The Light Cycle Debate

[DrPhoton]

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[Obi-Wan]

Another great article!! During my limited time growing indoors I have experimented with what I consider to be the 3 most common vegetative light cycles (18/6,20/4,24/0) for photoperiod plants and my experience has been on par with what you have written here. What I have found is that plants in my garden require 675-700 kWh of light to reach the growth that I desire before the flowering cycle. I have been able to achieve this growth in 28-30 days with a 24/0 light cycle, 34-38 days with a 20/4 light cycle, and 37-42 days with a 18/6 light cycle. In all 3 cases the difference in kWh used is marginal when taken in the context of the entire grow but growing a receptive plant under 24/0 vs.18/6 could save as much as 14 days of vegetative time . I personally want to make the switch to 12/12 as quickly as possible with photoperiod plants because the longer light cycle of the vegetative stage is the most costly. Having said that, I don't think you can simply compare the cost of light cycles when addressing Auto- flowers due to their need for increased light in order to maximize yield. While I agree that the increased yield of a 24/0 light cycle for an Auto-flower comes at the expense of increased cost, if compared to a photoperiod grow of the same yield the cost increase is almost negligible due to their overall rapid maturation rates. I can have a photoperiod plant in the vegetative stage for 28 days @ 24/0(672kWh) and flowering cycle for 70 days @ 12/12(840kWh) for a total of 1512 kWh used while an Auto-flower that matures in 60 days under 24/0 uses a total of 1440 kWh.

[DrPhoton]

Obi-Wan could you reread this article for me ?, i would much appreciate your thoughts whether the shared conclusion of ours is made clearly enough.

[Obi-Wan]

I cannot speak for everyone else but after reading it this time I am confident that we share the same conclusion.👍🏻

[DrPhoton]

Thank you much Obi Wan.

[Weed Pharma]

Some great information here- While I haven't looked at the light cycle debate for a very long time, I recall a couple of caveats which fly in the face of common "internet wisdom".

Anything over 17.5 hrs light/day increases cost vs the amount of product yield in photoperiod cannabis. (zero sum gain)
Vegging photoperiod cannabis beyond 5 weeks is the threshold for reduced efficiency.

Adjusting the light schedule for autos, changes the amount of time to mature- less light requires a longer crop time.

That said: There are reasons to alter light cycle for the overall grow.
For example, I increase light energy (watts or duration) to heat the tent during the winter months and decrease during the summer.
I have added auto's to a photoperiod grow to fill the tent.
Plant number restrictions.

Just an FYI- I have adopted the following light schedule for autos- assuming temperature does not need adjusting, etc

24/0 sprout until plant reaches 4-5th node.
20/4 vegetative/flower until the last 2 weeks.
24/0 to finish the plant through harvest.

[DrPhoton]

Yes i feel plants start providing diminishing returns with more than 18 hours. Its not worth the extra power for the little that you get in return.

[DrPhoton]

Improved readability and added further information.

[Redwasp]

Dangerdan what is your background? I just ask because i have always loved biology and took as much as I could in college. This just reminds me of those days. I love the technical side to your posts.

And Obi wan i keep up with alot of different aspects of my grows but never even contemplated keeping up with kwh used vs time vs yields.

Ive said it before but my family makes fun of me calling me the "Sheldon" of bud. Being Sheldon from Big Bang Theory.

[DrPhoton]

Electrician. I have read several books from several fields of science. Including electrical engineering, human anatomy and physiology, plant biology etc.

[Redwasp]

Very nice.

[whitebeard]

Doc, I can’t seem to find these articles, if they on your FB, drop me a clue where to look, please.

WRTYC on light-cycle controversy, I’m sure this is an easy one:
What’s the effect of inverting the day/night cycle?
Specifically going from 18/6 to 6/18 light/dark respectively for a case in point.

UPDATE - never mind, Doc, I found your wordpress site. Thanks!

[DrPhoton]

Other than inducing flowering ?. Just a linear reduction in biomass. There has been advanced research on the effects of pulsed lighting compared to continous lighting. Where the photon requirement can be found and used to asses the light utilization efficiency between different lighting protocols.

What this research has shown is that, changing the duty cycle (the on off rate) has no effect on the photosynthetic efficiency until the cycle goes below 2us. Where the pulses of light energy are too short to satisfy the electron transport in photosystems. Requiring excess energy in the dark cycle and reducing its efficiency. So the reduction of total light recieved over 24 hrs will reduce the biomass of the plant at a linear rate with respect with this.

So we can say with great certaintly that changing the duty cycle has no impact with respect to any change of photosynthetic efficiency. For example, going from 18/6 to 6/2.

However when you go from 18/6 to 6/18, you have a different set of questions. As you are no longer changing the duty cycle but the duty ratio. Now the change of the duty ratio has no effect up to 18/6. But when you increase the ratio of light to dark past this point, there can be negative effects which is created from this shift. The positive shift past 18 hours is more than unatural for plants, which many species react negatively to this positivie shift. Which is likely due to genetic expressions and or diurnal rythrms.

For example tomatoes that are grown with more than 18hrs of duty ratio, show reduced photosynthetic efficiency and chlorosis. The exact reasons for this is unclear, but current evidence suggests that photosynthate productions (source) could be exceeding photosynthate utilization (sink).

The conclusion of this is that, continous lighting can be beneficial for plants that are tolerant and do not show signs of leaf injury and can utilize this excess energy. Cannabis may be apart of this group but the possible benefits are outweighed by the possible negative benefits. Although using continous lighting may increase yield, it also uses more light energy input per unit of biomass produced. So there is no added benefit with continous lighting when talking about overall efficiency. Which is why i always recommend staying with 18/6 light cycles. The risk of reduced efficiency for said reasons when using higher duty ratios, is not worth it.

[whitebeard]

Thank you again for a fulsome reply!

let me stay with my tiny topic for amoment more, please. I want to make sure I understand: I am speaking only of photoperiod-reactive strains.

On second thought, I suppose my question to you was too vague, maybe too personal. I understand we cannot give plants 24/7 of high intensity light for 8 weeks, then plunge them into total darkness for 9 weeks and open the doors to perfect buds. I know that if we give them 12/12 from the beginning, then they will grow, then flower.
I also know that if we give them as much as 6 hours more light, they will benefit well from it.

I am curious as to whether there is a reverse phenomenon: whether experimentation in moving from 12/12 to 11/13, 10/14, 8/16, etc has been shown to change - damage or enhance - the maturation processes of flowering (and if so, what changes were observed). The question might be novel, it might be meaningless; however I’ve run into a few who swear by giving their plants anywhere from 24-36 hours of total darkness, then cutting in at 12/12. The experiment as I see it is straightforward, and I will certainly take it up as opportunity allows. It should seek to mimic a late-fall dark/light balance, to see if cannabis has any undiscovered tricks up her genes for this scenario.

not to be a drag on your thread, will wait your response

[DrPhoton]

No your questions are not at all personal, i always welcome a healthy discussion. I just thought you were referring to vegetative growth.

I assume you are talking about the effects of negative duty ratio light cycles during flowering. I did spend a bit of time on this. A negative duty ratio during flowering has been shown to reduce the flowering time and finish sooner. But at a cost to yields due to less overall input energy. Many many people who have tried this have found that this reduction in flowering time is very small, around a week at most. But longer flowering strains like white widow, may show a more significant difference.

The reason for this reduction of flowering time, is likely due to genetic expressions that trigger the plant to finish sooner as the lower duty ratio suggests that winter is coming sooner.

What you may not be aware of, is that you can actually increase the duty ratio from 12/12 to 14/10 by using far red light manipulation techniques. This is a process which activates the photoreceptors eariler, saving roughly 2 hours of required dark cycle to initiate flowering. But just like shifting the duty ratio negatively has an effect on flowering time, so does positive shifting. But instead it delays the flowering process. Making the overall grow longer. But again this does not change the over efficiency of the grow, as every additional gain in biomass is a result of added input energy.