Understanding and Applying Light Cycles for Growing Cannabis

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One tricky thing about growing marijuana is that certain strains are dependent on light cycles to produce weed. If you’ve read our post on The Growth Stages of the Marijuana Plant, you’ll be familiar with the terms photoperiod and autoflower. You’ll frequently see these when shopping for seeds because there is a big difference between the two. In particular, the biggest difference between the two is that photoperiod plants need a certain duration of light and dark to transition into the flowering stage while autoflowers will bloom after a given period regardless of changes in the light cycle.

Why should you grow a photoperiod plant?

Cannabis Tours, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons

It sounds like a hassle to have to keep a lighting schedule which, if you get wrong, will cause your plant not to bear flowers. Why not just go with autoflowers right?

Photoperiod plants are typically bigger, produce more yield, and pack more THC, CBD and other desirable cannabinoids in their buds compared to autoflowers. Thanks to modern breeding techniques, most photoperiod strains now have an autoflower version after being crossed with Ruderalis, a kind of feral cannabis. You can think of autoflowers as the “lite” versions of strains that are traditionally photoperiod plants. 

For example, here are some key differences between the Jack Herer Feminized and its Auto version.

Jack HererPhotoperiodAutoflower
Average THC level18-23%16%
Average yield350-450 gr/m2Indoors: 400 – 500 gr/m2
Outdoors: 190 gr/plant
Average height175 cm90 to 100 cm
Flowering time9 weeks60 to 70 days

If you’re wondering why the autoflower seems to offer a greater yield, this is because g/m2 denotes the amount per square meter, which means there could be 2 autoflower plants in a space occupied by one photoperiod plant. However, the JH photoperiod offers higher potency per bud. Considering that you can still increase this yield with proper training, a photoperiod plant may be a better option in states where you can only grow a certain number of plants legally.

Another good reason why you should get a photoperiod marijuana plant: you cannot use autoflowers for breeding whereas photoperiods can be revegetated and used as mother plants.  

How to use light cycles on your photoperiod plant

potted cannabis plant, hybrid of sativa and indica

Compared to the Ruderalis subspecies that endured harsher conditions like long winters, both Sativa and Indica originated from regions with a more favorable climate. Sativa thrived in countries nearer to the equator and enjoyed longer days and thus have a longer life cycle of 3 to 4 months. Meanwhile, Indicas come from temperate zones and grow faster at around 6 to 8 weeks.

Ideally, you would want to give your plant as much light as possible to make sure that it grows to its full potential during the vegetative stage before it shifts its energy and resources to making buds. 

How much light should I give my plant?

Photoperiod cannabis will only enter its blooming stage once it gets the sufficient number of hours in the dark. Most growers will recommend 18 hours of light and 6 hours of dark (18/6) during the vegetative stage and 12 hours of light and dark (12/12) to make them flower.

It is ok to exceed the number of hours of light during the vegetative stage. Some growers even give their plants a full 24 hours, arguing that this will cause explosive growth. However, those who go by 18/6 say that a few hours of dark is needed to let the plant rest and bring down cooling and electricity costs. 

It is possible to grow sativas on a 12/12 cycle since they come from regions where the length of days and nights are the same. However, this will prolong your plant’s vegetative stage. Indicas, on the other hand, respond quicker to light cycle changes and some will flower under a 14/10 or 13/11 cycle. One thing to keep in mind though, is that this can still differ depending on the strain, some naturally take longer than others. The kind of training you use will factor in as well since it affects how much light your plant gets.

A word of warning: once your plant has started flowering, avoid exposing it to light during the dark periods. This means no cellphone lights, no light leaks, nothing. During this period, your plants will be especially sensitive to red light so cover up any LED indicator lights in your grow room. If you must, use green light. Otherwise, it may take you an additional 4 to 6 weeks before your plants resume flowering.  

What kind of light does my plant need?

The kind of light that your photoperiod cannabis plant will change depending on which growth stage it is in. During the vegetative stage, it needs light with a color spectrum in the range of 6,000–6,500 Kelvins. This is considered a cool “daylight” color in the bluish end of the spectrum. Once you switch to a 12/12 cycle, you have to put on a “warmer” light around 2,800K at the reddish end of the spectrum. 

LED grow lights won’t have a problem adjusting to this change. Although LEDs emit a narrow color spectrum, most bulbs or arrays for growing will have a number of different LEDs on different color temperature ranges. If you’re using HIDs, you’ll want to get a metal halide (MH) bulb for vegging and a high pressure sodium (HPS) one for flowering. 

Frequently Asked Questions

How much light should a photoperiod marijuana plant get?

During its vegetative stage, a marijuana plant can get anywhere from 18 to 24 hours of light a day. A transition to a 12/12 cycle of light and darkness can make it go into the flowering stage.

Do autoflowers need a light cycle?

No, an autoflowering strain will switch to the flowering stage after a given period, regardless of the number of hours of light it gets in a day.


Once you know the in’s and out’s of lighting a photoperiod marijuana plant, you can use it to your advantage. If you are new to growing photoperiods, here are a few things to need to keep in mind: 

  • Your choice of strain also matters. Sativa and Indica have a number of key differences like plant height, vegging and flowering duration, etc. that factor into how they deal with light in general. 
  • Exposure to light during dark cycles can stress your plant. Not only will it go back to vegging, it can also turn into a hermaphrodite which may pollinate your other plants, ruining your grow.
  • You need to set up a backup system for power failures since a 12 hour period of darkness can prematurely send your plant into its flowering phase.
  • The only time you’d want to do 24 hours of light during the vegetative stage is if you need to raise the temperature of your grow room. Otherwise, the additional cost may not be worth it.
  • Abrupt changes in light cycles may stress to your plants so you’d want to make the change more gradual. For example, if shifting from a 24/0, change to an 18/6 cycle before going to 12/12.

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