Grow Lights

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Light is necessary for photosynthesis, the process by which plants make the food they need to grow and flourish. This light usually comes from the sun. Sometimes supplemental light is required, for instance, in winter and early spring when there are limited hours of daylight.  You may also need to consider supplemental light when starting plans off, or growing them on, indoors.  Using grow lights to extend the amount of time the plants receive light may boost growth.   A grow light is an artificial light source, generally an electric light, designed to stimulate plant growth by emitting light suitable for photosynthesis. Grow lights are used on an industrial level for indoor gardening, plant propagation and food production.  But they can also be used by amateur gardeners

Grow lights either attempt to provide a light spectrum similar to natural light from the sun, or to provide a spectrum that is specifically tailored to the needs/growth stage of the plants being cultivated.  The colour and intensity of natural light can vary and plants have varying light preferences.  Different types of grow lights can be used to mimic the variety in natural light and to satisfy the various needs of different types of plants or plants at different stages.  .

Type of Bulbs.

A range of bulb types can be used as grow lights. The most widely used lights for professional use are HIDs (High-intensity discharge lights),  fluorescents and LEDs

Bulbs that emit a bluish light (metal halide lights) are used for the first (or vegetative) phase of growth.  Blue spectrum light may trigger a greater vegetative response in plants.

Bulbs that emit a reddish light are (high pressure sodium lights) are used for the second (flowering or reproductive) phase of growth.  Red spectrum light may trigger a greater flowering/fruiting response in plants. If high pressure sodium lights are used for the vegetative phase, plants grow slightly more quickly, but may also become “Leggy” and weak.


Fluorescent lights are available in any desired color temperature in the range from 2700 K to 7800 K. Standard fluorescents are usually for starting seedlings to get a jump start on spring plantings and for growing vegetables and herbs indoors.  Cool white fluorescent lights are sometimes used as grow lights.

High-output fluorescent lights produce twice as much light as standard fluorescent lights. A HO fluorescent fixture has a very thin profile, making it extremely useful in vertically limited areas.

Compact Fluorescent lights are smaller versions of fluorescent lights and are available in cool/blue (6500 K) , warm/red (2700 K), and full spectrum/daylight (5000 K) versions.

High-pressure sodium (HPS)

High-pressure sodium lights enhance the fruiting and flowering process in plants. They are used for the second (or reproductive) phase of the growth. If high-pressure sodium lights are used for the vegetative phase, plants tend to be taller and “leggier. Sometimes the plants grown under these lights do not appear healthy due to the poor color rendering of high-pressure sodium, which makes the plants look pale, washed out or nitrogen starved.

High-pressure sodium lights have a long usable bulb life and six times more light output per watt of energy consumed than a standard incandescent grow light. Due to their high efficiency and the fact that plants grown in greenhouses get all the blue light they need naturally, these lights 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 are indicated for proper growth. HPS lights may cause distinctive infrared and optical signatures, which can attract insects or other species of pests; these may in turn threaten the plants being grown. High-pressure sodium lights emit a lot of heat, which can cause leggier growth, although this can be controlled by using special air-cooled bulb reflectors or enclosures.


Recent advancements in LEDs allow production of relatively inexpensive, bright, and long-lasting grow lights. Compared to other types of grow lights, LEDs are attractive to indoor growers since they consume much less electrical power, do not require ballasts, and produce considerably less heat. This allows LEDs to be placed closer to the plant canopy than other lights. Also, plants transpire less, as a result of the reduction in heat, and thus the time between watering cycles is longer.

Distance from Plants

The intensity of light from a bulb that reaches a surface decreases with distance. So, If your seed tray is twice as far away, it receives only a quarter the light. Grow light are often kept quite close to the plants and reflectors maybe used with grow-lights to focus the light where it is needed and to  maximize light efficiency. Plants may be grouped close together so that they receive equal lighting and that all light coming from the lights falls on the plants rather than on the surrounding area.  The distance from the bulb to the plant may vary according to the type of bulb used. Often it is as far as 60 cm (24 in) (with incandescent lights) but may be a close as up to 10 cm (4 in) with other lights (such as high-output fluorescent lights).

Light Requirements of Plants

Remember that while all plants require light to grow, some plants need less light than others. Natural shade lovers will grow flourish with lower light levels. For optimum growth artificial light must mimic the natural light to which the plant is best adapted. The bigger the plant gets the more light it requires.

In addition, plants also require both dark and light periods. So if you decide to use grow light , they should not be left on all the time.  The optimum light/dark period ratio depends on the species and variety of plant.  Some prefer long days and short nights and others prefer the opposite or intermediate “day lengths”.

If you use or experiment with grow lights, please report your result back to the gardening club.

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