The green light myth: It's not 'safe' at night

I always assumed green light wasn’t a ‘safe’ wavelength range to use at night during the flowering phase because it could affect the photoperiodic response and flower development of cannabis. In other words, green light at night could produce adverse effects on flowering cannabis. My thought process was it’s best to get as far from far-red light as possible. And now Dr. Bugbee’s team has proven so, along with other super exciting findings regarding the dark period threshold for PPFD (0.01!) and the effect of air temperature relative to PPFD at night.

We all know red, and far-red wavelengths control photoperiodism in cannabis (except autos for the most part). That far-red light is terrible at night because it prevents the inactivation of active phytochrome, inhibiting the flowering response and causing other issues.

But now, we also know that green light at night has adverse effects on flowering cannabis at night. At a minimum, it slows the flower growth and reduces flower diameter as a fuction of time.

I use Apogee’s light pollution quantum sensor to test my grow room light traps, and my grow rooms during the lights-off period. Quantum Light Pollution Sensors | Apogee Instruments


It’s worth noting that recently researchers found:

“that the photoperiod signal induces inflorescence development within the first five days of short day exposure”

New insights on flowering of Cannabis sativa

Another good and relevant study that is fairly recent:

New insights on the flowering of cannabis sativa.pdf (464.6 KB)


I have been using UV and DeepRed/FR (Emerson effect) for a few years now with great results. I’m not sure I would trust blue or green in lights out, I don’t trust any lights after dark except a little FR. THC-A actually shifts UV light into the usable 410+nm range(blue).

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LOL, too late! (j/k)

Bugbee points out that we need a lower intensity of green light to see than blue light because our eyes use green light more efficiently. So by using blue light, we would need higher PPFD, which could be worse than green light at lower PPFD.

The point of my post is to share this great video and point out we shouldn’t leave green lights running all night. It’s probably wise to avoid using green emergency exit lighting in commercial grows. And make sure the emergency exit signs are far enough away from the plants, so the PPFD is less than around 0.015-0.02 at night. The study he’s discussing used red LEDs to determine the dark period PPFD threshold for flowering. So if we use green or blue light, the threshold is probably greater than 0.01 PPFD. It’s probably no big deal to keep using green head lamps if we have to work on the plants for short periods because green light makes it easier to see at night.


this is the way

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Green light as far as I know is this penetration spectrum that carries through leaves and develops tissues deep within the canopy. It is only neglected by grow light manufacturers due to the expense of producing green light versus red and blue. I have always seen light leak effects in rooms that I entered with green light during lights off on sensitive genetics. Some genetics are totally impervious to this “light leak”

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