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Light Shock?

Is there any research data on the impact (or not) of instant lighting of plants vs. a gradual/phased-on lighting after plants have been in the dark for x hours. Is there a shock factor to the plants?

(Think of it as someone coming into your room in the morning and flipping all the lights at once when its time for you to wake up).

Natural lighting would be a gradual phase-on of illumination, not instant (and gradual phase-off too). Does it make any difference (besides the issue of inrush current on the lighting system)? If you are taking a phased-on approach, what are the phase-on/phased-off intervals?


This is a great question, @howardboothroyd. I also would love to see some research into the effect of gradual vs instant lighting on plants. Unfortunately, to date I have seen none.

Have any @Academic, @Educators, @Lighting, @GrowOpEmployees, @growopowners or @mastergrowers seen any research into this?


Instead of “research,” how about an extensive body
of actual real-world experience?

I’ll take it. Have you got some sort of anecdotal experience to support your position? What is your thought on the topic?


I’ve tested sunrise and sunset dimming, and a few other odd lighting schemes (checkerboards on a flip box, light movers, etc) and there has been one constant finding:

Anything that reduces your DLI will reduce your yields. Any light shock they experience pales in comparison to the yield drop that will accompany a change in their PPFD, I’ve even had plants Hermie because the light was too dim or inconsistently bright.


I don’t have a position, I have plants. They always
seem to do better in direct proportion to my having better
replicated the natural environment in which they evolved. After a
while, that “seem” becomes certain. And then when fifty other
growers follow suit and have the same experience, the certainty
becomes a fact.

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I have been working on putting a dimmer on my lights using PWM for this very reason. When I mentioned it to a person at the grow store he added that a plant really doesn’t fully absorb all of the light in the first hour and last hour of the day which made sense. It was like the plant needed to wake up as well. My other motivation was to save on energy cost and to replicate ma nature. We ran into incompatibility with the PWM (voltage level to do the dimming) and our Arduino boards so we haven’t implemented it yet but it is still on the road map. I think it is worth looking into and I wanted to take it a step further and overdrive the LED at say high noon so it would have even more light for a couple hours in the middle of the day or turn on a UVB. It is a bit off topic but do people think light movers really give you up to a 30% bump like the manufacturer’s claim? Moving the light around the plant like the sun also sounds like a great idea. I do have a light mover but not sure if I can say they give that big of a bump if anything at all.


Plenty of studies on this. It is called End-of-Day lighting, or simulating twilight. Across several studies, the ratio of red (600-700nm) and far red (700-800nm) affect growth via phytochrome variations, intensity also added to this, with more elongation with less intense lighting. A high ratio of red to far red reduces the rate of stem elongation, while a low ratio produces more stem elongation. LAI is lower with the lower ratio, total dry matter is the same, but this is total DM, so likely more stem weight than leaf weight, though I don’t think this was specifically stated. The studies are not on cannabis, but several types of flowering plants with the goal to eliminate PGR use through lighting. Some plants respond to this treatment, some do not. If you’re into experimenting, use some LEDs to produce a high ratio of red to far red for tighter stacking, but HPS lights are already a high ratio of red to far red, giving the result of tighter node spacing. I don’t know about all of you, but I want to see those nodes to stack.


Light movers eliminate shaded regions caused by static lighting, more than anything. I would gamble that yield would increase, but I doubt 30%. Why do so many products state 30% yield increase? I see that number a lot. :joy:


I have talked to a PhD Plant Physiologists about this and his answer is: NO. While firing up HPS at full power unleashes all of the Infrared heat from the fixture immediately and causes a humidity spike propelling pest and pathogens, there is no difference with a not heat producing source like LED. HID fixtures also have to fire at full power in order to heat the plasma without degrading light.

Look into ETR (Electron Transport Rates) and NPQ (Non Photo chemical quenching) in literature. Plants have a mechanism to direct excess light into heat if they are not ready for it. In most quick growing annuals, they are almost always able to handle the amounts of light we can throw at them ( since it is still a fraction of what sunlight can be).