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Fluorescent vs LED

8 years ago
last modified: 8 years ago

This is a subject that has come up repeatedly. How do Fluorescent grow lights compare to LED grow lamps?

First, let's clear up some terminology. CFL grow lamps are basically the same thing as fluorescent (CFL stands for 'compact fluorescent lamp). They both pass an electrical discharge through low pressure mercury vapor inside the tube, causing the mercury to emit UV radiation, which then hits the phosphor coating the inside the tube. The phosphor "fluoresces" and emits light. The type of light emitted depends on what particular phosphors are used.

Both regular fluorescent lamps and regular LED lamps commonly used for lighting are NOT well suited for growing plants. You will want to get special LED or fluorescent lamps designed for growing. The reason you need to get special grow lamps is because plants need deeper red wavelengths of light to efficiently convert to energy.

Take a look at the spectral graphs of the following two fluorescent grow lamps. The first is from a commercial induction grow lamp that emits a magenta-tinged color of light:

The second is from a high-end "full spectrum" grow tube:

You will notice that it looks different from the graph of ordinary fluorescent lamps, which can be seen here:


So why are all the LED grow lamps colored?

The reason LED grow light is not white is because LEDs can provide the exact wavelengths that plants need, without having to waste energy producing the wavelengths they don't. Ever noticed how plant leaves are green? This is because the chlorophyll pigment reflects green light while absorbing red and blue wavelengths of light. The wavelength that chlorophyll absorbs most efficiently is 660-670nm wavelength deep red.

So why not only just use all 660nm red LED light?

Plants need some other wavelengths as well. Blue light triggers auxins and tells plants where they should grow towards. Plants also can benefit from a small amount of 730nm infrared light, which activates phytochrome receptors in the plant, stimulating the plant to grow longer branches. The reason plants have these phytochrome receptors is that in nature, when the leaves of other plants shade out too much light, the plant needs to react and try to grow upwards as fast as possible. The leaves of these other plants are still very transparent to 730nm infrared, so if the ratio of 730nm gets much higher than the ratio of visible wavelengths, it tells the plant that there are other plants blocking its sun. However too much 730nm infrared is not good either, because then the plant can start to get too "leggy", overly elongated trunk and branches, fewer leaves, the plant will devote all its energy to trying to grow upwards, the branches will grow out angled upwards.

In my personal opinion, the best grow light combination is CMH (6500K) supplemented by plenty of 660nm red LED light (with more wattage allocated to the LED than the CMH lamp). For those of you who have never had experience with LED before, they are so efficient you don't need much light or wattage to get the plant to grow. Also keep in mind my recommendation here may not be so economical for those of you growing on a much smaller scale.

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