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Plants are the only organisms capable of the amazing feat called photosynthesis, by which carbon dioxide and water are converted into carbohydrates using light. Light from the sun, or other sources runs the greenhouse. Greenhouse should provide a space with optimal conditions (light, temperature, nutrition, pest control, etc.) for the plants so that they can perform photosynthesis. Often, natural sunlight is enough, and can be even too intense for some plants in terms of light but also in terms of temperature. Because the sun reflects through the glass of the greenhouse, the temperature can increase drastically and be above the temperature required for the plants. In those cases of excess light from the sun, shading or cooling by other means is required. Greenhouse mechanical cooling sytems (ventilation, etc.) are obviously important in this regard and will be discussed in Chapter 6. However, reducing light levels to control temperature is a task that should be studied carefully; for example, mechanical cooling may be better option for plant growth than reducing light so that plants are not suffering from lack of light. Actually, one of the most important challenge of a greenhouse managers will be to provide plants with enough sunlight to grow them optimally. This is particularly true in North temperate locations like the North Eastern or North Western US, where cloud cover during the winter months does not provide plants with optimal light for photosynthesis. Therefore, under these conditions, supplemental lighting is usually required.
The first part (I) of this chapter will deal with some of the light-related terminology used in greenhouse management, followed by a section on solar radiation (II), and on the different types of artificial/supplemental lighting sources (III), and finally a section (IV) on shading in the greenhouse. "
I. Light: its involvment in plant developemnt and its terminology.
I. A. Importance of light
In greenhouses, managers are facing
two situations in terms of lighting:
1- the light intensity is low and in that case supplemental lighting should be choosen judiciously, or
2- the lighting intensity is too high for the well being of the plants, then it will be necessary to find a way to reduce light intensity so that the conditions are optimal for plant growth.
Light is an essential element for the plant because it controlled numerous events in plant growth developemnt including plant form and structure (morphology), plant orientation (phototropism), and reproduction (flowering).
Light is indirectly involved in
transpiration (or water loss) from leaves. Early morning, light
is the trigger that causes opening of the tiny pores on the leaf surface known
Opening of stomates, in response to light, allows gas exchange between the plant
and the external environment. The process of photosynthesis depends on uptake
of CO2 through open stomates. At the same time, that CO2 is being taken up,
water vapor is being lost from the leaf through the process known as transpiration.
Actually, light is one of the component that should be highly controlled so that it does not participate to the overheating of the leaf, which would lead to excessive transpiration of the leaves. But the light environment should be controlled in association with temperature.
plants, called photoperiodic plants, are sensitive to photoperiodism,
the duration of day-nightPhotoperiodism
is a biological response to a change in the proportions of light and dark in
a 24-hour daily cycle. Plant can be grouped in three categories with respect
to how their flowering pattern responds to day length.
Some plants are short-day plants like chrysanthemum, Christmas cactus, and poinsettia. They flower naturally (outdoors) in the fall, when the daylength (photoperiod) is shorter than some critical length (different for different species). Long-day plants like rudbeckia and California poppy normally flower outdoors in the summer, when light duration is longer than the critical length. Day-neutral plants like rose and tomato cultivars flower without respect to day length. Artificial manipulation of photoperiod of short and long day species in the greenhouse environment allows these species to be flowered at any time of year.
John Kumpf says on photoperiodism:
"Learning how to manage the light environment in the greenhouse will help you to maximize plant growth (photosynthesis), initiate or delay flowering (photoperiodism), avoid excessive stretching/elongation (etiolation), associated with low light, and avoid plant stress (which inhibits growth) associated with overheating and excessive waterloss (transpiration). Broadly speaking, these responses to light are controlled by managing the brightness of light (intensity or illuminance) and/or the duration of light (daylength or photoperiod).."
I. B. Light
When the greenhouse manager needs to decide on the type and the amount of lighting required for his/her plants, it is important to be familiar with light terminology so that light recommandations can be accurately interpreted.
1. Wavelength refers to the color (and energy level) of light. Wavelenth is measured in nanometers (nm) which is one billionth of a meter (rainbow).
2 . Light, expressed in quantity, can be either consider as 1-photometric (luminous flux or illuminance) or 2-radiometric (irradiance or luminous efficacy).
3. Plant's energy requirement can be expressed as irradiance. The units are milliwatts per square meter or milliwatts per square foot (mW/m2, mW/ft2). This represents the quantity of energy received by a plant in the wavelength band 400-700 nm. However, foot candle are readily used, it is actually easier to evaluate lighting levels in foot-candles and to convert it to mW/ft2 using a conversion factor which is different depending on the type of lamp used.
4. Light measurement reflects three basic quantities:
- Luminous intensity represents the total output of light source and is measured in candela usually given by the lamp manufacturer. The luminous intensity provides the weighting factor needed to convert between radiometric and photometric measurements.
- Luminance or brightness refers to light that a surface emits. Measured in foot-lamberts, luminance of surfaces depends on their visual appearance ie if they are dark ir bright. Furthermore, the apparent brightness decreases as you go further from the surface.
- Illumination of a surface can also be measured and represents the amount of light that fall on a unit area. This is expressed in foot-candle. This value is commonly found for artificial light on plants.
Usually a photometer, also refered as light meter, is the instrument used to measure illumination.
5. Horticulturists estimate the instantaneous light level using the number of micromoles of photons in the PAR spectrum that reach one square meter each second (umol.ms). This represents the light energy used in photosynthesis. A measurement in micromoles actually describes the amount of usable light energy that a plant receive for growth. The PAR can be measured using a PAR meter.
II. Solar Radiation
Definition of the components of radiation.
Radiation from the sun can be described quantitatively or qualitatively.
The waveband of the light, as well as the distribution and intensity of the wavelengths within the waveband will determine the quality of the radiation received by the greenhouse.
The quantity of the radiation corresponds to the amount of energy within the radiation. This energy is quantified by the number of photons of light [moles of photons] per square meter per second, µmol m-2 s-1, or as a total energy value of the light, Watts per square meter [W m-2].
The wavelength or its frequency is another means to describe the radiation.The wavelength is typically expressed in nanometers (nm) or micrometers (µm) whereas the frequency can be defined as units of cycles per second,
Wavelength and frequency have an inverse relationship: as the wavelength increases, the energy of the light wave decreases, and on the contrary, as the wavelength decreases, its energy increases.
II. B. the different wavebands of interest in the radiation spectrum
The solar radiation can be divided into three wavebands:
- the ultra -violet (UV) corresponds to the wavelengths less than 400 nm and can cause skin damage because of their high energy.
- the visible light, within the 380-770 nm waveband, and contains the PAR (400-700 nm) waveband.
The different colors of the visible light, which corresponds to different waveband, may not have the same function towards plant's development.
For instance green (495-566nm) and yellow (566-589nm) light contributes to photosynthesis, orange (589-627 nm) will optimize for maximum photosynthesis and red light (627-770 nm) enhances flowering, stem elongation.
- the infrared (IR), greater than 770 nm and have an heating effect. Red: Far-red (R:FR) ratio is very important for plants because it influences plant growth response
The leaf is designed to absorb nearly 95% of wavelengths between 400 – 700 nm, but only 5% of the 700-850 nm waveband is absorbed. Of the remaining 95% of the 700-850 nm waveband, ~45% is reflected, and 45% is transmitted.
II. C. Sensors used to measure solar radiation
To measure the solar radiation, several sensors are available:
-pyranometer sensors measure the entire waveband from 280 to 2,800 nm
- quantum sensors only measure the PAR waveband ( 400-700 nm)
- spectroradiometers will measure soectral irradiance: it will measure the irradiance of photon in different wavelength that would have been previously splitted.
-to evaluate energy in the greenhouse, a net radiometer can be used: it measures the difference between the radiation that arrive from above and the radiation that is reflected from below.
Solar radiation and greenhouse
Greenhouse structures are an obstacle for solar radiation: frames, glazing bars, dirt, gutters, because they are opaque, absorb or reflect al the light that reaches them. Numerous greenhouse equipment are blocking the light from reaching the plant inside. In conclusion, a large portion of the solar radiation ( between 30 -50%) does not reach plants. The greenhouse cover, plastic or glass, will also have an important impact on light transmission.
Plant growth of plant can be manipulated with solar radiation. Current methods to regulate plant growth in the greenhouse may include daily exposure of seedlings to red or far red lighting at the end of the day for several weeks prior transplanting in the field. This treatment will alter the plant height and the total leaf area.
A daily cool-white lighting for several weeks for one hour before the end of the natural photoperiod will also alter plant height and total leaf area.
To reduce plant height, Far Red can be suppressed from the solar radiation using liquid copper sulfate filters.
III.Artificial Supplemental Lighting in the greenhouse
Supplemental lighting, to increase illuminance and/or to extend photoperiod can be a significant portion of the total greenhouse energy use. In addition to the investment in the lighting system itself, it is important to choose the most economical light source to fit the needs of the plants. All types of lamps convert electrical energy into both light and heat. Different kinds of lamps differ in the efficiency of their conversion of electrical energy into light. This affects not only the greenhouse energy budget in terms of the cost of electricity, but also in terms of dealing with the "waste heat" generated by the bulb. This can either contribute in a positive way to heating the greenhouse during the winter, or in a negative way if "waste" heat generated by the lamps increases the demand on greenhouse cooling systems (shade and/or mechanical systems)
III. A. Incandescent light bulbs.
Anyone who has ever touched a hot incandescent light bulb has experienced the fact that they are very inefficient in converting electricity into light. Hence they are not a good (economical) source of greenhouse supplental lighting when the objective is to increase light intensity. On the other hand, to control plant photoperiodic responses it requires only very low intensity to extend the photoperiod. For this purpose low wattage incandescent lighting may be a good way to inexpensively increase day length.
Some characteristics of incandescent lighting includes:
-Good source of red wavelengths
-Poor source of blue wavelengths
-Too hot for most plants, unless placed at least 3 feet above the plants.
- Easy to install
- One-third as efficient as fluorescent tubes
-Bulb's life is often only about 1,000 hours.
Incandescent Light With Aluminum Reflector can be used to redirect the light emitted by incandescent bulbs in order to increase their efficiency (brightness).
John Kumpf on Installation of Incandescent Lighting
John Kumpf says on Installation of Incandescent Lighting:
"Installation of incandescent lights is the easiest of all the lighting fixtures. Incandescent lights over plant material in the greenhouse often ends up being permanent even though that may not be the original intent. Ease of installation makes incandescent lighting especially popular for the small grower looking to control photoperiod. When installing this type of lighting be sure to use the proper size wire. To use the proper size wire, check local codes at any electrical outlet store in your location. Coated #12 solid copper wire works best. To connect the light to the wire use a screw on weather proof socket , making sure good contact has been made between the socket and the wire. The wire should be supported by structures that are free from sharp edges. They should always be wrapped around insulators rather than wood or piping. Also, be sure to tape any exposed ends. Sunlight can break down the coating on the wire, be sure to inspect the wire and all fixtures on a regular basis."
III. B. Fluorescent Lighting
Florescent lights are more efficient than incandescent lights in converting electricity into light, so they don't generate as much waste heat and their fixatures are cumbersome. However, incandescent, are limited in how much light intensity they can provide, take up a lot of space, and they are expensive. The wavelength distribution (color spectrum) of standard (cool white) florescent lamps is high in blue but low in red. Modified spectrum florescent lamps are available which are enriched in the red wavelengths so the overall spectrum more closely approximates that of sunlight (e.g. Grolux, Plant-Gro, and Optima). The downside of this modification is that it decreases overall light intensity. The advantage gained by the modified blue-red spectrum is more than offset by the loss of intensity and their increased price. Overall a grower is better off using standard cool white floresencent bulbs particularly when florescents are used in combination with incandescent (red rich) lighting or when florescents are used to supplement natural lighting which provides plenty of red light. Florescent lighting systems not generally used for supplemental greenhouse lighting when increased light intensity is the goal. Instead, they are primarily used in growth rooms or short term germination areas, either alone or in combination with incandescent lights.
Fluorescent tube's life is normally 10,000 hours or more and energy efficiency is 40-60 lumens/watt.
John Kumpf on Installation of Fluorescent Lighting
lighting is generally used in greenhouse only when low natural light is available.
More often, fluorescent lights are used in propagating areas or growth room
that receive no natural light.
Remembering that fluorescent light intensity weather it "very high output" or "high output" decreases dramatically as the distance from the plant increases. The support structures are usually designed to keep fixtures relatively close to plants ( i.e. 6 through 24 inches). Fluorescent light tubes are the lite part of the equation whereas the hallasts and reflectors can amount to substantial weight. The lights can be hung with boiler chain and shooks from constructed wood or pipe frames located over the bench. Donn't hang the fixture from the sash bars. In some situations, the reflectors can support the weight of the fixtures, however, this requires several cross stringer because most fixtures are no more than 8 foot long. As with any type of electrical set up, wether temporary or permanent, be sure on checking local wiring codes."
C. High Intensity Discharge (HID)
HID lamps are the most efficient at converting electricity into light and for this reason they are the most economical for greenhouse supplemental lighting when the primary goal is to increase light intensity.
There are two types of HID lamps - high pressure sodium (HPS) and Metal Halide (MH):
a). High Pressures Sodium (HPS) - these are the orangish lights you see illuminating shopping center parking lights. Their spectrum is rich in red wavelengths that provide an effect on daylength perception in plants, hence promoting both growth and flowering in long-day and day neutral plants including annuals. But this spectrum is comparatively poor in blues. They are used for supplementing lighting because they are efficient at converting electrical energy into PAR light (20-25%), and are available in high voltage. Bulb life is usually around 25,000 hours.
b). Metal Halide (MH). Most commonly used by professional greenhouse growers because their spectrum is more balanced than HPS. Because it provides more blue light than orange/yellow, and because plants are less disturbed under MH settings, these fixtures are mostly used for displaying plants in retails settings.
John Kumpf on HID Lamps
"These high pressure sodium lights are 400 watt lamps over some tomato plants in the greenhouse when you look at these one of the things you look for is light uniformity over the plant material and they should be setup on specific grids so that you have uniform light over the entire greenhouse. If you don't, you're going to have variations in how the plant material responds to the amount of light that the plant is receiving. Those that receive less light may be along the outside of the benches and will etiolate (stretch). So check the configurations of your lamps to make sure that you have good uniform lighting over all the plant material.
Be aware that light uniformity is extremely important when manipulating the light intensity in the greenhouse. In this particular situation we're not talking about photoperiod control, but we're talking about the available light that the plant has which in turn relates to photosynthesis. Looking at these lamps you notice that they are approximately 4 feet up off the top of the plant material. Pay attention to the distance between the light bulb and the growing point of the plant, it is essential to learn how close the plant can get before the heat and Hi light impact its gorwth. These are tomato plants and they are very sensitive to the heat, so we elevated the lights up off the plants and increased the number of lamps. This way we get the correct light intensity and uniform distribution. We also put white plastic down on the floor or paint the floor white to reflect light back up.
These lamps are set up to increase light intensity and they are used to extend the day length. Normally we would run these 18 hours a day from 6 in the morning until midnight, extending the day length to increase production. Whether it is tomato plants or roses, these lights do increase the growth and production in the winter time. We turn the lights off on April 15th and don't turn them back on until the 15th of October.
These are low profile luminaries. You want to make sure you clean the insides of these well. Take a piece cheese cloth and wipe them out before you put them up. This will ensure the most reflective light. So clean the inside of the lamp also take the outside of the lamp and rub them down as well, we want to keep these so that they will give you as much as reflected light as possible."
John Kumpf on Installation of HIDs
"High pressure sodium and metal halide fixtures are not easy to install and should be considered a permanent installation. The process requires two people, ladders tall enough to eliminate any excessive reaching, the proper weight chain, and hooks strong enough to hold the weight of the fixtures. I do not recommend hanging the lights from greenhouse sash bars because the accumulated weight of several lamps could damage the bars. The best way is to hang the lamps from sturdy interior structures or construct a grid out of heavy duty channel iron.
All wiring should be done by a qualified electrician. I would pay attention to the lay out of the electrical circuits which determines what lights are on or off. The importance of this reverts back to how much of the greenhouse will be lit at one time."
Example of calculation to determine fixtures location:
- A grower has a 9' x 128' bench over which he wants to suspend HID lamps of 400W.
- The light intensity desired for the plants that he is growing is 8600 lux or 800 fc. The effective flux of the HID lamps is 38,400 lumens.
-The number of fixtures (N) is calculated using this formula:
N = (light level x surface area)/ effective flux.
-For the grower in question the number of fixtures is then 24 [(800 x 9 x 128)/ 38,400].
- The pattern of those fixtures is defined as the horizontal spacing and the height above the crop surface. For each lamp the distance between fixtures along the line (L) and the distance between lines (B) is usually given. You can refer to Poot (1984) to find out those numbers.
In our example L = 1.55H and B = 2.7H with H the height.
Since Light level (E) = Effective flux (F)/ area (A);
A = L x B = 1.55H x 2.7H = 4.2 H2.
Therefore, H = racine carre of (A/4.2) = 3.38 ft
and L = 1.55 x H = 5.24 feet
and B = 2.7 x H = 9.13 feet
Uniform distribution of light is of importance for crop production in greenhouses. A lighting system depends strongly on the way light is distributed across the crops. The secret to obtaining a uniform blanket of light is to equip light fixtures with reflectors, which receive and reflect light in the desired direction. Its shape, reflecting angle and the emission of total light quantity between that angle determine the efficiency of each reflector.
The goal of using shade to grow crops is to reduce the amount of sunlight reaching a crop so that stress on plants caused by excessively high temperatures can be reduced. The use of shade is also an important way to reduce the temperature of the plants. The greenhouse manager needs to decide if light should be reduced to achieve temperature.
Greenhouse shading can be done using one of two methods: 1-either an external application of shading compound, 2- internally using a woven type of shading fabric. External shading is most effective in decreasing heat buildup in the greenhouse. However, applying a uniform coating of shade is difficult, as is removal, and once the shade is applied, it is semipermanent. Furthermore, cloudy days even during the summer means that plants may be receiving insufficient light for a long period of time. The problem of permanence can be overcome using internal shade, since shade material can be pulled and withdrawn as needed. However, the cost of internal shading is high and it does not prevent light from entering the greenhouse and then from being converted to heat. But using reflective or white material can reduce the problem.
John Kumpf on Shading
"Shading is a step in greenhouse
management that should be done very cautionely, especially permantent shading.
Shading has two main effects in greenhouse: it reduces the temperature and the
light intensity. THerfore it is important to consider both factors when decide
which shading technique to utilizes. For example if shading is used to decrease
temperature in greenhouse, it will also decrease the light intensity that plants
in the greenhouse will receive. And this may affect their growth. Therefore
it is important to find a balance between light and temperature when it comes
to install shading.
Shading plants in greenhouse varies considerably from small to large greenhouse.
Shade curtains in a large greenhouses often serve two purposes: reduce light in the summer and act as a thermal blanket in the winter. Modern shade screens can drastically change the greenhouse environment. They can be used to control temperature , reduce energy costs, and help cut down on the amount of water taken up by the plant to say nothing about creating a more comfortable environment for the greenhouse worker. Manual or computer controls activate the curtain giving the plant material growing under it the proper amount of light. Computer programs can control shade curtains so the plant gets just the right amount of light. When intensity is too high, the curtain closes and vice versa. Furthermore, those programs can also estimate the amount of light a plant will receive based on the accumulated light of the first few hours of the day determining whether the curtain should remain opened or should be closed. Always remember to install a manual over ride on this equipment. PICTURE.
Small greenhouses often use manual methods of shading plants. PICTURE The support structures for shade cloths in smaller greenhouse is usually a wire heavy enough (14 - 12 GagueA.).PICTURE
This wire is run from supports at each end of the bench and the sahde cloth is pulled on the plants. The same cloth can be used to control photoperiod. Using the greenhouse catalog, select the best material to serve the purpose, many different quality are available and this quality will modify the percentage of light reduction.
When you are finished using shade cloths be sure to take it down. It collects dust and can also fade, changing its light transmission reducing characteristics.
Shading Compound is one of the most famous external manner to control glass or polyethylene greenhouse shading. Shading compounds will diffuse light rays and reflect heat. If used with material other than glass, the shading compounds may harm the glazing and may not be easy to remove if not possible at all. Shading compounds come either in white or green and can be thinned using paint solvent. They are extremely effective in decreasing heat buildup in the greenhouse. However, it is difficult to apply and to remove the uniform coating of those shading compounds. Once applied, the shade is semipermanent, and during cloudy and dark days of summer, the plants in the greenhouse may receive insufficient light for several days. The second method to reduce light level is to block out light with some shading screens made of cloth, polypropylene, polyester or aluminum-coated polyester. These systems may be placed on the exterior of the greenhouse or on the interior.
When associated with cloth moving devices, movable black cloth will then offer some control of the greenhouse temperature. However, it is not always recommanded to utilize black cloth because it always increase the temperature because it ablsorbs light. It is also important to use good fan ventilation.
Used to control photoperiod, and especially to provide short day-light period. Cloth is pulled over the plants at 5pm and taken off at 8am. The cost runs 4 - 5 dollars per yard and comes in increments of 5' wide by any lengths. Extreme care should be taken when handling and setting up black cloth. Always make sure that cloth is in good shape (no holes and not faded). Any holes in the cloth should be repaired immediately because any external light reaching the covered plant material can cause delays in flowering as well as uneven flower bud initiation. Go over pinning to make sure that all holes are covered.
Also, don't leave cloth up after it has been used because sunlight will fade it when pulling the cloth shut. The area of most concern should be where the two cloths come together. Whether they are pinned together or one on top of the other, particular attention should be paid to this area. Be sure that all sharp edges are eliminated on the wire that the cloth is pulled over. Be sure that the wire is heavy enough to support the cloth, especially if your greenhouse has leaks that allow water to drip on the cloth. This should be avoided because water can cause the cloth to sag causing injury to the plants underneath."
Saran or polypropylene is used to reduce light over plants, especially when newly potted or recent cuttings. Comes in varying degrees of shade from 20% - 80% light reduction. Polypropylene film is used to reduce light over plants, especially when newly potted or recent cuttings. Black polypropylene is used to construct the shading cloth manufactured for today's greenhouses. This material is very strong and has a high U.V. protection. The grower has a wide range of shade percentages to choose from depending on their situation. A 30 - 95 percent shade cloth can be purchased. There are many links to commercial supplies of polypropylene film that will show the range of options available.
(Put in question here and at other question mark icons? Suggest using the links we have provided and any other **reputable** (define?) info they have access to in order to answer the question or develop some kind of strategy. Sort of interactive, not so passive?)
The information in this Web page is presented with the understanding that no discrimination or endorsment of any of the information linked to from this Web page is implied.