Section 3: Controlling Light in Greenhouses
In some situations, it may be desirable to specifically alter the light quality experienced by the plant and increase the relative ratio of certain wavelengths experienced by the plant. This is generally done to manipulate plant growth and development. For example, reducing the far-red light and increasing the blue light experienced by the plant results in shorter, darker-colored and stronger plant. Light quality can also affect the development of certain foliar diseases such as Botrytis. Light-emitting diodes that emit in very narrow wavelengths might be used for this purpose. Some commercially available light sources (i.e. low pressure sodium) have narrow emission spectra and may be used to increase the relative amount of selected wavelengths of light. Greenhouse glazings have also been developed with additives or pigments that filter certain wavelengths of light and allow for a shift in the relative ratios of wavelengths of light entering the greenhouse.
The most common way that greenhouse managers control the quality of light experienced by plants is through the selection of light sources used. Different light sources have a different emission spectra (emit different wavelengths) and thus the quality of light experienced by the plant can be controlled through light source selection. This issue is discussed in more depth later under light quantity and duration.
Light Quantity
The light level or quantity might need to be increased or decreased to maintain optimal levels depending on the plant species. Different plant species have different optimal light levels. However, for a given species, plant spacing, nutritional level and plant age can affect the optimal light level. For example, the optimal light level for a tomato seedling is lower than that for a well established and actively growing tomato. The table below gives some examples of recommended light levels (quantum flux density) for some common greenhouse-grown crops.
Table 2. Recommended Light Levels (quantum flux density) for Selected Plants in µmoles/m2/sec. |
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African violet |
150 - 250 |
Foliage plants |
150 - 250 |
Carnation |
250 - 450 |
Chrysanthemum |
250 - 450 |
Easter lily |
250 - 450 |
Geranium |
250 - 450 |
Poinsettia |
250 - 450 |
Cucumber |
250 - 450 |
Lettuce |
250 - 450 |
Strawberry |
250 - 450 |
Roses |
450 - 750 |
Tomato |
450 - 750 |
Adapted from: Plant Growth Chamber Handbook, Iowa Agriculture and Home Economics Experiment Station Special Report No. 99. Values are for actively growing plants. |
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Two methods are commonly used to reduce light levels in greenhouses. The first is the application of a shading compound to the glazing. There are several commercially available shading compounds (i.e. Kool Ray®). However, a mixture of 1 part white latex paint to 20 parts water works well. More than one layer may need to be applied depending on how much light reduction (exclusion) is desired. The shading compound is applied to the glazing (on the outside of the greenhouse) in the late spring and washed off in the fall (depending on latitude). Commercial products are available to assist in washing off of the shade compound.
The second method is to block out a portion of the light with some type of shading screen made of cloth, polypropylene, polyester, or aluminum-coated polyester. These systems may be placed on the exterior of the greenhouse or in the interior. They may be purchased in weaves that provide 10% to 90% light reduction. However, 30% to 60% is most commonly used.
A problem with these types of shading systems is that the shade remains in place in the mornings, afternoons, and on cloudy days when shading would not be needed. During these times, light levels fall below optimal levels. Mobile or retractable shade systems are being installed in many new greenhouses. These systems are placed in the gables of the greenhouses or outside above the greenhouse roof and are controlled by a computer that is in turn connected to a photometer (light meter). A desired light level can be programmed into the computer and the shade automatically pulled when light levels exceed the desired level. The shade will automatically be retracted when light levels fall below the desired level. There are several variations to these systems, but the end result is that they allow for a more uniform application of light as well as allowing for the optimal light level to be maintained for a longer period of time during the day.
Often, particularly in northern climates in the fall and winter months, increasing light levels is required. Light levels that are too low can cause flower bud abortion, reduced growth rates, longer internodes, lower plant quality, and increased disease incidence. Selection of a glazing that allows maximum light transmittance, minimizing obstructions, keeping the glazing clean, and increasing plant spacing are all ways of increasing the amount of light reaching the plants. However, these measures may not be enough and supplemental lighting may be required to increase light levels.
