Grow light

Grow light

A grow light is an electric lamp designed to promote plant growth by emitting an electromagnetic spectrum appropriate for photosynthesis. The emitted light spectrum is similar to that from the sun, allowing indoor growth with outdoor conditions. Natural daylight has a high color temperature (approx. 6000°K) and appears bluish. Through the use of the color rendering index, it is possible to compare how much the lamp matches the natural color of regular sunlight.

Light spectrums used

Different light spectrums are used for the different stages of plant growth. The initial vegetative stage requires blue spectrum of light, whereas the later "flowering" stage is usually done with red–orange spectrums. The lights can be bought by spectrum colour specifically, or some companies such as Sylvania Grolux produce a full spectrum bulb which caters for all stages of growth.

The light is usually used in conjunction with a reflector, to control and intensify the light emissions, and will include an electrical ballast to control the current to the light. This is required because of the high intensity of the light that is necessary to produce something akin to sunlight.

Types & set-ups

Lamp types used as grow lights include incandescents, fluorescent lamps, high-intensity discharge lamps, and LEDs.

Today, the most widely used lights for professional indoor flower and vegetable growing are HPS/SON (high pressure sodium) and MH (metal halide). These are types of HID or high-intensity discharge lamps. MH is used for the vegetative phase of growth, as it encourages short internodes (distance between sets of leaves), and inhibits cell elongation, creating a shorter, stockier plant. Unlike high pressure sodium lamps, metal halide lamps also produce ultraviolet radiation, which may play a role in increasing the amount, quality, and potency of herbs or other produce produced by plants. High pressure sodium lamps may trigger a greater flowering response in plants and are thus used for the for the second (or reproductive) phase of growth, or they are used by those people who only wish to purchase 1 single lamp. [ [ Growing Guide Part 2 - Grow lamps] ] If high pressure sodium lamps are used for the vegetative phase, plants will usually grow slightly more quickly, but will also have longer internodes, and may be taller. [ [ Lights - High Pressure Sodium AND Metal Halide] ]

Incandescent grow lights

Incandescent grow lights have a red-yellowish tone and low color temperature (approx. 2700°K). They are solely used to highlight indoor plant groupings and not as a true plant 'growing' light (although they may be labeled as such). Incandescent growing lamps have an average life span of 750 hours. In addition, they are less energy efficient than fluorescent or high-intensity discharge lamps, converting much of the electricity consumed into heat (rather than light). As such, a horticultural facility utilizing such sources of illumination may attract undue or unwanted attention from various pests, including insects or other other forms of wildlife, who can see into the infrared, just as many types of cameras and sensors can, in the present era.

Fluorescent grow lights

Today, (HID) lights are available in any desired color temperature in the range from 2800°K to 6000°K. Standard fluorescents are usually used for growing vegetables (as leaf lettuce, spinach, and herbs) or for starting seedlings to get a jump start on spring plantings. Standard fluorescents produce twice as many lumens per watt of energy consumed (compared to incandescents) and have an average usable life span of up to 20,000 hours. This is 25 times as long as an incandescent. Cool white fluorescent lamps are sometimes used as grow lamps. These offer slightly lower performance, a white light, and lower purchase cost.

High Output fluorescents produce twice as much light as standard fluorescent lamps. A HO fluorescent fixture has a very thin profile, making it extremely useful in vertically limited areas. High Output fluorescents produce about 5,000 lumens per 54 watt bulb and are available in warm (3000°K) and cool (6500°K) versions. Usable life span for High Output fluorescent lamps is about 10,000 hours.

Compact fluorescents are smaller versions of fluorescent lamps and are used for propagation, as well as for growing larger plants. Compact fluorescents work in specially designed reflectors that efficiently direct light to plants, much like the HID lamps below. Compact fluorescent bulbs are also available in warm (3000K) and cool (6500K) versions. Usable life span for compact fluorescent grow lamps is about 10,000 hours.

High Output Fluorescent/HID hybrids combine cool burning with the penetration of high intensity discharge technology. The primary advantages to these fixtures is their blend of light colors and broad even coverage. In addition, electricity costs are substantially reduced (compared to incandescent lighting).

Metal-Halide (MH) HID grow lamps

Metal halide HID lamps deliver a white light in the range of 2700 to 5500 Kelvin with a color rendering index of 65-75. As such, plants growing under most metal halide lamps appear almost identical to those growing outside. They are used where color rendering is important such as stadiums, malls, manufacturing plants, and supermarkets. Metal halide lamps are five times more efficient than incandescent lamps and last up to 25 times longer, with a usable lifespan between 10,000 and 20,000 hours, depending on the wattage. Horticultural metal halide lamps have an enhanced red spectrum, which is added for increased fruiting and flowering.

High-pressure sodium lamps

High pressure sodium lamps yield yellow lighting (2200K) and have a very poor color rendering index of 22. They are used for the second (or reproductive) phase of the growth. If high pressure sodium lamps are used for the vegetative phase, plants will usually grow slightly more quickly, but will also have longer internodes, and may be taller. They are used in less color critical applications such as parking lots, street lights and for 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. Orange/red spectrum HPS is the spectrum plants use in their reproductive processes, which generally produces larger harvests of higher quality herbs, vegetables, fruits or flowers. Plants grown under high pressure sodium however yields taller and leggier plants with a longer internodal length than plants grown under the bluer light of metal halide. Also, the plants grown under these lamps do not appear very healthy (although they usually are). This is due to the poor color rendering of high pressure sodium, which makes the plants look pale, washed out or nitrogen starved. Benefits to high pressure sodium lighting are their incredibly long usable bulb life (up to two years in many cases); and unparalleled efficiency at six times more light output per watt of energy consumed than a standard incandescent grow lamp. Due to their high efficiency and the fact that plants grown in greenhouses get all the blue light they need naturally, these lamps are the preferred supplemental greenhouse lights. But, in the higher latitudes, there are periods of the year where sunlight is scarce, and additional sources of light (aside from HPS illumination) are indicated for proper growth. It also ought to be noted that HPS lamps may cause distinctive infrared and optical signatures, which can attract insects or other species of pests; these may in turn threaten the various herbs, flowers, or vegetables being grown.

Combination HPS/MH lamps

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.

witchable, convertible & two-way lamps

Switchable, Two-Way and convertible lamps are used 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 used 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. To change between the lamps, only the bulb needs changing and a switch needs to be set to the appropriate setting.

LED grow lamps

Recent advancements in LEDs have allowed for the production of relatively cheap, bright and long lasting grow lights that emit only the colors of light required for plant growth. These lights are attractive to indoor growers since they do not consume as much power, do not require ballasts, and produce a fraction of the heat of HID lamps. The lamps consist of arrays of many wide-spectrum red and a few narrow-spectrum blue LEDs of specific wavelengths. Although LED grow lights have shown promise through plant research by NASA and many universities, it is unknown whether the results are applicable to the cultivation of all plant-species, as their luminous efficiency is much lower.


Grow lights are most used for indoor gardening, plant propagation and food production, including indoor hydroponics and aquatic plants. Although most grow lights are used on a industrial level, some small-scale/domestic usage of these lamps has also been found.

According to the inverse square law, the intensity of light radiating from a point source (in this case a bulb) that reaches a surface is inversely proportional to the square of the surface's distance from the source. So if an object is twice as far away, it receives only 1/4 the light. This is a serious hurdle for indoor growers, and many techniques are employed to use light as efficiently as possible.
Reflectors are thus often used in the lamps to maximize light efficiency. Plants or lights are moved as close together as possible so that they receive equal lighting and that all light coming from the lamps wind up on the plants (rather than partly besides it). Often, the distance between lamp and plant is in the range of 24 inches (with incandescent lamps), up to 4 inches (with other lamps as compact, large and high-output fluorescent lamps). [ [ Grow lights and specing between plants] ] Some home gardeners cover the walls of their grow-room with some type of reflective material, or alternatively, white paint to maximise efficiency.

A commonly used covering is 6 millimeter (150 µm) PVC plastic sheeting that is white on one side and black on the other. The plastic is installed with the white side facing in to the room to reflect light, and the black facing the wall, to reduce fungus and mold growth. Another common covering is flat white paint, with a high titanium dioxide content to maximize reflectivity. Mylar sheeting is also sometimes used, along with Astrofoil (which also reflects heat), and Foylon (a foil-laminated, reinforced fabric).

Light requirements of plants

To determine the appropriate lighting (and the lamp to be best used), the specific needs of the plant need to be determined. To arrange optimum lighting, the lighting present in the plant's natural environment need to be imitated. Of course, the bigger the plant gets the more light it requires; if there is not enough light, a plant will not grow, regardless of other conditions. [ [ Determining appropriate lighting] ]

For example vegetables grow best in full sunlight, which means in practice that as much light as possible must be supplied to grow vegetables indoors (fluorescent lamps, or MH-lamps are thus preferred). Foliage plants (eg Philodendron) grow in full shade and can therefore grow normally with relatively little artificial light (thus for the latter, regular incandescents may already suffice).

In addition, plants also require both dark and light ("photo"-) periods. [ [ Plants requiring dark and light periods] ] As such, lights need to be timed to switch them on and off at set intervals. The optimum photo/dark-periods is specific depending on each plant (some prefer long days and short nights an others preferring the opposite, or something in between) For indoor gardening one of the most important topics is light density, measured in lux. Light density is the amount of light incident on a surface. One lux equals one lumen (unit) of light falling on an area of one square meter. A brightly lit office would be illuminated at about 400 lux. In Imperial (pounds-feet) terms, a foot-candle, or the intensity of a standard candle on an area of 1 square foot, is about 10.76 lux.


ee also

*Indoor plant cultivation

External links

* [ Choosing grow lights depending on its purpose]
* [ Grow Lights Lesson Plan]

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