Light is the source of energy a green plant uses to sustain itself; without it, they cannot survive. Additionally, it often is the limiting factor in greenhouse production, especially during the winter months and into early spring. In most cases there is a direct correlation between the amount of light a plant receives and its growth and productivity. As spring progresses, however, the days lengthen and temperatures become warmer. It is at this time of the year when excess light actually can be detrimental to plant growth because of the heating affect it has in a structure such as a greenhouse or high tunnel.
Light is a form of radiant energy and, as such, brings with it an increase in temperature in the interior of a closed structure with a transparent cover. Most of us are familiar with the “greenhouse effect” in nature, currently being blamed for global warming. The term was borrowed from the greenhouse industry since it is well known that shorter, more energetic wavelengths of light penetrate the covering material of a greenhouse, strike an object, and are converted to longer, in-fared wavelengths that lack the energy needed to escape through the covering material. Instead, they cause the interior of the structure to become warmer.
In winter and early spring the greenhouse effect is welcome since it helps heat greenhouses and reduce fuel costs. In the late spring and summer we would rather avoid it since it causes temperatures to rise to levels that make many greenhouse species less productive than they would be at lower temperatures. Plant productivity can be expressed as the difference in rate between the processes of photosynthesis (the manufacture of food) and respiration (the usage of food manufactured). It has been well-established, that the optimum temperature for photosynthesis is considerable lower than that for respiration. Therefore, as temperatures increase above the optimum for photosynthesis less food is manufactured but more is consumed because of the increase in the rate of respiration.
Additionally, many species grown in greenhouses or high tunnels (including tomato) are “light saturated” at levels about one-half the intensity of direct sunshine. The end result is the additional light cannot be used by the plant and actually begins to decrease its productivity because of the temperature increase that accompanies it. Other tomato disorders such as poor fruit-set, white core and yellow shoulder have been linked to high temperature stress.
Shading a greenhouse or high tunnel to exclude excess light is a management practice that helps to limit the rise of its interior temperature. In days-of-old, shading compounds such as white wash were applied to the structure’s covering to limit the amount of light allowed to enter. Shading compounds were relatively inexpensive to apply, could be used on any covering material and structure, and reflected light outside the greenhouse before it entered and caused temperatures to rise. One disadvantage of shading compounds is the amount of light blocked tends to decrease during the summer as rain slowly washes it off. Newer types of shading compounds are a bit longer lived and usually require physical removal at summer’s end.
Upon the arrival of the plastic age, shading compounds gave way to the use of shading fabrics (shade cloth) as a method of limiting the amount of light entering a greenhouse. Shade cloth is rated according to the percentage of light it blocks from passing through it. The later, in turn, is dictated by the tightness of the weave of the fabric. Shade cloth initially is more expensive than a single coat of shading compound but can be used for many years given proper care. Additionally, it can be applied in a fraction of the time needed to apply shading compounds.
Modern shade cloth is comprised of (usually) black polypropylene plastic strands either woven or knitted together to form a fabric. Woven shade cloth must be hemmed in order to keep it from fraying. Hemming can add significantly to the initial purchase price of a shade cloth panel. Knitted shade cloth does not need to be hemmed but some care must be taken not to stretch it out-of-shape for fear of reducing its sun-blocking ability. A newer type of shade cloth is comprised of polyethylene strips that have been laminated with aluminum. The aluminum tends to reflect the sun and the energy it contains whereas polypropylene blocks and absorbs it.
Two management decisions that must be made when using any type of artificial shade include: 1) what percent of the light intercepted by a structure should be blocked, and 2) when should the shade be applied? We will answer these questions using tomato as an example.
During the summer the sun’s intensity can be as high as 8000 or 9000 foot candles (f.c.). Using the latter value as an extreme, tomato is light-saturated at half the sun’s strength (about 4,500 f.c.). The covering material of a plastic greenhouse or high tunnel often blocks up to 20 percent of the light striking it depending on its age and clarity. Therefore, shade cloth that is rated at 30 percent blockage is a good choice for tomato growers. Together, the plastic covering and shade cloth should block 50 percent of the 9000 f.c., allowing the desired 4,500 f.c. to enter.
When to apply shade cloth is a bit more subjective. An unusually warm (early) spring would dictate an early application; the opposite would be true for a cool (late) spring. Applying shade before excess temperature becomes a problem can lower productivity. Generally, late May or early June is the time of the year many tomato growers apply shade to their structures. Depending on their cropping regimen, the shade usually is left in place until late September or early October when excessive temperatures are no longer a problem.
REVISED: November 30, 2015