By Theo Tekstra

Marketing Manager at Gavita Horticultural Lighting

The biggest response I ever got on a social media post was April 1, 2017, when I “introduced the Gavita Laser Lights” on Facebook. Obviously, this was an April Fools post, but it was incredible to see how long this thing lasted, and I still get (serious) questions and inquiries about it. 

Mixing fiction with credible data can make a compelling (false) argument, and sometimes reality is stranger than fiction. So, here is the challenge: I will write a short article every edition, and it is up to you to decide whether this is fact or fiction. Call it an exercise in critical thinking and fact-checking. 

Episode 1: Green Light


So why are plants green? This is an easy one. Your eyes don’t lie: Most plants are in fact green. They absorb mostly the red and blue light, reflecting the green light. The scientific proof which predominantly influences our ideas about plant response to different wavelengths of light was produced by Keith McCree in 1970 (published in 1972), in his famous paper “The action spectrum, absorptance and quantum yield of photosynthesis in crop plants” – leading to the definition of PAR light and the, now famous, McCree curve (fig 1).


Fig 1: McCree curve, quantum yield, original publication 1972.

The McCree curve clearly shows a dip in the spectral response around the green light, which also shows that red light is most efficient. Most horticultural lighting manufacturers nowadays acknowledge this research, and with the introduction of LEDs it is now much easier to create a spectrum which is much more like the McCree curve, providing optimal lighting for the plant at maximum efficiency. You can see this in greenhouses and indoor facilities alike all around the world nowadays: This purple kind of glow is a result of mixing only red and blue LEDs, as they are the most efficient for plant lighting (fig 2).


Fig 2: Red and blue LED lighting in a greenhouse. (Source: Colorado State University)

It is reflected (no pun intended) in the spectrum of most LED plant light products on the market today: they mostly only have predominant red and blue light. You can also see that effect back in the green LED products which are used as a safelight during the dark cycle of a generative short-day plant, without disturbing the flowering cycle. (fig 3)

LED Bulbs

Fig 3: Green light options in a grow room. (Source: MonsterGardens YouTube video)

So there you have it, both supported by science and products evolving from this science. Plants are green because green light is very inefficient for photosynthesis, and it can be used during the dark cycle as a safelight, not interfering with the flowering cycle of short day plants. 

Open for Debate! Bring your knowledge and join the conversation…

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Fake or Not?

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  1. Peter Gill

    Not fake. I’ll use a metal halide (Blue) and a High pressure sodium (red) and a green light during the lights off cycle and would love to use a ceramic metal halide for a better PAR. Not forgetting the LEP. The horticultural lighting industry has come a long way since I first started with phosphor coated 1500 watt metal Halides in the early 80s. JKP.

  2. mike

    green light will affect your dark cycle


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