Will a Shade Cloth in a Greenhouse Increase the Leaf Temperature of Plants?

Leaf Temperatures

• Plant leaves do not usually have the same temperature as the air around them. Sunlight, particularly the infrared or heat wavelengths, warms up the leaf tissues to be warmer than the air. The leaf naturally cools itself through transpiration, the release of water from pores in the form of water vapor. Leaf temperatures get hotter when in direct sunlight, but on cloudy days, leaves do not get as warm, as the clouds block a percentage of the light and heat energy. Shade cloth acts like a cloud, blocking a portion of the sun energy reaching the plants growing in the greenhouse. Through shading and transpiration, the plants' leaves may be physically cooler than the air.

Shade Cloth

• Traditionally, greenhouse managers often used a temporary opaque white paint wash on greenhouses in the summer to reduce the amount of sun energy that goes through the glass panels. A less expensive and easier strategy is the use of a shade cloth. A shade cloth is made of fabric or more durable nylon fibers loosely woven in a net-like matrix. Shade cloth, based on the distance between the fibers, blocks light. A 60-percent blockage occurs on a cloth with many more fibers with tighter spacing than a 10-percent weave with fewer fibers and greater inter-fiber distances. Shade cloth may be black, white or metallic in color. Black materials absorb and radiate heat more than reflective white or metallic fibers.

Decreased Temperatures

• Placing a shade cloth over plants inside the greenhouse structure often results in a drop in leaf temperatures. However, if too much shading occurs, the plants cannot photosynthesize, and their metabolism falters. Weak plants may not transpire well and therefore not cool down as efficiently if the air temperature is warm but the light is too low. White and metallic shade cloth is especially helpful in reducing both greenhouse interior temperatures and the temperatures of plant foliage.

Increased Temperatures

• Black shade cloth may decrease or increase leaf temperatures in greenhouses. While the increase in shade blocks more sunlight energy and cools the leaves, the increase in black-colored fibers absorbs more heat. The result is the shade cloth releasing heat into the air in the greenhouse, resulting in a spike in temperature. While the leaves are cooler from shading, their temperatures may still increase because of the air temperature increasing more than usual. This warming effect may be offset by using reflective cloth fibers or mounting the shade cloth on the exterior of the greenhouse structure.