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Everything You Need To Know About Photoperiod Cannabis

Imagine you’re a cannabis plant in the summertime. The days are long, the sun is shining and you’re in photosynthesis heaven. You grow tall, you grow bushy and you’re sure the good times are never going to end. Until you notice a change in the air. The days start getting shorter and you can no longer gorge yourself on sunlight. Maybe you don’t want to believe it at first, but eventually you have to accept that autumn is approaching and your time is running out. You need to start making plans for after you’re gone, so you go into bloom, developing flowers so that you can reproduce and pass on your genetics (I forgot to mention, you’re a female).

Now imagine you’re a human who grows cannabis (you can be any gender). You know that your plants grow in size when they have lots of light each day, and that they flower when they receive less than a certain number of hours of daylight. Naturally, you want to take advantage of this in order to maximise your harvest, which means you need to understand a thing or two about photoperiods.

What Is A Photoperiod?

Indoor cannabis plant

The term photoperiod refers to the balance between daylight and darkness in a 24-hour period. You’ll often hear cannabis growers talking about a 12-hour (or 12/12) photoperiod, which means 12 hours of light followed by 12 hours of darkness.

Some plants, like garden peas, are known as long-day plants, which means they flower when the days are long, but cannabis is a short-day plant, and produces bud only when the number of daylight hours dips below a critical level. This means that if you’re growing indoors, you’ll need to control the lighting so that your plants get the correct photoperiod in order to produce a good harvest, while outdoor growers will need to consider the seasons very carefully when deciding which cultivars to plant and when to sow their seeds.

Of course, if that all sounds a bit too complicated, you can always just buy autoflowering cannabis seeds, which have been specially bred in order to flower after a certain period of time, regardless of how much light they are exposed to. These are particularly suitable for less experienced growers who want a guaranteed harvest without having to worry about photoperiods, and you can read about this in more detail in our previous post.

What’s The Right Photoperiod For Cannabis?

Generally speaking, most photoperiod cannabis (ie not autoflowering) varieties need 18 hours or more of sunlight when they are in the vegetative stage. This is the growth phase that precedes flowering, when the plant grows in size and develops its foliage. Most cultivars will be ready to go into bloom around eight to ten weeks after germination, although they can be kept in the vegetative state indefinitely if this photoperiod is maintained.

A 12-hour photoperiod is then typically considered to be optimal during flowering, which can last for anything from seven to 14 weeks, depending on the cannabis strain. Too many hours of daylight during this phase will stop your plants from flowering, while too few will stunt their growth, although there has been very little proper research done in order to determine the perfect photoperiod for different cannabis varieties.

That said, studies have shown that French hemp can flower with as much as 15 and a half hours of sunlight per day[i], while another variety that is typically grown in Southern Europe requires less than 10 hours of darkness in order to produce inflorescences[ii].

An as yet un-reviewed study looked at the impact of different photoperiods on high-THC weed, and found that maximum yields can be maintained with just 10.8 hours of darkness per night during flowering, but that just half an hour less results in significantly fewer flowers[iii].

Another interesting study found that it takes cannabis plants five days to start flowering once they are exposed to a short enough photoperiod[iv]. However, you’ll need to maintain this light-dark balance throughout your crop’s flowering phase in order to end up with a maximum harvest.

Working With Photoperiods

Outdoor cannabis plant

If you’re growing outdoors then you will need to make use of the seasons in order to ensure your plants get the right amount of light at the right times. As a rule, you’ll want to time it so that your weed is vegetative throughout the summer, when the days are long, and then flowers in the autumn, when daylight becomes more scarce.

Naturally, this will depend massively on where you live. For instance, if you reside in Northern Europe then you may have to wait until the height of summer before it is warm enough for your plants to grow, which won’t give you long before the days start getting shorter and you start seeing flowers. It may also be too cold for your plants bloom once autumn gets into full swing, which means you’ll have to harvest them quite early.

In cases like this, you’ll need to find a cultivar that flowers early and for a short period. Indica strains are likely to be the most appropriate, as these evolved in temperate climates and have therefore learned to get a move on, going into bloom faster than Sativa varieties.

In contrast, Sativa types developed in the tropics, where the photoperiod is close to 12/12 for most of the year. As a result, they generally have long vegetative and flowering phases, with some strains able to remain in bloom for a full three months.

Clearly, a non-tropical climate will struggle to support some of these slow, long-flowering cultivars, which is why many people living in temperate regions tend to grow indoors. This allows for more control over temperature, humidity and, of course, light. Cultivating inside also makes it easier to ensure that no unwanted light sources disturb your cannabis during the dark phase, which is important as just a few minutes of light per night can prevent a plant from going into bloom.

On that note, it’s worth bearing in mind that marijuana is most sensitive to red light wavelengths, so if you do need to switch the lights on during the night then you should use a green light, as this will disturb your plants the least.

Cannabis tends to flower with a 12-hour photoperiod, which means 12 hours of daylight followed by 12 hours of darkness.

What Is Photoperiod Cannabis & How To Grow It Outdoors

For thousands of years, photoperiod-sensitive cannabis was the only toke in town. Despite the advances in artificial lighting and the recent ascendance of autoflowering hybrids, growing photoperiod strains in the great outdoors still rewards farmers with the heaviest harvests.

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Photoperiod, explained

When you come across the words photoperiod, photoperiodic, and photoperiodism, think of the hours of light in a given 24-hour period. Cannabis strains that bloom indoors under the standard 12-12 light-dark cycle do so because they are triggered to flower with a longer dark phase.

Outdoors, most photoperiod strains will transition into flowering as the hours of daylight dip below 15. Thus, a longer flowering period is to be expected. 12-12 is optimal for indoor cultivation. That’s why the flowering-time data on the reverse of most cannabis seed packs reflects bloom periods somewhere between 8–12 weeks.

Typically, heavy indicas like Northern Light finish flowering the fastest, sativas the slowest, and indica-sativa hybrids somewhere in between.

Watch Out: Light Pollution!

Because photoperiod cannabis relies on undisturbed hours of darkness to flower, light pollution (such as from street lights nearby) or interruptions in the dark cycle, even if just for a short time, are a problem.

If photoperiod plants don’t get the required hours of undisturbed darkness, it can cause them to revert back to the vegetative phase, display hermaphroditic qualities, produce inferior yields, or fail to flower altogether.

How do you know if your location is experiencing light pollution? If you can still read the larger headers in a magazine at night, your growing spot may not be ideal.

How to Avoid Light Pollution When Growing Weed

Fortunately, there are ways to avoid light pollution when growing photoperiod cannabis outdoors:

  • Choose a growing location away from light sources (street lights, busy streets, etc.).
  • If growing in a greenhouse, light-proof your windows with blinds or tarps at night. You can even automate this system to keep the schedule as consistent as possible.
  • If growing “under the stars”, consider erecting a basic structure that allows you to cover your ladies with a blackout tarp at night.
  • Even a short interruption in the dark period can be a problem. Don’t even shine your phone light on your plants during the dark period.
  • If you need to see your plants at night, get a “dark night” lamp. These emit a green light that doesn’t stress plants as much.
  • Consider growing autoflowers. Autoflowers are not sensitive to light pollution or disrupted darkness hours.

Photoperiod vs Autoflowering Cannabis Strains

The main characteristic of photoperiod cannabis is that its growing stages (vegetative, flowering) are influenced by the hours of light plants receive. In contrast, autoflowering plants flower automatically based on age, which also limits their growth.

Here are the main differences between photoperiod and autoflowering cannabis:

Cultivating cannabis outdoors has changed for the 21st century grower. This is your modern guide to growing great photoperiod marijuana outdoors.