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Best Lighting regimes

I have a question regarding optimum day length at an indoor grow.

Thinking of using 12 days with an additional 6 hrs of night interruption as I’ve always felt plants need some down time to conduct metabolic processes and 24 hrs of light does not allow for that.

Also, since my background is in ornamental production, I have a question about the Flowering ie. Blackout, Short days, Light dep. etc…phase. I hear people saying you need 12/12 days to flower. However, in my experience with Chrysanthemums which are a light sensitive crop, transition form vegetative to reproductive growth occurs when the day length is less than 12 hr…

Therefore we will give the plants 13 hr of darkness. This helps ensure that all the plants will transition at the same time and initiate flowering uniformly .

I would appreciate some advice on this in case I’m off base.


The 24 hour light question is a common debate in this industry, but I believe most of us who are educated on the topic would say to give 6-8 hours of dark in veg for respiration, cell elongation, etc. For flowering - 12 hours of an uninterrupted dark period has been fine for every strain I’ve grown or heard of. You’ll miss out on some valuable photosynthesis if you go any longer.



Yes you’re correct to note that plants need a minimum of 3 hours of darkness, and as much as 6 hours for veg. As for your photoperiod, yes a longer night will help uniform flower initiation like chrysanthemums. This may come at a cost to production, as that 1 hour is 8% of your lighting time, still, a stronger signal could result in more flowering.

Any questions about lighting feel free to reach out to us at LumiGrow, we work with growers experienced in all types of crop production.



Thanks for sharing @AmericanWeedwolf!

What about autoflowering strains? Is the darkness period still important for them? I’ve read some growers will keep the lights on 24/7 throughout the entire growing cycle to try and make best use of the limited lifespan, but I’m skeptical.

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Here is a response from one of my friends Jordan who runs an autoflowering forum:

The three most commonly used and accepted light cycles for autoflowers are 18/6, 20/4, and 24/0.

True autoflowers by nature will flower out on any light cycle but there are points of diminishing returns by giving too much light (meaning it’s simply not being utilized efficiently or provides diminished gains,) and not enough light means that while the buds will still grow and put on bulk, they will not grow to their optimum.

Out of those three cycles listed, two cycles are used more often than not for autoflowers and those are 18/6 and 20/4.

We actually ran a poll since 2011 on the AFN forum asking growers what they prefer for lighting cycle with over 900 members giving feedback on it:

24/0 (lights on/off) 163 vote(s) 17.3%
20/4 (lights on/off) 424 vote(s) 45.0%
18//6 (lights on/off)) 345 vote(s) 36.6%
Other - please post an answer 49 vote(s) 5.2%

So if you were judging that based on community response alone, most growers typically choose 18/6 and 20/4. On top of the power savings, there is also some stretch that happens during lights out that appeals to some growers. 24/0 lighting with most lights typically will produce slightly shorter, more squat autoflower plants with tighter nodal spacing (attributed to lack of stretch.)

Personally I have grown 24/0 for YEARS and have grown hundreds of autoflowers this way. There is a healthy debate on whether autoflowers need rest or not, I firmly believe there is enough documented proof to say “no they don’t,” but I also think that there is a point of diminished return with more light and it doesn’t necessarily equate to larger plants or bigger buds, more potency, etc. You also run into the potential of some autoflower strains that may not have been worked to fully autoflower (breeder needs backcross a few times,) or just a freak phenotype that has photosensitive leaning characteristics. In nearly every case of seeing this online, the solution is to reduce the amount of light hours to the point of what would be more akin to photoperiod flowering.

Long story short, autoflowers have proven to be able to be grown successfully and bountifully on 24/0 lighting. However, this is no concrete proof that more light hours equates to larger harvest or better growth and some of the larger plants recorded that have been grown have used the more traditional lighting style. Ultimately it is down to grower preference, experience, and way the plants react to the lighting environment and from a traditional sense the documented and proven 18/6 or 20/4 will give just as much optimal result as 24/0 lighting when it comes to autoflowering cannabis in most scenarios.

I hope that helps, happy growing!

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Awesome. Thanks for the detailed response. My gut tells me there should be some benefit to having some lights out time. And if there is no noticable difference when leaving on 24/7, then I’ll probably opt for a 18/6 schedule. That’s what I’m currently using to veg my photoperiod plant.


I did not see the responses until now, but I feel it is important to continue this debate for the betterment of the community. I am simply trying to keep the conversation going and educate, not claim that I know better. I encourage anyone to continue the conversation if there is evidence to support a different theory.

AmericanWeedwolf, you do make a good point that there is a cost savings when turning out your lights.

AND, I definitely agree that Cannabis does not require a dark period to live. However, the cost savings you mention are a prime example of why 24-hour light is detrimental to your grow. The savings seen by turning out the lights, assume no further yield by keeping them on. It at least assumes that yield/cost of lighting diminishes over time. Otherwise, it would certainly benefit the business to keep the lights on. To me this indicates an obvious limitation in the cellular processes when the lights are left on 24/7.

This limitation becomes clearer when you consider the pathway that the Calvin Cycle follows during photorespiration vs nonphoto(dark)respiration. I can’t explain it any better than this article…

All that said, my argument assumes Cannabis is a C3 plant. If it is a C4 plant, as some claim, there is a lot more to consider. However, I think the savings seen by turning your lights off for a few hours, coupled with the preference for high humidity, would indicate to me that Cannabis is in fact a C3 plant…

Generally, hot/dry conditions cause stomata to close. In C3 plants this causes CO2 levels to diminish, which increases photorespiration and decreases the plants efficiency in producing AND using sugars. C4 plants on the other hand separate the processes in space, preventing excess photorespiration even when the stomata are closed.

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If you want to calculate the heating load of any fixture, regarding of the light you use, this light is going to be absorbed by the plant, walls, ceiling or any object that touches. When the light touches this objects turns into heat. So you need to consider the heat per watt. I found the following formulas that may be useful for your cannabis operation. So for example the ECLIPSE LED has 65 WATTS you multiply that for the constant 3.41 to give you 222 BTU’s.

The same math works for any lighting fixture.

1 watt at luminaire = 3.412 BTUs per hour
1 watt at luminaire = 0.4 watts of air conditioning
1 watt at luminaire = 0.0015 pounds (0.00068kg) of CO2 per hour


Good topic to bump, and I appreciate your objectivity! I have a few thoughts on the matter and a question of my own.

I have seen a pretty convincing research paper about 24 hrs of light vs a 4 to 6 hour “resting” period (sorry I did not save the link, but the research was as credible as you could hope for). They saw a significant increase in dry mass with the 24 hrs of light over the course of weeks. (Whether or not there are diminishing returns per watt of light beyond a certain point is a really interesting question.) This makes sense physiologically via how we currently understand photosynthesis to function - purely based on inputs being present for the reaction to take place (mainly light and CO2); these reactions have been shown to take place in the light, no matter how long it’s been on. As mentioned before, if your plants continue to grow without damage, that’s a good indicator that darkness is unnecessary. And I haven’t seen any scientific data to support the “needs rest” theory, just a lot of people with feelings… so please post something if you’ve seen anything. I’d love to read it!

My other thought on the subject is that if it indeed made a significant difference in dry mass, more people would have noticed and would be doing it… I think that it might be a good idea to run lights 24/7 if you are limited in veg space/time and need maximum growth per day. On the other hand, I find that a few hours of darkness comes in handy for any foliar sprays (mainly oils) or sulfur burning. Do note also that various insect species need particular light requirements to complete their life cycles, which has some interesting implications.

What I think can be argued in favor of some dark time would be that it naturally helps the temperature and RH swing a little bit, which is a good thing for building a bit of tolerance in your plants to various conditions they might later encounter (on purpose or not). Good research is out there on secondary metabolite and phytochemical production in various crops in response to abiotic stress (to include temperature and RH swings). As a living soil grower, I also must take into account what the microbes might like, which is obviously hard to pinpoint. I would think more varying conditions would encourage diversity. I do feel strongly that if your lights and environment are constantly the same without much fluctuation, you’re asking for a bit of vulnerability in your crop, so that’s something to look out for.

My question is a little off topic, but fits well in this thread: Have any of you ever tried a 24 to 48 hour period of uninterrupted darkness at the beginning of flower? My thought is that it might help convert more phytochrome to its red (Pr) form and initiate a stronger hormonal response to help speed things along, (but if all the phytochrome is already converted within little more than 12 hours, we would just be losing out on photosynthesis time…) Photoperiodism is very complex with multiple steps within the process. I was mostly wondering if anyone has tried it and noted a significant difference…?

One more light question: What ppfd or ppf have you all found to work best for rooting cuttings? What about specific spectrum? Feel free to include temperatures as well!

Good stuff!

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Thanks for your input! I’m interested to hear more success stories with 24 hour light. As far as phytochrome goes, my understanding is that it is sort of like a switch (in regards to flowering in short day plants). Once the concentration of converted phytochromes reaches a certain limit, flowering is triggered. So, extra dark time will not make anything happen faster. However, one way that I’ve heard of phytochrome conversion being manipulated is by using far-red light at the beginning of the dark cycle to induce a faster conversion, thus allowing the plants to flower under 14 hours of light or more. The argument is that this allows for more time photosynthesiss and potentially greater yields.


I’m quickly learning that there is just so much I don’t know! I understand the basics of what the plants need to do well, but I have to say I have a lot of respect for everybody who has already done this and other research. It’s very cool to know that there is a science here and that science is continually being debated professionally and improved upon. I’m already having so much fun learning what I’m learning, and I’m excited to continue to learn!


While photosynthesis has both “light” and “dark” reactions, the “dark” reactions are collectively known as the Calvin Cycle. The metabolic steps of the Calvin cycle are sometimes referred to as the dark reactions, or light-independent reactions, because none of the steps requires light directly In other words, they can happen with or without light. Nevertheless, the Calvin cycle in most plants occurs during daylight, for only then can the fight reactions regenerate the NADPH and ATP spent in the reduction Of C02 to sugar. If Cannabis plants “need a minimum of 3 hours of darkness, and as much as 6 hours for veg”, then it seems the plants would die or even show stress under 24 hour light. Yet they do not.

The main benefit to cutting off the lights for a few hours for vegging plants is the savings that come in your reduced power bill, nothing more.