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Lighting for plant growth

This article looks at lighting for plant growth with special emphasis on lighting for fish tanks.

Light is essential for plants as it provides the source of energy used for photosynthesis. Photosynthesis is the process by which plants make their food. If plants cannot make enough food, they will not survive, it’s as simple as that. However, just ‘providing light’ is not sufficient. Plant growth is influenced by light quality, intensity and duration.

Chlorophyll

Plants harness the energy provided by light in chloroplasts. Chloroplasts contain chlorophyll which absorbs light. However, not all wavelengths of light are absorbed efficiently. Chlorophyll/light absorption curves indicate the efficiency of absorption and by looking at the curves you can see that red and blue light are more effective than green light. If you think about it, that is completely logical as plants do not use all of the green light. Instead they reflect it, which is the reason plants look green.

Electric light and wavelength (colour)

The wavelength or colour produced by light is controlled by many factors. For example, a tungsten filament in an incandescent bulb sends out more light with long wavelengths (the red colours) when it is cool and more short wavelengths (the blue colours) when it is very warm/hot.

Other sources emit light in other ways. For example, metallic vapor lamps are considered most efficient for high intensity lighting on larger areas. The common fluorescent tubes are considered the most cost effective and easy way of lighting a  limited areas.

How do fluorescent tubes work?

Fluorescent tubes work like this. When electricity is supplied to the tube, special phosphors become excited and give off energy in certain wavelengths. The interesting thing is that is possible to design fluorescent tubes to send out primarily red, green or yellow. In fact they can be designed to emit any colour you like. The benefit of this property is that they can be designed with plants in mind, since plants do not use all wavelengths with equal efficiency.

In response to this, fluorescent tubes have been designed which send out proportionately more blue and red light. This means that the green-yellow-orange fraction has been reduced. The loss green-yellow-orange, even though it is slightly efficient, is not compensated enough by the blue-reds and increased plant growth is not possible.

Maximum growth using fluorescent lights

Research has shown that that maximum growth of most plants is better achieved with cool white fluorescent lights rather than the blue-red phosphors. Experts say that there is no advantage in using blue-red fluorescent tubes except to make the tank look good. Examples of such tubes on sale are Grolux, Plant-Gro, Plant Light, Vita Light and Optima. These tubes cost more and may make you tank look better for aesthetic reason, but the extra cost cannot be justified in terms of plant growth.

The benefits of light intensity

The intensity of the light is another factor that affects plant growth. The greater the intensity of light, the more light the plant received, therefore the more efficient the plant can be in energy production. To understand this concept better, think about where plants grow in nature. Most grow best in positions of high sunlight and will do their best to find such spots.  Most flowering plants need a high light intensity of 25 to 30 lamp watts per square foot. By comparison, a standard fluorescent tube emits 10 lamp watts per foot of length.

Photoperiodic plants

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