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General Lighting Requirements For Plants
To determine how much light a plant will require, consider where and how it grows best in its natural environment. Most vegetables, for instance, grow best in full sunlight, which means as much light as possible must be supplied to grow vegetables indoors. Foliage plants like the Philodendron grow in full shade and therefore can grow normally with relatively little artificial light. Exotic plants, such as Bromeliads, grow in varying conditions depending on the species. Some grow in deep shade in the jungle, while others grow in bright sunlight. The lighting level required for growth indoors depends upon the characteristics of the particular plant being grown.
Plants NEED Darkness
Plants need dark periods. Light (called photo-periods) and dark periods and their relative lengths have an effect on plant maturity. Recent studies have conclusively proven that it is not just the length of the day which affects growth, but the duration of the dark period which follows. The dark period of each day affects flowering and seeding of most plants. Although many plants can grow under continuous light, nearly all plants prefer a dark period each day for normal growth. All plants need some darkness to grow well or to trigger flowering. The ideal photoperiods of plants vary, some preferring long days and short nights; other the reverse; and some do best when the length of the night and day periods are equal.
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Color Temperature
In scientific terms, Kelvin temperature is a measure of the color of a light source relative to a black body at a particular temperature expressed in degrees Kelvin (°K).
In simpler terms, it is the degree of warmth or coolness of a light source, not with regards to the physical temperature, rather to the visual temperature of the light. The higher the degree K, the more blue, or "cooler" the lamp appears. The lower the degree K, the more "warm", or red the light appears.
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Incandescent lights have a low color temperature (approximately 2700°K) and have a red-yellowish tone; natural daylight has a high color temperature (approximately 6000°K) and appears bluish. Today, the phosphors used in fluorescent and high intensity (HID) lights can be blended to provide any desired color temperature in the range from 2800°K to 6000°K.
Below you will find a brief explanation of the various artificial light sources used in horticultural applications that are available today along with the postives and negatives associated with each type.
Color Rendering Index (CRI)
A rating scale for light sources (lamps) from 0 to 100 to indicate how accurately colors can be perceived under a light source. The higher the CRI, the more accurately colors appear. Technically, CRI ratings should only be compared for lamps with similar color temperatures (Kelvin ratings).
Incandescent Plant Grow Lights
Most incandescent plant lights are best used to highlight indoor plant groupings and have very little use as a true plant 'growing' light, even those that are labeled as a "grow light".
Newer color corrected incandescent spot grow bulbs are suitable for adding supplemental light or highlighting to a single plant, such as an orchid or indoor bonsai tree.
However, you should never place an incandescent bulb closer than 24 inches over your plants. Incandescents burn very hot and will burn plants when placed close enough for them to appreciably affect plant growth.
Most incandescent bulbs have an average life span of 750 hours.
Fluorescent Plant Grow Lights
Recent advances in fluorescent technologies provide the indoor gardener with a multitude of fluorescent plant growing options.
Standard Fluorescents are an excellent choice for starting seedlings to get a jump start on spring plantings. They also are excellent for cultivating a cool weather harvest in the basement year round. Leaf lettuce, spinach, and herbs all will do well year round under fluorescent lamps.
The trick to using them successfully is to keep standard fluorescent bulbs no further than 4 inches away from the tops of the plants. Since the tubes burn relatively cool to the touch, you do not have to worry about burning plants unless they are actually touching the bulb. Fluorescent tubes are an excellent choice for the display and grow lighting of african violets, small cacti and succulents, and many kitchen herbs.
New advances in color blending make full-spectrum fluorescents one of the truest color rendering lamps on the market today. Standard fluorescent tubes are twice as efficient as incandescent bulbs, producing twice as many lumens per watt of energy consumed and have an average usable life span of up to 20,000 hours... more than 25 times as long as an incandescent.
High Output Fluorescents produce almost twice as much light as standard fluorescents while still burning very cool to the touch. HO fluorescent fixtures have a very thin profile, making them extremely useful in vertically limited areas. High Output Fluorescent fixtures are available in 2, 4, 6, and 8 tube models, making them ideal for a wide variety of horticultural applications. An 8-tube 4-foot model produces approximately the same amount of lumens as a 400 watt Metal Halide fixture, but with much less heat and the ability to more evenly spread that light over the coverage area. High Output Fluorescents produce about 5,000 lumens per 54 watt 4-foot tube and are available in warm (3000K) or cool (6500K) versions. Also available are 2-foot 28 watt tubes that produce 2,450 initial lumens. Average usable life span for High Output Fluorescent tubes is about 10,000 hours.
Compact Fluorescents take the cool burning fluorescent technology and pack it into a focused, high output "bulb", that you can use not only for propagation, but for growing larger plants as well. Compact fluorescents work in specially designed reflectors that efficiently direct light to the plants, much like the high intensity discharge lamps described below. Compact fluorescent bulbs are also available in warm (3000K) or cool (6500K) versions. Average usable life span for Compact Fluorescent bulbs is about 10,000 hours.
High Output Fluorescent / HID Hybrids combine the incredibly bright cool burning power of high output fluorescents with the penetration that can only be achieved through high intensity discharge technology. The primary advantages to these fixtures is their blend of light colors and broad even coverage.
Metal Halide (MH)
High Intensity Discharge (HID) Plant Grow Lights
Metal halide is a highly efficient light source capable of delivering a white light in the range of 2700 to 5500 Kelvin degrees with typical Color Rendering Index (CRI) ratings in the mid-60's to mid-70's. Some lamp chemistries even obtain CRI's in the 80s. The CRI is an index that measures how closely an artificial light source matches the natural colors of sunlight, with 100 being a perfect score. In contrast, high pressure sodium lamps yield yellow lighting (2200K) and have a very poor color rendering index of 22.
By varying the blend of chemicals in the arc tube, metal halide engineers are able to alter the characteristics of the light output.
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This flexibility in design is what makes metal halide lighting so versatile. White light is a very important attribute of metal halide technology, because it is the closest to the natural sunlight that people prefer.
Metal halide lamps are widely used where color rendering is important such as stadiums, malls, manufacturing plants, and even your local supermarket.
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For horticultural purposes, metal halides tend to produce a shorter, more compact plant habit that more closely resembles plants grown outdoors. A plants appearance growing under most metal halide lamps is almost identical to those growing outside, which makes it the preferred lamp for use in living areas. Horticultural metal halide lamps have an enhanced red spectrum, which is added for increased fruiting and flowering without sacrificing plant appearance.
Metal halide lamps are about five times more efficient than their incandescent counterparts and last up to 25 times longer, with a usable lifespan between 10,000 and 20,000 hours, depending on the wattage.
High Pressure Sodium (HPS)
High Intensity Discharge (HID) Plant Grow Lights
High pressure sodium lamps have been used for years in less color critical applications such as parking lots, street lights and as supplemental greenhouse lighting. The benefit of high pressure sodium lamps to the horticultural industry is their ability to enhance the fruiting and flowering process in plants. The orange/red spectrum HPS produces is the spectrum plants use in their reproductive processes, which generally produces larger harvests of higher quality fruits or flowers.
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The major drawback to growing under high pressure sodium alone is that the plants tend to be taller and leggier with a longer internodal length than plants grown under metal halide.
Another minor negative is that plants generally do not appear very healthy growing under HP Sodium light. The operative word being 'appear', because in fact most plants do grow very well under HPS lighting.
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Due to the poor color rendering of high pressure sodium, the plants tend to look pale, washed out or nitrogen starved.
Additional benefits to high pressure sodium lighting are incredibly long usable bulb life... up to two years in many cases; and unparralled efficiency at six times more light output per watt of energy consumed than a standard incandescent bulb.
Due to their high efficiency and the fact that plants grown in greenhouses get all the blue light they need naturally, HP Sodium lamps are the preferred supplemental greenhouse light.
Combination HPS/MH lamps combine a metal halide bulb and a high pressure sodium bulb in the same reflector, either with a single integrated ballast assembly or two separate ballast assemblies. The combination of blue metal halide light and red high pressure sodium light creates an ideal spectral blend and extremely high outputs.
Convertible, Switchable & Two-Way lamps are configured to burn either a metal halide bulb or an equivalent wattage high pressure sodium bulb in the same fixture, but not at the same time. These fixtures are ideal for propagating and vegetatively growing plants under the metal halide, then switching to a high pressure sodium bulb for the fruiting or flowering stage of plant growth. Generally, all that is required to change between one and the other is to change the bulb and set a switch to the appropriate setting.
Light Emitting Diodes (LED)
While NASA and many universities are actively engaged in studying this exciting new technology for use in growing plants, many people are reporting very disappointing results when using most of the LED plant grow lights currently being marketed to home gardeners today. We have tested many versions in our store and have reached the conclusion that the only ones that come close to even considering are priced in the $2,000.00 range, and in our opinion there are much better performing conventional technologies available today that outperform LED's and do so at a much lower cost.
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