouse is less common but can be accomplished by erecting several tall poles outside, stringing wire or cable between these poles and using wire, cable or rods to pull the curtain. While installation inside the greenhouse protects the curtain material and hardware from wind, rain and UV light, there are also benefits to exterior installation.

Outside shade curtains keep energy out of the greenhouse, which significantly reduces cooling requirements and the solar load on plants. Another reason to install shade curtains outside the structure is the lack of headspace inside some greenhouses. Furthermore, curtains installed inside generally result in some shade even when completely retracted, whereas in outside installations the curtains can be pulled completely back so that they do not block light when fully retracted.
 

Many growers use exterior shading compound as an inexpensive alternative to shading fabric. Shading compounds contain latex paint and other chemicals that can be toxic to aquatic organisms. It is important to apply and remove exterior shading compounds so that runoff containing the shading compound remains on the property and does not enter the municipal storm/sewer system or waterways.

Shading compounds
Commercial shading compounds, whitewash and materials used as protectants for sunscald on fruits and vegetables have been used for greenhouse shading with good results. This is considerably cheaper than fabric shade curtains, but it does have some limitations.

Typically, the entire greenhouse or greenhouse range is sprayed, so this method is not an option if just a few benches need shade. Secondly, the sprayed area will be continuously shaded for an extended period — usually from the mid-spring to early fall for most crops.

During cloudy and low-light days in summer, the plants in the greenhouse may receive insufficient light. Moreover, shading materials can be difficult to uniformly apply and remove, although most of the compounds will eventually wear off in the fall.

If used with material other than glass, shading compounds that are formulated specifically for non-glass surfaces should be selected to avoid damaging the glazing and to facilitate removal of the shading compound when shading is no longer required.

Julie Newman is an environmental horticulture advisor at the University of California Cooperative Extension.

Have a question? You can write Julie at jpnewman@ucdavis.edu.