28 hour photoperiod

Im running an experiment. Im flowering my girls with a 28 hour photoperiod. Im currently running a 16/12 cycle. I had 1 of three expectations.

  1. It would shorten the flower period due to receiving 96 hours of light per week, rather than the 84 hours per week that a 12/12 offers. Thereby gaining one day per week of daylight.

  2. It would have the same flowering period, yet yield more flower due to the extra week and a half of extra daylight that a 12/12 would offer.

  3. It would actually extend the flowering time, and hopefully produce 33% more bud due to all of the excess of daylight.

I am currently on week 8, and its looking like the 3rd possibility is the one that is occuring. The buds are getting pretty big (especially for the strains), but almost all of the pistils are still snow white. These are clones, so I know where they should be by now. 2 strains finished in 9 weeks last time. The other is a Haze that i had to pull early because its health was deteriorating. However last time, they were on a 12/12 schedule. Based on this information, what is everyone’s predictions on the outcome?


Good luck with the experiment. Have you done any research as to whether this type experiment has already been done in a controlled laboratory environment?


I imagine the plant will never go into flower as the days will remain long enough that it won’t trigger a plant to produce buds.

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“Long-night” plants would have been a better label than “short-day” plants, since the length of darkness, not the length of light, is the stimulus. - Dr. Ernest Small, Cannabis: A Complete Guide pg. 67

In scientific literature the hours of light is called the photoperiod and the hours of darkness is called the critical photoperiod.

My guess is the only measurable difference will be in the electric bill :slight_smile:


I went directly into 16/12. It took 2 weeks for flowering to start. It was about the same as 12/12. I already knew that 16/12 worked. I did it in my first grow 20 years ago. Flowering isnt the issue. Its almost 8 weeks since the flip. Thos is probay my last grow for awhile, so i had to make it count. These things had to veg forever because i had nowhere for them, so they stunted really bad at first. I think the poor things vegged for 3 months before i could get them into a tent.

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Ill take a picture tonight when they wake up. These are clones, so i have an idea of how well these did before, but it is different growing conditions from the prior grow. I have 2 grape apes, 3 bay 11, and 2 pineapple haze. Its way overcrowded.

Cannabis plants, if exposed to more light than they need, will not utilize it for photosynthesis. If you have adequate PAR values for your lights, the light delivered after a certain point is rejected.

I find it surprising that your plants are flowering under a 16 hour ‘day’.


Oh they are flowering.


If we are talking about starting the count at the occurrence of real flowers, these are just over 5 actual weeks old. If we take that same standard, and apply it to “days” (one-28 hour cycle) then these are just over 4 weeks old. As far as the number of hours of light they’re almost 6 weeks old.

Now we know that 16/12 works as to being able to put them into flower, now we just need to see where they finish. Will it be a week and a half early? Right on time? Or will it be a week and a half late? This will determine whether it takes a certain number of “days” of flower, or if it takes a certain number of hours of light, or if it just takes the same amount of actual time. I think its a worthwhile experiment.

Its adding 12 hours a week of light. 600 watt HPS would use about 7 Kwhrs a week. At 14 cents a Kwhr thats about a dollar a week.

HOWEVER! Since my lights are peak hours half the time, and off peak the other half, its about even, so probably no extra cost for electricity, unless the alternative is running 12 on at night. Then there would be a difference.

Sure. I was just joking around about the bill. I get that at the scale we operate at even 24 hours of light won’t break the bank.

I’m interested to see what you learn. Did you do the first grow on 12/12 with the intention to compare it with the 16/12?

If it would flower a plant was never in doubt. What I doubt is that it will make a significant difference or offer any important improvements in the outcome. A few days here or there for when to harvest isn’t a big deal to me as a home grower.

I did grow the last time at 12/12. The growing conditions were not identicle however, because i did not intend to do this. I think there may be plenty to gleen from this though. There is really only one thing that makes a big difference to any of us (2 things kind of) quality, and quantity. The 2 grape apes seemed to be different from each other (2 different mothers). They have the same terp distinctions, however one grew larger, but airy buds. The other grew smaller more dense buds. The bay 11’s were both similar. Im not sure how concerned i am with the pineapple haze. I guess the question i would be wondering right now is where do they look like they are? 4 weeks, 5 weeks, or 6 weeks? Compared to the last grow, id have to say it does look to be about 30% heavier. Trichomes is the next question of yield. Does the extra light make the plant produce more protection from the light in the form of trichomes?

One thing to realize is that the flowers themselves are still tiny. They have not started to swell yet by much measure. The one that is in my hand, is already almost the size of a 24 ounce can. I can say that that is much better than the last grow already. Plus im not the greatest grower. This is walmart soil, with a Scotts bloom enhancer, and a fermentation bucket for CO2, and an aquarium ph tester. I truly believe that a grower with some real experience, could really do something with this. Im just trying to not only show its possible, but it could have some real benefits.

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This was my bay 11 2 weeks before chop last time. I probably harvested a bit early but i can see my buds are much bigger this time around. But the age is close. I doubt i will be chopping in 2 weeks from now.

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Please look at the thread. They are way into bud. Ive got probably 40 buds that are over a foot long.

I did look. Very interesting and can’t wait to hear your final results

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Interesting experiment @Labweed. It looks like it’s working as it should. As @poly pointed out, as long as they get 12 hours of dark, they will flower, so a 16/12 cycle will achieve that.
But as @Myfriendis410 also points out, once you hit the maximum DLI, any excess is wasted. Do you know what your light numbers are? PAR, PPFD, DLI? Are you hitting those magic numbers? You have one (??) HPS 600w bulb? How big is your tent? Looks 8x4 or 10x5, or is it 10x10? Maybe these buds ARE bigger than previous grows under 12/12 because they weren’t getting enough light under 12/12. Which I don’t have an issue with, it would prove that folks with poor lighting (I’m NOT saying yours is!) can still get good buds by extending the hours (and therefore DLI), instead of buying additional lighting (which some growers can’t afford. Reality sux at times!). I’m wondering that IF you had sufficient light under 12/12, would it improve budding under 16/12?
Also, I think that would be a real PITA changing MY schedule to water/feed them, with their mornings shifting 4 hours later every one of our days!
But cool experiment, all the power to you! Very interested in final results!


I would like to see this experiment done with clones with one room 12/12 and the other 16/12. I can achieve DLI in 12 hours of light so not sure what difference it’d make. Maybe if I didn’t have the light power to achieve DLI this would be beneficial.


I was running 1000 HPS in 12/12. It was too much. I couldnt move the light high enough, so this time i decided to put it on the 600 setting. DLI doesnt really mean thats all the light it can handle, and its done for the day. Otherwise you couldnt veg it 24 hrs a day. DLI is a preference of the plant, not a maximum. I wish i had a well spread LED light instead of HPS.

You definitely can veg it 24 hours a day. Doesn’t mean the plant is going to process light 24 hours a day. They will come to a point where they’re done and will go to sleep even with lights on. Typical identifier is the droop.

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