Grow Lights for Plants

Plants require red and blue light to produce sugar using photosynthesis. Historically, humans have grown plants outdoors where abundant sunlight is available. Indoor gardening requires a different approach since sufficient natural light is usually lacking.

Plants perform best in an environment with a combination of red and blue light of sufficient intensity. Incandescent lamps (as in the traditional Edison light bulb) are good for providing light for everyday activities, but are inappropriate for driving photosynthesis, since they have little output in the blue range and much of their energy is converted to heat.

Standard fluorescent tubes are adequate for some plant growing applications but specially designed plant grow lights produce high outputs in the wavelengths required for maximum biological and energy efficiency.

High Pressure Sodium Lights

High pressure sodium (HPS) lights produce an intense illumination and are slightly biased toward the red end of the electromagnetic spectrum. This makes them ideal for plants that require a red shift to promote flowering. Sodium lamps may be employed singly, or in combination with metal halide lamps if a more balanced color spectrum is needed.

Like most conventional style bulbs, HPS bulbs emit light in all directions, so as much light as possible is directed toward the plants by using a reflecting enclosure over the bulb. Sodium lamps get hot so they cannot be placed too close to plants. They also emit ultraviolet light, so adequate safety precautions (UV absorbing glasses and skin protection) should be observed when working around them.

In terms of the amount of photosynthetically active radiation (PAR) produced per watt of electrical energy converted, HPS lamps are the most efficient type of plant grow light. For this reason they are a favorite of large scale commercial growers.

Metal Halide Lights

Metal halide lamps (MH) offer a high intensity light source biased more toward the blue end of the electromagnetic spectrum. For this reason they are excellent for promoting vegetative growth.

Similarly to HPS lights, metal halides are mounted in reflectors to direct light toward plants. They also get hot in operation, so they cannot be placed too close to plants. Once again eye protection should be worn when working around metal halides since they have a notable output in the ultraviolet range.

Fluorescent Lights

Compact fluorescent lights (CFL) are suitable for promoting plant growth in small home applications. They are energy efficient and available in many wattages, so they can be used singly or in combination to provide balanced spectrum PAR in a range of intensities. They do not generate a lot of heat so they can be located close to plant surfaces. They also have the advantage of fitting into standard screw-base light fixtures.

The T5 fluorescent tube looks similar to a conventional fluorescent tube but usually has a smaller diameter. These lights are superior to standard fluorescents in plant growth applications, since they have much higher output of PAR. Like CFLs, T5 lights run at a relatively cool temperature so they can be located fairly close to plants without risk of causing heat damage.

LED Lighting

Light emitting diodes (LEDs) are a recent addition to the grow light market. LEDs have the lowest power consumption of any grow light system but their PAR outputs are quite low. An advantage is, that since individual LEDs are small, they can be combined to construct grow lights in many configurations They also produce very little heat so LEDs can be placed very close to plants for maximum efficiency to make the best use of their lower light intensity.

Red and blue LEDs can be combined in a fixture to provide any ratio of red-to-blue wavelengths. Grow lights constructed with LEDs are still quite expensive, but the price is going down. They are durable, do not emit ultraviolet radiation, and last for up to a decade. So, despite their high initial cost, LEDs remain a viable option when selecting a plant grow light system.