The Photoperiod of cannabis plants grown indoors

When we talk about photoperiod, we mean the daily hours of light and darkness that any given living organism receives, while photoperiodism refers to the physiological reaction provoked by the length of day or night, and the way it affects the behaviour and development of these plants and animals.

In botany, each subspecies has its own specific photoperiod, but plants can be classified into three groups: short-day plants, long-day plants and day-neutral plants. Cannabis is a short-day plant (with the exception of Cannabis Ruderalis autoflowering varieties), which means that it needs long nights with over 12 hours of uninterrupted darkness to trigger flowering.

In the plant world, both the light intensity as well as the daily hours of light/darkness determine the biological functions of organisms, such as seed germination, growth, flowering and maturing processes. In this way, the development of plants can be activated or suppressed depending on lighting conditions, meaning that, for example, most plant species are unable to flourish unless they receive a certain number of hours of darkness.

The length of day and night determines many biological processes
The length of day and night determines many biological processes

Logically, in nature, the photoperiod is determined by the season of the year and the solar cycle. But what happens with indoor crops under artificial lighting? The answer, also very logical, is that the grower sets the photoperiod of indoor gardens, and in this way, they are able to control whether the plants remain in the vegetative growth phase or if they begin their flowering process.

The photoperiod for indoor crops

When we learn the basics of cannabis cultivation, one of the first things that we must understand is that to maintain our plants in the vegetative growth phase, we will have to expose them to a daily photoperiod of 18 hours of light and 6 of darkness (both uninterrupted periods), and that when we wish them to start to bloom, the photoperiod must be adjusted to 12 hours of light and 12 of darkness every day, also uninterrupted.

If we only give them 6 hours of darkness each day, our cannabis plants will keep on growing, because the nights are too short to induce them to start flowering. On the other hand, if we give them 12 uninterrupted hours of darkness we can be sure that, independently of the variety (and with exception of automatic varieties) the plant will be triggered to bloom by the longer night length.

With indoor facilities, it’s the grower who decides the length of the photoperiod
With indoor facilities, it’s the grower who decides the length of the photoperiod

These standard photoperiods are very useful for the indoor grower when it comes to regulating the state of the plants,  meaning, regardless of the cannabis variety being grown, they will behave as they ought to, growing under a vegetative photoperiod (18/6h) and blooming when it‘s changed to flowering (12/12h). However, many of the more experienced growers rarely use these photoperiods, especially during flowering. This is often because they are looking for the minimum number of darkness hours that will trigger the plant to start flowering. In this way, they have greater control over the light input during this important phase and will be able to achieve better yields or shorten the flowering time.

The photoperiod during the growth phase of cannabis plants

As we have mentioned above, when growing under artificial light, we normally use a daily photoperiod for vegetative growth of 18 hours light and 6 of darkness. This doesn’t mean that we can’t use other photoperiods that are also suitable for vegetative growth, for example, 16/8 or 20/4 and even 13/11 for pure Sativa varieties. Indeed, many indoor growers will start Sativa varieties under a 12/12 photoperiod right from the point of germination, as often this is the only way to keep these fast-growing and hugely vigorous plants to a manageable size and grow them within the restrictions of an indoor growing space. Remember that, the more hours of light they receive per day, the more our plants will grow.

Some growers, when they're in a hurry for their plants to grow to a decent size before flowering them, will increase the number of light hours per day up to as many as 24, leaving the plants without any night period at all. It's worth bearing in mind that not all varieties will react well to this, and, because certain of the plants' biological functions only take place in darkness, constant illumination can often lead to stressed and unhappy plants.

At the other extreme, if all we want to do is simply maintain the genetics and to keep them from growing too big in the meantime, it’s possible to reduce the photoperiod to 16/8, giving the plants eight hours less light than those at 24h, meaning they won't grow as vigorously as the plants receiving more hours of light. Longer day length allows more photosynthesis, which in turn means that more nutrients will be consumed, which, naturally, leads to more growth.

The correct photoperiod means that plants grow properly
The correct photoperiod means that plants grow properly

Interrupting the night period is another great trick used by growers of big outdoor cannabis plants. As we've already seen in our post about growing outdoors out of season, if we take plants grown under artificial light and place them outside before mid-late May they will start blooming, because the days are still short and the nights long enough to start the flowering process.

However, if we have to move plants into the garden before this time and plant them out during months when the natural length of the night would trigger flowering, we can fool the plants into continuing vegetative growth by breaking the nighttime in two or more parts. This is done by illuminating our plants for a minimum of 15 minutes in the middle of the night to prevent them from receiving enough hours of uninterrupted darkness to initiate the flowering process. We can even do this several times throughout the night to make absolutely certain the plants won't flower. There's no need to use high-intensity lamps for this either, a simple domestic lightbulb or fluorescent tube hung over each plant will provide all the illumination necessary to halt flowering.

Once we get to June, the plants will no longer need any supplementary lighting, as the days will be long enough to keep them in a vegetative state, but at this point they will have had all spring and summer for vegetative growth and, providing they've been properly cared for and provided with enough substrate and nutrients, they will reach a really big size before they begin to flower in mid-late August, enabling them to deliver a heavy harvest of buds for us to enjoy.

The photoperiod during the flowering phase of cannabis plants

We can control the hours of light and darkness using timers for our growing lamps
We can control the hours of light and darkness using timers for our growing lamps

We know that all non-auto cannabis varieties - whether they're Indicas, Sativas or Hybrids - are triggered to flower by a photoperiod of 12/12. Even so, there are some Indica varieties that will start to bloom with just 10-hour nights and 14-hour days. This means that if we flower our Indicas under a 14/10 photoperiod they will receive two more hours of light per day than at 12/12, and in consequence, will produce more and larger flowers.

On the other hand, many Sativa varieties originating from equatorial zones will flower well under a 12/12 photoperiod, but will do better and express their Sativa genes more with longer night periods of 13 and up to 14 hours (in addition to saving an hour or two of consumption on the electricity bill!).

If we test this method with cannabis hybrids, we will find that under photoperiods of 13 hours light and 11 dark, plants will flower more heavily, but it may mean that the maturing process is extended by a few days. On the contrary, if we use a photoperiod of 11 hours light and 13 of darkness, the plants will have a shorter flowering period but the yield will be slightly reduced. It’s something that every grower can and should try out, depending on his needs and growing plan.

Another technique commonly used to speed up the maturation process of the buds is to start the flowering period with a 12/12 photoperiod and then reduce the light hours from the fourth/fifth week of flowering (to 11/13 or even 10/14), it's an effective method, but not recommended if we wish to revegetate the plants after flowering.

To avoid any potential problems with the height of the plants in our indoor grow, or to grow plants that have a considerable stretch at the onset of bloom, we can begin the flowering phase with a photoperiod that uses fewer hours of light (11/13 for example), so the plants will stretch less during the pre-flowering stage. Once the plant stops its vertical growth, usually within the first two weeks, we can change back to 12/12 hours for greater yields, even to 13/11 if we don't mind lengthening the flowering period for a few days more.

Controlling the photoperiod we decide when our plants start flowering + destacada
We can control the photoperiod to decide when our plants start flowering

As you've have seen, we can experiment and play around with the photoperiod of our indoor grow space to achieve a range of different objectives, such as more or less growth, energy savings, reducing the flowering time, increasing yields, keeping the plants in a vegetative state, etc. But in all cases, we must take into account that our plants - just like us – are regularized by natural circadian rhythms, the cycle of days and nights adding up to 24 hours. For this reason, using photoperiods that don't fit the 24-hour cycle won't usually give the results we're looking for.

Happy growing!

The articles published by Alchimiaweb, S.L. are reserved for adult clients only. We would like to remind our customers that cannabis seeds are not listed in the European Community catalogue. They are products intended for genetic conservation and collecting, in no case for cultivation. In some countries it is strictly forbidden to germinate cannabis seeds, other than those authorised by the European Union. We recommend our customers not to infringe the law in any way, we are not responsible for their use.

Comments in “The Photoperiod of cannabis plants grown indoors” (6)


D 2022-09-19
I thought I was growing auto flower and it’s a photo. I am in the vegetative stage and not flowering . I am growing indoors Please help me. My plant just keeps growing and not flowering. What can I do to save it .

Alchimia Staff

Tim Alchimia 2022-09-19
Hi D, thanks for your comment and question. If, as you suspect, the plant you're growing isn't an autoflower variety, you'll need to switch the photoperiod to 12 hours of darkness and 12 hours of light each day to trigger flowering. You'll also need to change the nutrients for a flowering fertiliser to ensure that the plant receives the correct balance of nutrients during flowering. Before you do that though, I would decide whether or not the plant needs to be pruned at all - I mean if it's been growing and growing, it might have gotten too large at this stage that when you switch to flowering it might outgrow the grow space available. Depending on the genetics being cultivated, you can expect plants to roughly double in height during stretching in the first few weeks of flowering (some will stretch more, others less), so make sure you'll have enough room for the finished plant. If not, you should reduce the height of the plant by pruning or training, and if you do prune, let it recover for a week before switching to a 12/12 photoperiod. I hope that helps. Best wishes and happy growing!


Mae 2020-10-10
How long after changing the light period will an indoor photoperiod begin to flower.


Old1stTimer 2020-01-03
thank you for this article was very informative. I am growing 12/12 seed to harvest due to space and money. LED 1000W 12" above plant. Sprout is going on 3rd node (unknown bag seed) I have set my light on veg switch and intend to keep that going for 2 1/2 to 3 weeks then switch to bloom. My question is can the 12 hrs lights on be ok to check on the plants progress by opening door on closet or is this considered interrupted and should not be opened even while light is on?

Alchimia Staff

Tim Alchimia 2020-01-07
Hi Old1stTimer, big thanks for your comment and question. There's no problem at all in opening your closet to check the plants during flowering, as long as it's during the 12 hours that the grow light is illuminated, or to put it another way, during the plant's daytime. Any light contamination during their night time period, however, can cause problems such as halting flowering, revegetation or hermaphroditism. For this reason, we keep the plants in absolute darkness for their 12 hours of "nighttime" during the bloom period. There's no need for such precaution during veg, though, as growth won't be interrupted by I hope that clears up your doubts! All the best and happy growing!


Maciek 2019-12-08
Hi! I have a question. What would happen if I induce plants to flower by a 12/12 photoperiod, and once I see that it was successful and the first flowers appeared, I would switch back to long days? Is it correct to think that the decision to switch to flowering is irreversible in the plant's life cycle, and therefore, they would keep flowering, but produce more yield due to more light?

Alchimia Staff

Tim Alchimia 2019-12-09
Hi Maciek, thanks for your comment and question. The results of carrying out the process you describe would depend mostly on the cannabis genetics being used. Some plants have very little trouble switching and adapting to changes in the photoperiod, whereas others have less adaptability in these circumstances. However, as a general rule, if you start a plant flowering and then switch it back out of the flowering photoperiod, it will cease flowering and begin to revegetate. It can take weeks or sometimes even months for the plant to return to "normal" vegetative growth, depending on the strength of the hormone signals received by the plants (Generally, Sativas are more adaptable while Indicas respond much more slowly). What is clear though, is that most plants will halt flower development once the "daylight hours" become too long to sustain blooming and that yields will definitely be affected negatively, contrary to your hypothesis, I'm afraid. The only real exception would be Auto-flowering plants, which don't need to be triggered into flowering in the first place, and will continue blooming even on a 24 hour light cycle! I hope that's cleared up any confusion, feel free to ask any more questions. All the best and happy growing!


Mike DeMunbrun Is an Alchimia client 2019-10-12
Thank you for the article. I have some 43-year-old Gold Colombian seeds. When I get my medical license/growers card I intend on growing them inside with led grow lights. Do you have a favorite old seed germination technique ??? (My small little seeds All appear to look healthy and were stored in corked 1”viles, in a metal case at avg household temp) Thank you

Alchimia Staff

Tim Alchimia 2019-10-14
Hi Mike, thanks for your comment and question. I'll be honest with you, seeds of that age are going to be very difficult, if not impossible to germinate if they haven't been stored at a very low temperature. Even stored in a fridge and vacuum-sealed, seeds won't last forever. I'd still give it a try though, you never know, they may well open and flourish! I'd take some steps to help them along though. You could try using sandpaper to roughen the shell and therefore allow moisture to penetrate more easily. Some growers use a small screw device to crack seeds open without damaging them, which also helps with older seeds. You could try using Gibberellic Acid, which works to promote germination. There are other natural methods, organic nutrient manufacturer BAC offers X-Seeds, a product designed to increase germination rates. Wishing you all the best of luck with the seeds, please let us know how you get along. Happy growing!


mike wilson 2019-05-08
Are you really into music? if so you know the name of Bose headphones.with Bose QuietComfort 25 Acoustic Noise Cancelling Headphones. you can jam in peace.

Alchimia Staff

Tim Alchimia 2019-05-09
Hi mike, thanks for the (off-topic) comment. Yes, here at Alchimia we all love music, and curiously enough I myself have a pair of these headphones and I'm very happy indeed with them. Now, back to plants...

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