Shading is a good first stage for cooling the greenhouse. It will reduce light levels and heat gain while saving energy. It can also reduce leaf temperature. Although it is best over the glazing, it can be effective inside with a retractable screen system.
The most common materials are woven or knitted shade fabrics made from polypropylene, polyethylene or polyolefin. These are lightweight, easy to apply and available in several degrees of shade from 10% to 90%. They usually have to be custom-fabricated to fit the greenhouse. Most materials are ultraviolet-stabilized. Knitted materials have an average life of seven to 10 years and woven materials a life of 10 to 12 years.
Polypropylene is strong, tough and highly resistant to flexing, abrasion and chemical attack. It will shrink about 1% when placed on the greenhouse. The knitted material resists tears and will not unravel at the edges. Aluminum strips can be added to reflect the heat. When used outside, this results in a lower roof temperature. Inside use of an open weave allows heat to pass through to escape through the roof vents.
The woven polyproplene requires that the edges be taped to prevent unraveling. Grommets are usually placed along the edges for attaching to the exterior of the greenhouse with rope or tie-downs. Woven material is usually heavier than knitted material.
High-density polyethylene is usually made as a knitted material with monofiliment yarn for strength and longevity. It resists tears, molds and mildew. An aluminized knitted material is also available. It reflects sunlight and doesn’t transmit heat to the glazing when used outside. Both indoor and outdoor materials are available. They are UV-resistant and recyclable.
Polyolefin is made of monofiliment yarn and interwoven with aluminum strips to reflect heat. It is also available with a combination black surface for heat retention and white surface for heat reflection. Polyolefin provides strength and longevity with limited stretching.
Colored shade material is a new tool with specific advantages. Photoselective shading can affect plant morphology and physiology. Shade material can also provide some pest control. By changing the ratio of the color, vegetative growth, flowering, fruit quality and yield can be enhanced.
Research has shown that red and pearl color is the best for many crops. On tomato, lettuce and ornamentals it stimulates the growth rate. On cut flowers it produces stronger stems. On strawberries it enhances anthocyanin content. Yellow and blue produce dwarfing and can reduce the need for height control chemicals.
Pest control can also be affected by the screen color. Aluminized and white screen material reflect light and confuse insects. Yellow and blue have been reported to lower populations of silver whitefly, thrips and aphids. White and gray can lower mite numbers. Where only light shading is needed, the use of an insect screen material will provide physical protection against insects. Before selecting a color material, check the internet for the latest research reported.
Shade level — Most shade materials are available in several light reduction levels. When selecting the degree of shade, remember that the greenhouse structure and glazing already reduce the light level significantly. Typically in a single-layer glass greenhouse, the glazing and structure may reduce light transmission 20 to 30%. In a double poly house, Goldsberry and van der Salm found reductions of 35 to 40%. Adding a 50% shade will reduce the level an additional 20 to 30%.