Marijuana Photoperiods

What are they? How to induce Flowering?

Night time vs day time hours

Hi there and welcome (back) to Free the Tree 🙂
In today’s article we’re going to cover Marijuana’s photoperiods. We’ll look at exactly what photo periods are, their importance, how to utilize them and much more!

Cannabis Photo Periods – Table of Content

Let’s get into it

What is a Photoperiod?

A photoperiod is the relationship between the duration of the light period and the dark period.  In other words the photoperiod is the length in time of the day and night.
Depending on the distance from the equator the photoperiods change on earth which means that strains from different sides of the world won’t have the same photoperiod needs.

Photoperiods and Marijuana

Photo periods are crucial to help the cannabis plant grow, especially by determining which stage of the lifecycle is in (exception for Marijuana Ruderalis).

Most marijuana strains will stay in the vegetative growth stage as long as their exposed to a cycle of 18 to 24 hours of light followed by 6 to 0 hours of a dark photo-period.
18 hours of light will give the plant all it needs to sustain a good vegetative growth.

Once the light photoperiod reduces under a certain time, the plant starts entering the flowering stage. Around the 12 hours of darkness most (if not all) strains enter the flowering stage.

Photoperiod recap

The marijuana plant basically uses the photoperiod as a timer for her lifecycle, it indicates “stay in vegetative stage” or “alright it’s time to get those flowers going”.  She ‘knows” that once light reduces under a certain levels there x months left for her to live, thus she must start her reproductive cycle to prepare the next generation (pistil&flower development).
Depending on which region of the world the strain is from this minimal light hour photoperiod will vary.

Photoperiods to start the Flowering Stage of Cannabis

As we said in the previous section the Marijuana plant will enter the flowering stage once there’s about 12 straight hours of darkness in a 24-hour photoperiod.

Research has proven that less than 12 hours of light does not induce flowering any faster and reduces flower formation and yield. Although, exposing the plant to a straight 36 hours of darkness to force flowering stage has shown very positive results (more on that after)

Exposing the plant to more than 12 hours of light often prolongs the flowering stage, although some growers have seen an increase in yield when extending the day photoperiod to 13/14 hours about 3 weeks after starting the flowering stage.
Flowering is prolonged by about 1 week and yield increases of about 10%. if you try this out keep in mind that different strains will react differently to this.

What is the best Photoperiod for Marijuana?

As a general rule of thumb you’ll want to have 18/6 day/night hours during the vegetative stage and a photoperiod around 12/12 for the flowering cycle.
Once you start knowing a strain you can test out different photoperiods, just do make sure not to bounce back and forth within the same grow.

Furthermore, some growers have tested reducing the day/nigh photoperiod gradually, imitating nature, in order to induce flowering. These tests don’t seem to show faster flowering nor bigger yields.

Photoperiods and Strain Genetics

The relationship between genetics and photoperiod response has been proven by many growers, and is pretty logical. The different reaction are due to the fact that these strains are indigenous of different regions of the world, thus different photoperiods.

Little scientific documentation is available so far so we’ll can make some generalizations grouping marijuana in its 2 main families, sativa and indica.

Sativa strains response to Photoperiods

Sativa-dominant plants that originated in the tropics respond to long days better than their indica cousins.
On and around the equator day and night periods are almost the same length, thus sativa plants tend to bloom when they’re chronologically ready after completing the vegetative stage and are less sensitive to photo-period change.

For example, you can grow a pure Haze strain on a 12/12 night/day schedule, she will still go through the seedling and vegetative stages before spending her 3 months (or more) developing flowers.
It’s important to note that, even though they will go through every stage naturally, the plants will grow slower and inducing the flowering period will take longer.

Indica strains response to Photoperiods

Indica dominant varieties, which originate from northern latitudes, tends to flower sooner and respond much quicker to a 12 hour photo-period.
This is most likely due to the fact that in latitudes further from the equator cold weather comes faster after the day hours start reducing.

Many indica varieties will flower under a 14/10 or 13/11 day/night photoperiod, but this varies depending on the strain so you’ll have to do some tests.

More hours of light during the flowering stage can cause some strains to produce bugger plants with reduced flowering time, but some growers have reported looser, leafier buds as a result.

Giving indica-dominant marijuana plants less than 12 hours of darkness will not start flowering faster, instead it will take longer to mature, the buds will be smaller and the over all yield will be smaller.

Ruderalis strains response to Photoperiods

So we put this one just because it exists ^^ but you really don’t want to be growing this indoors, you can read up more over here on the Ruderalis strains.
The interesting thing to note here is that Cannabis Ruderalis does not depend on photoperiods to go into flowering.
In order to make the autoflowering seeds growers used her genetics.

Unstable Strains

Genetically unstable strains could express hermaphroditic tendencies if the photoperiod bounces up and down several times. If you plan to give plants a 13/11 day/night schedule stick with it, don’t change your mind and bounce it up to 15/9.
Variations like this will stress the plants and could produce herma’s

Dangers of Light exposure during the night period

Marijuana plants won’t stop flowering if the lights are turned on for a few minutes once or twice during the flowering stage. The danger starts to come if a light is turned on, even a few minutes, during at least 2 or 3 consecutive nights.
For well bred indica dominant plants the revert will be done within 3 days. Sativa’s are a bit longer and will take 4 to 5 days before showing vegetative growth.
If you plants started re-vegetating it can take between 4 to 6 weeks to be able to restart flowering.

When trying to start the flowering stage, less than one half of a foot-candle of light will prevent cannabis flowering, that’s just a little more than a full moon on a clear night.

Lights during night period tips

  • If you absolutely need light, use a green light. This will not bother the plants.
  • Even small lights will bother the plants, tape up LED’s on electric appliances
  • Red lights are the worst! Marijuana is especially sensitive to this light spectrum

Photo-period flowering vs Auto flowering strains

You might have heard of the famous “autoflowering” strains, that seem to flower by themselves. These are strains that have been crossed or genetically modified in order to not depend on photoperiods for their flowering, instead they depend on their root development.

These strains are possible thanks to a third family of Cannabis, Cannabis Ruderalis. Growers noticed that this strain, while not really smokable due to its, let’s say, un-favorable effects, had the particularity of depending on the development of their root system to start flowering. Basically, once the roots have grown in the space available to them, the plants starts to flower. 

By crossing cannabis Ruderalis with other strains, grow banks were able to take this genetic train from Ruderalis and create auto-flowering variations of famous strains

Alright folks that’s it for this one! We’ll update the 3 missing sections in the very very near futur
Until next time, Be safe and grow easy!

Couple words about the grower

This article was written by Plant Lover, Free the Tree's founder and a passionate grower of Cannabis, as well as all other plants.
Mostly from the city, I work on my computer all day so about 4 years ago, on my off time, I started growing to have some more green in my life.

I never imagined I would fall in love with botany and discover so much about this plant. Since then I keep learning everyday, meeting new awesome people of our community and grow constantly

You can always get in touch with me on our Instagram!

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