Section 1: Introduction

Light is considered by many plant scientists as the single most important variable with respect to plant growth and development, and light is often the limiting factor in plant growth. Light essentially provides the energy required to run photosynthesis which is the process by which a plant utilizes carbon dioxide (CO₂) and water (H₂0) to form carbohydrates [(CH₂O)_n] such as sugars and starch. In fact, this process is the basis for almost all terrestrial life on Earth.

Light also affects plant growth and development by its involvement in functions other than photosynthesis. Changes in plant growth and development that are controlled by light, but not necessarily a function of photosynthesis are referred to as being photomorphogenic responses (photo = light, morpho = change and genic = growth). For example, a poinsettia plant will only form leaves (vegetative growth) if the length of the night is less than approximately 11.5 hours (long day). However, if the length of the night exceeds 11.5 hours (short days), physiological changes occur in the meristematic tissues and the plant forms floral structures. This process is an example of a photomorphogenic response and more specifically would be referred to as a photoperiodic response (photo = light and periodic = duration).

Before discussing greenhouse light control, a few basic concepts regarding light should be reviewed. Discussing light can be a little confusing because light displays both particle and wave properties. This simply means that light behaves as particles (photons) when it is measured in certain ways and as waves (wavelength) when measured in other ways. Photons are discrete particles of visible light with varying energy levels. The wavelength of light is indicative of the energy level of the light and may be expressed as a specific wavelength or its corresponding color (as perceived by the human eye). The shorter the wavelength, the higher the energy level of the corresponding photons.

Section 2: Three Important Attributes of Light Affecting Plant Growth

Light Quality
Because light energy is transformed to chemical energy (and thus physiological responses) when photons interact with energy level-specific (or wavelength-specific) photoreceptors, different wavelengths of light affect different processes in the plant. Light with wavelengths of 400 to 700 nm (or photons with the corresponding energy levels) is referred to as photosynthetically active radiation (PAR) and can be used by the plant to drive photosynthesis. Within this range, red (corresponding to 650 - 700 nm) and blue (corresponding to 460 - 480 nm) light are most efficiently used by plants for photosynthesis. Green light (corresponding to 490 – 520 nm) is primarily reflected away (that is why plant leaves are usually green).

Light quality also affects plant growth through photomorphogenesis. Probably the most well known case where light quality affects photomorphogenesis is through the phytochrome photoreceptor (a specific light perceiving/reacting molecule). Phytochrome has been shown to be involved in many photomorphogenic responses including seed germination and photoperiodism. In addition to the example described above regarding flowering and photoperiodism in poinsettia, a common photomorphogenic response that is often observed is that plants grown under low light conditions such as in a forest or under other plants tend to stretch and have longer internodes than plants grown in bright light. Leaves filter red light more than far-red light thus creating a far-red light enriched environment within a leaf canopy. This high far-red light environment causes plants growing under the canopy to stretch or elongate. Additionally, plants grown under a light source high in blue light will be shorter and have darker-colored leaves than plants grown under a light source high in far-red light. These are all examples of photomorphogenic responses by plants to light and are dependent upon or controlled by the light quality perceived or encountered by the plant.

Light Quantity
Light is the energy source that powers photosynthesis. For each plant species there is an optimal light level. Below this level, photosynthetic activity is reduced to a level below the maximum photosynthetic potential of the plant. In fact, if light levels are too low, the rate of respiration (using up of carbohydrates for energy) may exceed the photosynthetic rate and result in a net loss of carbohydrates. Above the optimal level, photosynthetic rate becomes static (saturation light level) and adding light does not result in a corresponding increase in photosynthetic rate. In fact, further increases in light level can cause damage the photosystems of the plant and actually reduce the photosynthetic rate. The goal in greenhouse crops production is to optimize light levels so as to maximize the photosynthetic rate.

Light levels (or light quantity) may be measured and expressed in numerous ways:

Radiant flux is the energy (of all wavelengths) emitted by a light source and is measured in watts.

Radiant emittance is the energy received from a light source per square meter and is measured as watts per square meter.

Irradiance is a measure of the amount of light energy intercepted by a surface and is measured in watts per square meter.

Lumen is a unit of light energy output (in a directions) from a light source. A 100W incandescent bulb produces approximately 1,740 lumens.

Foot candle is equivalent to 1 lumen per square foot. Foot candles are often used to report light levels in greenhouses. However, devices designed to measure foot candles are designed to mimic the human eye (being more sensitive to yellow and green light) and are not the most appropriate for measuring or reporting light with respect to plant requirements (where blue and red light are more important).

Illumination intensity is measured as the number of foot candles striking a surface (100 lumens striking a 1 square foot area is equivalent to 100 foot candles).

Lux is equivalent to 1 lumen per square meter.

Quantum flux density is the number of photons (measured within the corresponding 400 - 700 nm wavelength range) intercepted by a surface over a period of time and is measured as Einsteins or moles per second per square meter (1 Einstein = 1 mole). An Einstein (or a mole) is a very large number so when measuring light in a greenhouse, the unit µEinsteins/m²/sec (or µmol/m²/sec) is usually used. This is the most appropriate way to measure and report light levels with respect to plant growth and development because it is an actual measure of the number of photons available (particles of light) within PAR to drive photosynthesis and control some photomorphogenic responses.

If only a footcandle meter is available to measure light, and the light source is known, rough conversion factors exist for converting footcandle measurements to quantum flux density (as µmol/m2/s). These are listed in Table 1.

Another useful term related to light quantity is that of daily light integral (DLI). Where quantum flux density is a measure of light level (number of photons) at a point in time (1 second), the daily light integral measures the total sum of photons received by the plant during a 24-hour period. The daily light integral is measured as mol/day/m²/m² (sometimes the shorthand version of moles/day is used and per square meter is assumed)and is the number of moles (mol) of photons accumulated over a 24-hour period irrespective of the actual quantum flux density at any given point during the day. The optimal daily light integral varies by species (just as does the optimal quantum flux density). However, for low-light requiring species such as African violet, low-light foliage plants and ferns the optimal daily light integral is 5 to 10 mol/day/m²/m². For most bedding plant species and potted plants, a daily light integral of 10 to 20 mol/day/m²/m² is desirable. Lettuce and herbs require 12-17 mole/day/m².

Photoperiod (light duration)
The duration that light is received by or provided to a plant (photoperiod) can significantly affect plant development. The major horticultural interest in controlling photoperiod is that photoperiod often controls when a plant shifts from a vegetative to a reproductive phase (when it flowers). Long-day plants (i.e. Easter lily) become reproductive when the length of the night is less than some critical value (when days are long). Short-day plants (i.e. poinsettia, chrysanthemum) become reproductive when the length of the night exceeds some critical value (when days are short). When growing greenhouse crops, it may be necessary to artificially shorten or lengthen the night in order to control plant development and when the shift from vegetative to reproductive growth occurs (which is critical to scheduling crops to flower at a specific time).

Section 3: Controlling Light in Greenhouses

Light Quality
In commercial greenhouse production, light quality is important when selecting a light source for supplemental photosynthetic lighting or photoperiod control. A broad emission spectrum within the 400 to 700 nm range is desirable especially when adding light to increase photosynthetic rate. A light source that provides a very narrow spectrum (i.e. low pressure sodium lamps) is generally not desirable since this will hinder plant growth and development and may result in undesirable photomorphogenic responses. Light sources being used to extend daylength and create artificially long days (short nights) must provide sufficient light in the red range (650-700nm) in order to affect the phytochrome photoreceptor that controls photoperiodic plant responses.

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. However, light-emitting diodes are somewhat expensive and primarily used for research purposes at this time (although they are beginning to be used in some commercial growing situations). 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.

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.

Section 4: Supplemental Lighting in Greenhouses

Before selecting a light source for greenhouse (or growth chamber/germination chamber) lighting, numerous factors should be considered. Among these are the total energy emitted by the source, efficiency (% of electrical energy converted to light energy), wavelengths emitted (especially in the 400 to 700 nm wavelengths), cost, life expectancy (of bulbs and fixtures), and the fixtures required (including ballasts). The following is a discussion related to general types of light sources (i.e. types of bulbs), their properties and how they may be used in greenhouses, germination chambers or growth chambers.

Types of Lamps Used in Greenhouses and Other Controlled Environments:

Incandescent lamps (tungsten-filament) - These lamps are generally not used for supplemental lighting in greenhouses for photosynthetic purposes. A large portion of the radiation given off by these lamps is in the form of infrared (heat). Because of this, their efficiency rating is only 7%. Lamps range from 40 to 500 watts. Life span ranges from 750 to 1000 hours. In order to produce enough light for effective photosynthetic lighting, a large number of these lights would be required. This would require a large number of fixtures and would result in large amounts of heat being produced. Further, most of the visible radiation that these lamps produce is in the red and far-red wavelengths that cause plants to become tall and to have weak stems. However, because relatively low light levels are required for photoperiodic lighting, incandescent lamps are suitable for photoperiod control (creating artificially long days) and are commonly used for this purpose in greenhouses.

Incandescent lights are often used in growth chambers. This is because most light in growth chambers is provided by cool white fluorescent bulbs (see below) which are high in blue light but relatively low in red light. A few incandescent bulbs are added to provide additional red light and thus broaden the light spectrum experienced by the plants in the growth chamber (provide light quality closer to natural sun light). Incandescent lamps are not used in germination chambers due to their high heat load.

Tungsten-halogen lamps - These lamps combine tungsten filament with iodine vapor. This allows for the output to remain constant throughout the life of the lamp. These lamps are available in up to 1,500 watts and have a have a 2000-hour lifespan. Again, there lamps are typically only suitable for photoperiodic lighting in greenhouses or in growth chambers.

Fluorescent lamps - These lamps are most commonly used in growth chambers and seed germination chambers. They are rarely used to produce crops in greenhouses. As with incandescent lamps, a large number of lamps would be required to produce enough light to benefit the crop. Fluorescent lamps require ballasts that provide adequate voltage to start operation and limits continuing current to the lamp. Ballasts can be heavy and thus increase the dead load on a structure. Additionally, ballasts generate significant amounts of heat. The required fixtures (ballasts and reflectors) cost money, require additional wiring and block natural sunlight. Fluorescent lamps are more efficient than incandescent lamps (20% efficiency) and provide their light over a broader spectrum (more in the blue region) than incandescent lamps. Both cool white and warm white lamps are used, but cool white lamps are most common. Although not commonly used in greenhouses, fluorescent lamps are commonly used in growth chambers and in germination chambers.

• Cool white lamps have a lifespan of 12,000 hours, and come in lengths of 4 ft and 8 ft, and 75-watt bulbs are most commonly used for horticultural purposes. Warm white lamps are similar to cool white lamps, but warm white lamps emits more light in the red region of the spectrum and less in the blue region than cool white lamps.
• High output (HO) and very high output (VHO) fluorescent lamps are also available. Whereas cool white lamps generate 91 watts of light energy, HO lamps generate 126 watts and VHO lamps generate 225 watts.
• There are other types of fluorescent lamps designed for plant production. These are used in hobby situations since the same limitations apply as to other fluorescent lamps. These "plant lamps" or "grow lamps" usually have slightly broader emission spectra than standard cool white incandescent lamps. These lamps generally generate 46 watts of light energy and have a life span of 10,000 to 12,000 hours and cost more than traditional cool white lamps.

Light emitting diodes (LED) are devices that are much more like computer chips than actual light bulbs. They are small solid state semiconductor devices that emit light and the wavelength (or color of light) that is emitted depends upon the type of semiconductor material and the impurities or additives (i.e. phosphor coating is added to make blue) used to make the LED. Light emitting diodes are highly efficient and emit light in a very narrow range (i.e. as a single color). They can be designed to emit in specific colors such as blue and red, and they can be mixed to provide a broader spectrum of light. In addition to emitting in narrow specific wavelengths, advantages of LED are a very high efficiency, very low heat output, and a long life span. Light emitting diodes are currently used primarily for research purposes but a great deal of research and development is going on to bring LED forward for use in horticulture, and they are increasingly being used in commercial horticulture situations.

High Intensity Discharge (HID) Lamps - These are the most commonly used lamps for supplemental photosynthetic lighting in greenhouses. They are referred to as high intensity discharge lamps because they have a much higher quantum flux density than incandescent or fluorescent lamps (they give off more photons). As with fluorescent lamps, these lamps require ballasts that can be heavy and generate significant amounts of heat. Reflectors are used to direct the light generated downward and to improve uniformity of light distribution. These reflectors are manufactured in many different shapes. Numerous types off bulbs are available for use in HID lamps:

• High-pressure mercury bulbs have an emission spectrum similar to fluorescent lamps but with a higher concentration of their radiation being emitted in the red region. Light energy is produced by these lamps using a two-step process. First, the filament gives off UV light. This UV light excites a phosphor powder inside of the tube. This powder fluoresces and gives off visible light. Because of this two-step process, these lamps have an efficiency rating of only 13% and have a lifespan of about 10,000 hours. These lamps are more commonly used along roadways and in parking complexes. They may be purchased in wattages of up to 1000W.
  
  
• High-pressure metal-halide bulbs cost more than high-pressure mercury bulbs and have a shorter life span than high-pressure mercury bulbs. However, they have a broader emission spectrum than mercury bulbs. Their efficiency rating is 20%, and they have a lifespan of 8,000 to 15,000 hours. They may be purchased in wattages of up to 2000W.
  
  
• Low-pressure sodium bulbs have an efficiency rating of 27% and have a lifespan of 18,000 hours. However, these bulbs have a very narrow emission spectrum (nearly all light is emitted around 590 nm). This narrow emission spectrum can cause adverse effects on crop development. These bulbs are available in wattages of 35, 55, 135 and 180W.
  
  
• High-pressure sodium (HPS) lamps are the most common type of bulb used for HID lighting in greenhouses. They have a broader emission spectrum than low-pressure sodium bulbs and are lower in cost than high pressure mercury bulbs (approximately $150 -$250 per unit or $2.25 - $3.00/ft2 to purchase and install). These bulbs have efficiency ratings of 25% and a lifespan of 24,000 hours. They are available in wattages of 250, 400 and 1000W. The 400 watt and 1000 watt bulbs are most commonly used in greenhouses.

Using Supplemental Lighting in Greenhouses
As discussed above, supplemental lighting in greenhouse may be conducted for two purposes. The first being to alter the photoperiod experienced by the plants (photoperiodic lighting) and the second being to increase to amount of light available for photosynthesis (HID lighting).

Controlling Photoperiod in Greenhouses
The duration that light is perceived by the plant (photoperiod) is usually controlled in order to time flowering or to maintain plants in a vegetative condition. During long-day photoperiods (i.e. late spring and summer), black out cloth (a 100% light excluding cotton polypropylene fabric) may be pulled to artificially shorten the length of the photoperiod if required. Black out cloth may also need to be pulled over the crop during naturally short days in order to block out light pollution coming from external sources.

During short-day photoperiods, supplemental photoperiodic lighting may be used to increase photoperiod (specifically to shorten the length of the night experienced by the plants and create a long day). Photoperiodic lighting requires only very low levels of light (typically 10 µmol/m²/s are recommended), but does require that the light source emit in the red region of the spectral distribution. Incandescent lamps work well for this purpose because they emit enough photons (have a high enough light level), in the red region and they are low in cost and simple to install. If already available and installed in the greenhouse, high pressure sodium or metal halide HID lamps may also be used for photoperiodic lighting. However, they will produce more light than required and will cost more to operate than incandescent lamps.

Compact fluorescent bulbs have become increasingly common in home and commercial applications. Researchers have tested these types of bulbs as an alternative to incandescent lamps for creating long-days. For some plant species, it has been found that compact fluorescent lamps are not as effective at creating the long-day effect as incandescent lamps. It has been speculated that this may be due to the differences in the light quality emitted by the two light sources. Mixing incandescent lamps with compact fluorescent lamps at a 1:1 ratio was effective for photoperiod control.

When manipulating photoperiod through lighting, the lights may be turned on for 4 - 8 hours at sunset or before sunrise (extending the day). However, more commonly, the lights are turned on during the middle of the night (night interruption). This places a light period in the middle of the night, thus creating two short night periods rather than having a single long night. Night interruption has been shown to be the most effective method of creating a long-day photoperiod. For this method, lights are usually turned on at 10:00PM and turned off at 2:00AM. Cyclic lighting may also be used to interrupt the night period. There are several potential lighting cycles that can be used, but a common method is to turn the lights on for 5 - 10 minutes per hour during the night. Finally, if HID lighting is being used on the crop, and long days need to be maintained, the lights can simply be turned on for 18 hours as usual. This period is sufficient to create long days for all greenhouse crops.

High Intensity Discharge Lighting in Greenhouses
Supplemental HID lighting is specifically used to increase the light level (number of photons) available for photosynthesis. Because of the costs associated with HID lighting, it is not economical to use HID lights with all crops even if a positive growth response can be achieved. The increased plant quality, increased production or reduced cropping time resulting from the increased light levels must compensate for the increased cost associated with HID lighting. Therefore, HID lighting is most commonly used for higher value crops such as roses, vegetative stock plants, ornamental and vegetable plugs or seedlings, and some greenhouse-grown vegetable crops. In some cases, HID lighting might be used on containerized crops if ambient light levels are very low and limiting.

It is best to work with the manufacturer to decide on the best placement of fixtures to achieve desired light levels with maximum efficiency. In some cases, 1000 watt lamps are used. However, tall greenhouses are required with these lamps to insure uniform light distribution. The reflector used with the light source is important. The goal is to achieve the most uniform distribution of light that is possible while using as few lamps as possible. HID lamps using the older style circular reflectors are generally spaced apart at 1.5 times the distance between the lamp and the plant material. New types of reflectors allow spacing to be up to 4.5 times the distance between the lamps and the plants.

As discussed above, greenhouse managers may look to maintain an optimal quantum flux density (light level) throughout the day. However, another useful way to control light level is to measure the daily light integral (DLI) and make sure that the crop receives its required daily light integral. On days with more natural light, little supplemental HID lighting might be required. On other days with less natural light, more supplemental HID lighting may be required. The required daily light integral may be met by increasing the quantum flux density or by increasing the length of time the crops receive light. However, the quantum flux density must be higher than the light compensation point or no net photosynthesis occurs. The best way to manage light quantity is to provide light at a quantum flux density in the optimal range for a period of time required to provide the optimal daily light integral. When conducting supplemental HID lighting, light should be provided for no longer than 16 - 18 hours per day. Providing supplemental light for 24 hours per day can be detrimental to plants.

© M.R. Evans, 2008, 2009, 2011, 2014