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21:36/12 photoperiod


New Member
Hola peeps...Just looking for hard data or experience on this. Any one have a friend of a friend of a friend of an Uncle who grew three foot buds using this light cycle.
I'm only finding the same article (which is posted here at the overgrow faq's) on the web at a bunch of different sites.
Looking for a university study if anyone knows about one.
Two things that strike me about this...
1. If this works, (which I doubt it does because of point number 2) why isn't there more data on it, and why isn't everyone using it. Like Co2 for instance...we all know the hard data to support Co2 enrichment. It's a proven fact that it works and most people use it in grow rooms.
2. Plants will , for the most part, (excluding plants on the equator) start their flowering cycle when the daylight hours get shorter. It seems to me you are just re-vegging it for a week or two.

But hey...WTF... if it helps the plant I'd do it. Any help on this would be greatly appreciated.


Well-Known Member
Usually #/# is used to refer to how many hours in one day are used for light/dark. 18/6 and 12/12 both add up to 24 hours total in a day as they should.

There aren't 33 hours and 36 minutes in a day so I have no idea what you are trying to convey here. :confused:

Smokin Moose

Fallen Cannabis Warrior
I ran across something that is in the experimental stage if you are interested.

[DISCLAIMER]: This information is provided as experimental data and not fact.

The only photoperiod manipulation from years of experiments that offered discernible improvements was the following photoperiod adjustment made for 1 or 2 calendar weeks at the point of maximum flowering rate: Daylength of 21 hours, 36 minutes with a dark period of 12 hours. To accomplish this, you need a 7 day, 24 hour digital timer. During a 7 day calendar week on Earth, the "sun" only cycles 5 times. This permits easily switching back to the regular 12/12 at your discretion. You may want to only alter during peak flower production to stimulate the plant's metabolism. Using this photoperiod throughout the flowering cycle will cause this:

A variety that takes 49 days of 12/12 to mature, won't see 49 - 12 hour dark periods under 21:36/12 until almost 10 calendar weeks have passed.

The total increase in light energy is almost 80%, which will produce larger yields, if all of your other enviromental conditions are kept optimal.

The total increase in flowering period is only 40%, half the potential room for improvement. This means you don't have to be perfect to win out.

Selective application of the 21:36/12 photperiod for only 1 or 2 weeks extends the wait only 2 to 4 Earth days, which makes up the missing 2 complete day and night cycles each week on Planet Ito. This permits the additional light energy to be provided without purchasing additional equipment or overloading existing circuits, which maximizes the existing system's capabilities. The main advantage is that matched with co2 and optimal nutrition, the plants metabolism will increase dramatically. I have only successfully tested this photoperiod for two weeks. The potential for a net increase of 40% over the entire cycle (80% increase in light energy vs. 40% longer wait) is worthwhile. Don't be afraid!

Day 1 - Sunday, 6:00am til Monday, 3:36am
Day 2 - Monday, 3:36pm til Tuesday, 1:12pm
Day 3 - Wednesday, 1:12am til Wednesday, 10:48pm
Day 4 - Thursday, 10:48am til Friday 8:24am
Day 5 - Friday, 8:24pm til Saturday 6:00pm

Smokin Moose

Fallen Cannabis Warrior
And this from Cannabis Culture

Timing is Everything

Getting your plants to produce fine buds is a simple as night and day.

When I first started growing pot in the early 1970's, the relationship between light timing and flowering was virtually unknown by the apprentice grower. Even though High Times began publication in 1974, the concept of 'bud cycle' was not apparent until Ed Rosenthal and Mel Frank published their first works in 1976. Prior to this, many of us simply grew big plants, either outdoors or under some form of artificial light, and just consumed whatever presented itself.

Those lucky enough to be able to grow outdoors and all the way into October were blessed with some pleasant surprises. The rest of us often learned the hard way what bunkweed was - a harsh and unpleasant tasting substance that produced little high and plenty of headache. Once Mel and Ed set us straight, our product's quality began to increase.

Cannabis needs to properly mature in order to be of value. The young plants will grow with vigor, like a weed. Young cannabis will flourish practically anywhere, under almost any conditions – but it takes a special environment, and a specific set of circumstances, in order to properly mature. The key element is the timing of the light cycle. Like humans, plants have two worlds in which they exist: night and day. Day is when it is light and night is when it is dark. A thorough understanding of this simple fact is crucial to more fully understanding the nature of cannabis.

Vegetative stage

Sprouts, fresh clones and young plants live in what we refer to as the vegetative stage. This period has a long daytime and a short nighttime, like summer. It is during this vegetative stage that the plants send out much new growth. Large shade leaves form and act as sugar factories for the plant, turning sunlight into fiber for new growth. The plant needs to use as much food from the available light as it can, while it can.

This is why high nitrogen fertilizers are so beneficial during this period. The nitrogen, coupled with the extra light, acts as building blocks to the overall structure of the plant. As a general rule of thumb, in the indoor garden the average light cycle for the vegetative stage is 18 hours on and 6 hours off.

Bud cycle shift

At some point in the young plant's development it becomes time to begin the shift to what is called the flowering, or bud, cycle. It is during this period that the plants declare their sex and produce large floral clusters that become the buds. This stage has a shorter daytime and an increased nighttime, such as late summer and fall.

The large shade leaves begin to die and fall off as the plant shifts its energy from producing leaves and stem to producing floral clusters. Food from light and nitrogen decreases, and the demand for phosphorous and potassium increases to fuel the process. During the early flower stage the plant will go through what appears to be a growth spurt as the stems stretch to catch the light that, if outdoors, would be coming at a lower angle as the season progressed.

The floral clusters sprout from the areas where the leaves attach to the stems, called "nodes." The buds fill in the nodes and progress out. And, as we all know, it is strictly the female plants that develop into our high quality and most desired sensimilla. Indoors, the typical light cycle used in the bud stage is 12 hours on and 12 hours off.

Outdoors, the change in light cycle timing is gradual and slow, a little bit more every day. The transition between the plants' stages is therefore more drawn out and gradual. Indoors, the change in light cycle is usually instant: one day is 18 hours long and the next (and all those to follow) is 12 hours long. Under these indoor conditions the plant is forced to make the shift quickly, which is why the average length of the indoor flowering cycle is eight to nine weeks. This forced flowering has its advantages as the plants are made to finish up quickly, thus aiding production.

Oddly enough, plants do most of their fiber production at night, which may help to explain why such small vegetative plants are capable of producing so much bud in such a relatively short period of time. A well formed, six inch tall veggie plant (a plant in the vegetative stage) placed immediately into the bud cycle, is capable of producing an ounce or two of finished product in two short months, given adequate light, food and root space.

Nights of total darkness

An important thing to remember about the indoor bud cycle is that the dark period must be absolute and uninterrupted. The room must be thoroughly sealed to be completely dark when the lights are off. The only way to test this is to sit in the room in the dark, either during the day or with any lights outside the room on, to check for light leaks. It is advisable to allow your eyes to adjust to the darkness before declaring the room adequately sealed.

Once the bud cycle has begun it is important to never interrupt the dark period with any light, even for a short period of time. Doing so may interrupt the long, slow process of change that the plant had been working on up to that point. The plant may react by having to restart the process and seriously delay the scheduled maturation time.

I don't understand why it is that outdoor plants are not as sensitive to these nighttime interruptions. Perhaps it has to do with the unmatchable light intensity of the Sun. Stars, the moon and streetlights glowing through the low clouds over an urban area don't seem to hinder the outdoor plant all that much. For whatever reason, indoor plants tend to be ultra-sensitive to nighttime interruptions of light. So remember to make it dark and keep it dark.

Sativa variations

Another aspect to consider is that Indica and Sativa varieties differ in their photo period expression, or photoreactive rate. The typical 18/6 and 12/12 light cycles are primarily beneficial to Indica varieties. Indica became the herb of choice early on in the industry due to its fast maturation and large production abilities under the HID lights. Indica is a variety from the 30th parallel and above, and this timing cycle is more akin to locations north of the 30th latitude (or south, in the southern hemisphere).

Sativa originates from equatorial regions, between 30 degrees north and 30 degrees south. Around the equator there is a much smaller difference between seasonal day lengths. The vegetative stage may be 13 hours of day and 11 hours of night, whereas the flowering cycle may be the opposite, 11 hours of day and 13 hours of night. There are pure Sativa strains that require three to four months to mature in the flowering cycle indoors. And although outdoor equatorial crops take such a long time to mature, it is often possible in the right areas to produce two to four crops per year, thanks to the tropical environments.

It will be interesting to see what specialized grow rooms, developed to provide a wider range of photo periods, would produce. More interesting will be what the wonderful and great outdoors will produce in all of her various environments. Light fortified greenhouses are capable of producing high quality herb just about anywhere on the planet. Once implemented, the global environment will surely reveal interesting and desirable variations, via careful selective breeding. In the meantime, further experimentation and research using different indoor light timing cycles would be very worthwhile.

DJ Short


New Member
Thanks Moose...You're always loaded...With info
My first grow is going to be PPP. No one really knows where it comes from but it's from some part of S. Africa. I plan to simulate the summertime conditions of Durban S. Africa. I guess I should find out the daily sun rise and set times but in the mean time I'm going to use the 18/6 and 12/12 just to keep the record keeping simple. 18/6 and 12/12 will be the reference point in the future when I tweak the light cycle.
Besides, the only real way to grow big buds is to copy those of you who grow big buds.
After I get a couple of grows under my belt I will try 21:36/12 just to see what happens. for now K.I.S.S. totally applies to me.


Well-Known Member
I am really hoping that the world can save some regional strains in their pure form for just this reason.

The beauty of mother natures bud is what I am sure there is another name for, but I call it Environmental Genetic Traits. Traits that are specific to a plant, for example, something grew in a valley with a lot of rain, but equal sunshine; With bats flying over during season, but only when the temperature was less then 80 degrees.

If strains like this could be saved and worked with, there is no telling what we could do.

By learning how environment effects Cannabinoid production and distribution amounts-informing us on whats changes need to take place in the grow environment for certain effects in the Cannabinoid(i.e. Pain Management). In order to have a true baseline for analysis, we need the data from these pure strains to give us the tools we need to put the puzzle together.

All that said, I think it is important to remember that the closer you can get to reproducing or enhancing the environment that produced the plant and all it's effects, the closer you will get to having strong, healthy plants.

Of course in the real world, Cannabis has been cross bred so much that you can plant almost any common seed indoors with minimal support, and it will at least grow. Unfortunately this leaves us with desirable and undesirable traits, some of which can be manipulated in growth, but many that cannot.

You may be able to tell, this area fascinates me, and there is still much research to read and perform. But one thing I have learned so far is that clean genetics are very important in the race to cure and alleviate many medical conditions. So when you breed, take good notes, and label well. That special light cycle may just be the cure for a form of Cancer :)

Damn that was a great bowl this morning :-0


New Member
I'm with you Boss... I like the science that goes along with growing any organism. I got my beans from Buy Dutch Seeds...I hope the genetics are close to being pure. We shall see.

Here's what we know so far. PPP is a sativa which has long lanky leaves. It grows well in tropical environments.
So summer time temps in Durban are well over 80 degrees and RH average is around 63%. I also took the summertime temps and RH for Maui and for the coastal mountains in So Cal. where I grew up. Fallbrook to be exact. Averaged them out and POW!!!....77 degrees and 65.3% RH. Avg. visible light 13h 8m. Time and temps were taken on June 21 07 and Dec 22 07 (for S. Africa) the summer equinox (longest day of the year) for each region.
However, for my first grow the temp is dialed in at 80 deg. RH for veg is 60% and visible light will be on a 18/6 rotation. By the end of April I should have it up and running. These numbers will give me a starting point of reference. As you can see the average visible light is only 13 hours on the longest day of the year. And a little over 11 hours on the shortest day.
What would it do to the plants if they were put on that light cycle? Veg 13 on 11 off, Bloom 11 on 13 off....Unlike the super-charged plants that get 18/6 I think it would slow growth. However...is that a bad thing....It may turn out like a fine aged single malt scotch. We'll have to play with this after the first grow.
As far as production goes...I guess that even if you slow grow you can still be on a two month span between harvests.


Well-Known Member
Smokin Moose: Ya know, after reading that you jogged my memory a bit. I do remember that popping up in another grow site I was a mod at that has since shut down. I remember a few people doing it but it wasnt many. They tried to get me to do it but I just couldnt string enough timers together to get it done.

jackhammermik: I grew out some PPP a few years back. Im sure you will be pleased with its performance and potency. I really liked that strain. Perhaps I should order some again soon.
:19: The old 6 day week huh? :19: I think I remember that post as well.
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New Member
Has anyone tried this 21:36/12 photoperiod since the topic was posted? I'm intrigued by it and was wondering what you did for the veg stage. Did you do a constant light period with no darkness through veg until you switch to the 21:36/12 or what? Also curious if you achieved the yields you expected, for example the potential 40% increase as stated above.

Also a question for "Medical Marijuana", why did you only use the cycle for two weeks? i realize all this was posted in 2008. wondering if you ever did a whole flowering cycle with that unique photoperiod. Any thoughts on this would be great.

I had a couple thoughts why it wouldn't have taken off as the go to method.
1) difference in strains, maybe not suitable for all strains and some lost their crop.
2) people just don't understand the concept that time, photoperiod, and earth days in a controlled environment are what we make it.
3) the only thing that's hard to wrap my head around is whether or not day lengths on earth has ever been 33.6 hours long.

How could a plant have evolved to flower under such a cycle. If it works it works, if it works well we should all be doing it. (we should all be growing killer bud too, considering time and effort but that doesn't happen either).

If this works it proves that darkness truly initiates flowering regardless of day light and that day light has no effect on flower initiation.

Or its a ratio explanation, that is that a light/dark ratio of 1.8 or less initiates flowering. Also in the natural environment, I know flowers start before day light is 12 and 12. But back to the ratio hypothesis, a 14 hour day light followed by 10 hours of darkness may initiate flowering outside, it also works out to be 1.4 which is less than the 1.8 ratio of 21:36 (or 21.6)/12.

What we know is that something dictates flower initiation and production, not necessarily that plants know, understand, or can perform math. But a well educated biologist told me that plants don't lie, animals do. seeing a plant growing somewhere allows us to instantly make some sort of assessment of that piece of land. Perhaps it is the plants response to the ratio, "daylight is decreasing, i better reproduce before I die". Not trying to personify plants here but it's an easier way to explain. After all whatever traits were present during the past 300 million years and the ability to adapt is what has led them to be here now.

Let's revive this old thread and see what folks have to say about it.
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