photoperiod cannabis

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.


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 fastest, sativas slowest, and indica-sativa hybrids somewhere in the middle.



Cannabis is an annual plant. And as a short-day plant, it will not be ready for harvest until autumn. In general, cannabis planted in the spring after the equinox will gradually shift from vegetative growth to bloom some time after the summer solstice, and finally be ready for harvest sometime during autumn. How much sunlight each plant receives will set the pace of the crop.

Greenhouse growers can use tarps and light-proof plastic sheeting to reduce the hours of sunlight and trigger plants to bloom faster. The only downside is it turns the grower into a walking timer plug. Don’t forget to remove the shade material before sunrise.


In the Northern Hemisphere, the outdoor growing season is April–November. The summers become shorter and winters colder the further north you go. In the Southern Hemisphere, it’s the complete opposite, with the season running from August–April. Of course, there will be some regions that are outliers. But unless you are living close to the equator where a near-constant natural 12-12 day and night cycle prevails year-round, you must do some research.


Data is digital gold. Use an online sunrise-sunset calculator. You need to study the historic monthly analysis for your local area. This is absolute need-to-know information. The sun is your grow light in the sky. You must be certain of how many hours your plants will receive before you germinate a seed.

First, you must pinpoint when 18+ hours of light will be available for vegetative growth. Then you must check approximately when sunlight is less than 12 hours to estimate a harvest window. Don’t get too hung up on a perfect 12-12. Remember, outdoor photoperiod cannabis will bloom slow and steady as the nights get longer. Some late-blooming super sativas like Amnesia Haze can grow to enormous sizes and not finish flowering until sunlight is down to 11 hours per day in late-autumn.


18+ hours of daily sunshine is not enough to get growing. Check the weather forecast regularly and study the historical data for the last few years. Spring in Northern Europe is still pretty cold. For best results, cannabis needs 20–28°C temperatures to grow to its full potential. The reality is if you live too far north of the Mediterranean, you have a very short growing season. A hardy autoflowering hybrid like Blue Cheese Automatic is better suited to cooler northern climates. Lucky continental growers can really go for it with exotic strains like Kali Dog and Chocolate Haze.


OK, so you have completed your desk research. Now it’s time to do some field research. You must inspect your grow site. Make sure no obstacles are casting shadows to block sunlight, and scout around the area to make sure it’s safe from prying eyes too. The best outdoor grow space is a private one. Use chicken wire and fencing to keep out any weed-munching herbivores or rodents.


You must tailor your choice of cannabis seeds to your microclimate. Use the RQS strain filter to help make this important decision. One of the big advantages all photoperiod strains have over autoflowering cannabis is you can take cuttings to preserve the genetics. When you find a winning mother plant that thrives in your cannabis garden, cut clones to repeat the success.


Climate change is already destroying outdoor cannabis farms. Growers in Northern California saw crops go up in flames in 2017. Smoke from the wildfires tainted marijuana growing for miles around. This is an extreme example, but extreme weather events are a real threat to outdoor cannabis cultivation everywhere. All you can do is be aware of the current weather patterns in your region.


Getting started indoors is always a smart idea. If you have good weather, a sunny windowsill is a good spot for delicate seedlings. Or, if it’s still too cold and raining outside, you can use a 200W cool white CFL to see plants through vegetative growth. A hybrid indoor-outdoor grow is not a compromise, it’s getting the best of both worlds.

We recommend growing outdoors using cannabis-specific soils in white plastic pots or Smart Pots. Pick up some brand name dope-growing dirt from the local grow store. You can do little more than staking plants with bamboo for support if you grow in the ground. Containers give you the option of moving plants. This is usually required as the seasons change. The Earth moves around the sun, so if you are growing on a balcony or terrace, you may need to move plants around as the day progresses to keep them in direct sunlight.

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

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.