| Intro | Light | Nutrients | Propagation | Systems |
HORTICULTURAL LIGHTING
Color
HID lighting
Metal halide lamps
High pressure sodium lamps
Ballasts
Light intensity
Light carts
Light movers
Side lighting
Light reflectors
Light duration
Summary
Hydroponic gardening is the way of the future for environmentally controlled agriculture. By carefully monitoring and controlling nutrients, lighting, temperature and humidity, phenominal yields can be achieved in a relatively small space.
In most grow rooms, lighting is the limiting factor for optimum yields. Nutrient levels can be easily manipulated by adding fertilizer salts, but light and temperature control the rate at which those nutrients are used by the plant. When environmental conditions are right, endodermal cells in the plant roots allow more water and nutrients to enter the vascular system to be used by the plant. Without the right quality and quantity of light, adding more fertilizer will not produce better growth.
Today, artificial lighting is used in the production of all kinds of crops. Some of the advantages include:
higher output per acre
better plant quality
extended growing seasons
and better control over the growing environment
This is true for cut flowers, potted plants, vegetables and other crops.
Great advances have been made in the field of horticultural lighting over the last few years, and the future promises even greater results.
Three factors must be considered
for successful horticultural lighting: color, intensity,
and duration of light. When these factors are properly balanced,
artificial lights, such as fluorescent, Metal Halide and
High Pressure Sodium, can help yield spectacular gardens.
COLOR
The sun's spectrum contains visible light in every color
of the rainbow. Blue and violet rays appear at one end of
the visible spectrum, at about a 380 nanometer wavelength,
and red light appears at the other end of the spectrum,
at about 760 nanometers. If all of the colors of the spectrum
are absorbed by an object, it will appear black. If all
of the colors are reflected by an object, it will appear
white.
Plants appear green because they reflect green light, while absorbing the other colors of the spectrum. The green pigment in plants is chlorophyll, essential for photosynthesis. The green portion of the light spectrum is relatively unimportant to photosynthesis, since it is mostly reflected by the plant. Chlorophyll absorbs light energy from the blue and red portions of the spectrum to convert carbon dioxide and water into sugars to be used by the plant. Without red and blue light rays, the plant will weaken or die prematurely.
The orange-red portion of the light spectrum is very important to plant reproduction. Pigments called "photochromes" absorb the red and far red portions of the light spectrum. Photochromes regulate seed germination, root development, tuber and bulb formation, dormancy, flowering and fruit production. Therefore, the red portion of the light spectrum is essential to plant growth, particularly during the flowering, fruiting and reproductive stage.
Plants need light from the blue end of the spectrum as well. Blue light stimulates chlorophyll production more than any other color, promoting thick leaves, stocky stems and strong vegetative growth. Carotenoids, the yellow-orange pigment in plants, absorb blue light and control leaf fall and fruit ripening. Riboflavin, containing another pigment, absorbs violet light and influences "phototropism", the movement of plant foliage in response to a light source.
"Positive tropism", is the movement of plant foliage toward a light source. "Negative tropism" is the movement of plant foliage away from a light source. Positive tropism is greatest in the blue end of the spectrum at about 450 nanometers. At this level, plants lean toward the light, spreading their leaves out horizontally to absorb the most light possible. Lack of blue light will make plants tall and spindly, since the plants will S T-R-E-A-C-H out in search of blue rays. Adequate blue light helps ensure each plant's natural shape and prevents "legginess", or unnatural elongation of the stems.
Because of their color spectrums and wattage outputs, Metal Halide and High Pressure Sodium lamps are the most popular artificial growing lights.
High Pressure Sodium lamps are strong
in the orange and red spectrum bands. The HP sodium lamp
emits an orange-like glow that is sometimes compared to
that of the harvest sun. These colors promote flower production
and stem elongation. When using an HP sodium lamp, flower
production may increase by 20% or more. Unfortunately, HP
sodium lamps lack in the blue portion of the spectrum. Conventional
HP sodium lights should not be used alone since their lack
of blue light will make plants tall and spindly. When used
in a greenhouse setting, however, HP sodium lamps are all
that are required for supplemental lighting, since sunlight
provides all the blue light necessary.
Metal Halide lamps have a fairly balanced color spectrum,
with a higher percentage of blue and violet light rays than
High Pressure Sodium lamps. Therefore MH lamps are excellent
during the vegetative growth stage. MH lamps help keep plants
shorter and stockier, with more compact vegetation. Stockier
plants are stronger, better able to resist disease, and
generally more productive than spindly plants.
When clear Metal Halide and High
Pressure Sodium lamps are used together, a winning combination
is achieved. Blue light in the Metal Halide promotes strong
vegetative growth, and the red light in the High Pressure
Sodium lamp promotes strong fruiting and flowering. Some
growers like to use a carousel system, alternating MH and
HPS lamps, to provide a balanced light spectrum during the
plants complete growing cycle.
HID LIGHTING
Metal Halide and High Pressure Sodium lamps belong to the HID family, or High Intensity Discharge. In HID lamps, light is produced by passing current through a vapor at relatively high pressure, normally 1 to 8 atmospheres. Fluorescent and Low Pressure Sodium lamps, on the other hand, produce light under low pressure conditions.
Lets look more closely at a Metal Halide lamp.
The center of a Metal Halide lamp contains a quartz arc tube. Light is produced by arcing electricity through combined vapors of mercury and argon, with other metals, introduced into the arc tube as compounds of iodine: thorium iodide, sodium iodide and scandium iodide. Spring supports in the dome and neck of the outer bulb hold the arc tube in place. The outer bulb acts as a protective jacket. It protects the arc tube, while absorbing harmful ultraviolet radiation. If you spend a lot of time in the grow room, protective goggles that filter out UV are also recommended.
Phosphor coatings are also an option. Phosphor provides more diffuse light which is easier on the eyes. Plus, the phosphor coating tends to tip the color spectrum towards the red end of the spectrum, providing slightly more red light for fruiting and flowering. A phosphor coated bulb is recommended if a "single source" Metal Halide lamp is required.
High Pressure Sodium lamps are shaped differently than Metal Halide lamps. A ceramic arc tube contains sodium and mercury, with a little xenon gas for starting. The sodium discharge dominates the color, producing the orange-red light. Electricity passes through electrodes at the ends of the arc tubes. If the lamp is turned off or a power surge occurs, the gasses will need to cool three to 15 minutes before restarting is possible.
Son Agro is a special kind of HP
sodium lamp, with a 30% increase in the blue end of the
light spectrum. The Son Agro bulb also burns brighter than
a regular 400-watt HP sodium lamp, and it is recommended
if a "single source" HP sodium lamp is required
in a grow room. Although Son Agro lamps do not come in 1,000-watt
varieties, two 430-watt lamps are generally more desirable
than one 1,000-watt regular HP sodium. The color spectrum
will be more natural, and the light will be more evenly
distributed throughout the grow room.
Ballasts
MH and HPS lamps both require special ballasts. Each ballast contains a transformer and a capacitor, and regulates the voltage for the lamp. Since most HID bulbs come in either 250, 400 or 1000-watt versions, make sure the ballast you use is designed for the proper lamp and wattage. Using the wrong ballast could result in a burned out lamp or worse!
Conversion bulbs are available for some metal halide ballasts. The bulb looks like a metal halide, but produces a light spectrum similar to a HP sodium lamp. The 360-watt conversion bulb works on a 400-watt metal halide ballast, and the 940-watt bulb works on a 1000-watt metal halide ballast. Although not as bright as regular HP sodium lamps, conversion bulbs are more economical than buying two complete systems.
Light intensity is dependent on two factors: the brightness of the lamp and the distance to the plant.
The brightness of a lamp is measured in lumens. One lumen is the amount of light emitted from one candle that falls on 1 square foot, one foot from the source. For example, a 1,000-watt standard metal halide emits 110,000 initial lumens, and a 1,000-watt HP Sodium lamp emits 140,000 initial lumens, measured one foot from the source.
The light intensity falls off exponentially the further the plant is from the light source, as shown by the "Light and Distance Chart". Light intensity is calculated using the formula l = L/D2 or "light intensity equals initial lumens divided by distance squared. If a plant is two feet away from a light source, 1/4 of the initial lumens are available. At three feet, 1/9 of the lumens are available. And at four feet, only 1/16 of the initial lumens are available.
A light meter is a good way to test the light intensity in any part of the grow room. Simply point the meter toward the light source and take the reading. Most light meters measure in lumens, foot candles or miliwatts per-square-meter. The U.S. Department of Agriculture can provide data about specific light level requirements for various plants during different stages of their growth.
Generally speaking, HID lights should be placed as close to the growing tips of the plants as possible, without burning the leaves. Ideally, HID lamps should be hung about 12 to 18 inches above the garden. Tender clones, seedlings and transplants need more space, usually about 24 inches.
If the plants start to become tall and spindly, they are S-T-R-E-C-H-I-N-G for more light. Bring the light source closer to the plants. If the leaf tips become "sun bleached" or begin to curl, back them away from the light source.
Proper spacing is also important. Hundreds of seedlings can be huddled together under one HID lamp. But as the foliage begins to spread and grow, make sure that the leaves do not touch and shade each other. By altering the spacing every few days so that plants remain as close together as possible without actually touching, shade will be kept to a minimum and growth will be maximized.
High light intensity helps keep plants short and stocky, with smaller but denser and heavier leaves. The cuticle and cell walls are thicker, and the leaf stomata, through which transpiration of carbon dioxide, oxygen and water take place, are denser. These changes help make the plant more resistant to high temperatures, drought and infection.
To better utilize space during the propagation stage, light carts are sometimes used. Light carts make it possible to stack several trays of seedlings in a very small space.
Most light carts use fluorescent grow lamps instead of HID's. Many fluorescent "grow lamps" have a balanced color spectrum, closely approximating natural sunlight, but their low lumen output make them impractical for growing plants with high light requirements.
Fluorescent lighting is excellent for rooting cuttings, however, since tender young cuttings and seedlings do not need as much intense light as more mature plants. Even so, the lamps should be kept as close to the young plants as possible, preferably within a few inches.
Although wide spectrum fluorescents are recommended, cool white fluorescents can also be used if necessary. Their strong blue light is good for initial growth, but their lack of illumination, particularly in the red part of the spectrum, slows down root development by a few days. For gardeners on a budget, using a fixture with one standard fluorescent bulb and one wide spectrum fluorescent bulb may be a good compromise.
Once the plants start to develop a good root structure, however, they should be moved under HID lighting.
To better utilize available space as the plants mature, track lighting may be used. The light is adjusted to the proper height above the plants, and the motor drive is started. The light moves along the full length of the track, providing even lighting to all of the plants in the row. Light movers can increase the amount of intense light plants receive by 25 to 35%, and the plants will receive light from more than one direction.
To further increase even light distribution, a smart box may also be used. When the light reaches the end of the track, the light will pause for 30 seconds so the plants on the ends of the row receive as much light as those in the middle.
Side lighting can also be beneficial to plants. If a light is hung directly overhead, the upper leaves tend to shade the lower leaves. Eventually, the lower leaves will yellow and die.
To provide additional side lighting,
some light movers have a T bar so that lamps can be hung
on both sides of the row. The angle of carousel lights can
also be adjusted so that more side lighting is available.
Low pressure sodium lamps may also be used for side lighting.
The lamp is three feet long, and extremely energy efficient,
producing 180 lumens per watt in the 180-watt size. Unfortunately,
all of the light produced is in a very narrow color spectrum,
almost entirely in the yellow-orange band. Furthermore,
its low wattage rating severely limits its overall intensity,
producing only about 32,000 initial lumens.
When used for side lighting, however, LPS lamps can be very useful. The lamps can be placed low to the ground and close to the plants, compensating for their lack of light intensity. Furthermore, since phototropism is initiated by violet light, their narrow color spectrum will help prevent the plants from leaning unnaturally toward the sides. Experiments at the USDA labs in Maryland have yielded promising results for LPS lamps when used as a supplement to full spectrum lighting.
Light
Reflectors
Reflective light can also be used by plants. Chrome Mylar
coating can be added to grow room walls, reflecting 95%
of available light. Chrome Mylar helps eliminate shadow
areas, and it is the most reflective sheet product available.
White plastic sheeting is another alternative. It can be
added to both the walls and floors, providing some reflective
light to lower leaves.
A low cost alternative to sheet reflectors is flat white paint. Surprisingly, flat white paint is more reflective than a glossy finish, and almost as reflective as mylar.
Duration of light and dark periods, or photoperiodism, is also important to a plant's growth and reproduction.
During the propagation stage, lights should be left on as long as possible, up to 24 hours per day. Strong initial growth will make the young plants healthier and more resistant to disease, ultimately providing better yields. As the plants begin to mature, however, light duration should gradually be reduced.
Most plants do well with fourteen to sixteen hours of light per day. In a greenhouse setting, supplemental lights can be used on cloudy days or to extend daylight hours in the winter. With supplemental lighting, plants can be kept actively growing all year long, making out of season fruits possible. Dormancy can be prevented in some crops, and winter fruiting can be induced by providing extra light in the fall.
Plants use periods of darkness, too. During dark periods, plants continue to take in oxygen and give off carbon dioxide in a process called respiration. During this time, the plant uses some of the energy that was stored as sugars during photosynthesis, producing proteins, hormones and other complex products. Therefore, lighting mature plants more than seventeen and a half hours per day will not increase production significantly, since photosynthesis will slow down after a certain number of hours. In fact, too many daylight hours can actually be counterproductive, delaying or preventing flowering in some crops.
Most plants are sensitive to day length. Some plants flower during the spring and summer, when days are longer than nights, and some flower in the autumn, when nights are longer than days. Therefore, plants are classified as short-day, long-day and day-neutral.
A hormone called "florigen" controls budding and flowering. Long day plants require about 14 to 18 hours of light to produce just the right amount of florigen to flower and reproduce. Short day plants require about 10 to 13 hours of light. If short day plants are exposed to too many hours of light, florigen can be destroyed, preventing blooming.
Short day plants include poinsettias, chrysanthemums, and gardenias. They require long nights in order to flower. If even a small amount of light interrupts the dark period, flowering can be hindered. Some greenhouse growers purposely interrupt the dark period to delay flowering in Christmas poinsettias, then, at the appropriate time, cover the plants with a dark tarp to induce flowering.
Lettuce and spinach are also short day plants. Once mature plants are exposed to just one 12 hour day, there is nothing that can stop them from "bolting" and producing flowers, even if the harvest isn't ready.
To precisely control the duration of light, timers can be used. In a greenhouse setting, timers should be set to come on at dusk, when the intensity of sunlight is about twice that of the artificial lights. Light sensors can also be used to turn on supplemental lights on cloudy days. But since turning HID's on and off too much can reduce their life, once the lamp comes on, it should be left on for at least an hour.
Careful regulation of artificial
lighting is well worth the effort. When the color, intensity
and duration of light are properly balanced and controlled,
plants will be healthier, the growing season will be extended,
and productivity will be increased.
SUMMARY
Let's take a moment to summarize the most important features of various horticultural lights:
*Metal Halide lamps are strong in the blue end of the spectrum,
*producing stronger vegetative growth.
*High Pressure Sodium lamps are strong in the red
end of the spectrum,
*producing stronger fruiting and flowering.
*Son Agro lamps are HPS lamps with a 30% increase in the blue end of the spectrum,
*making them an ideal "single source" lamp from
seed to harvest.
*Fluorescent grow lamps also
have a balanced color spectrum,
*but because of their low lumen output,
they are only practical
*for growing tender seedlings and
cuttings, or for "low light" plants.
*Low Pressure Sodium lamps have a very narrow color spectrum,
*but they may be used for "side lighting".
*Light movers provide more even lighting when used
with high intensity lamps.
*And timers can be used to control the duration of light and dark periods,
*allowing better control of the growing season.
When the proper combination of horticultural lighting is
used, along with the right levels of carbon dioxide, nutrients,
temperature and humidity, hydroponically grown plants can
flourish in any grow room!
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