Flowering Stage: How to Flower a Cannabis Plant

A person holding cannabis buds in a hand

Successfully flowering cannabis plants is one of the trickiest aspects of cannabis cultivation. At this delicate stage, the plants require meticulous care and attention if they are to achieve worthwhile harvests—and more than ever, finding the right balance is key, as mistakes can be far more difficult to correct.

What makes cannabis plants flower?

Cannabis is usually a short-day plant, meaning that it begins to flower when the hours of daylight noticeably drop—in a natural, outdoor environment, this occurs when the seasons shift from summer to autumn. Plants will typically begin to flower up to a month prior to the autumnal equinox, and are usually ready to harvest by the end of October (in the northern hemisphere)—although some long-flowering varieties may need until late November or even early December, if the climate is warm enough to support it.

Indoor flowering can be activated by regulating the hours of light

The above statement holds true for most commercial varieties of cannabis, as such varieties tend to contain a significant proportion of “indica” genetics—i.e., they descend from plants indigenous to the temperature upland regions of India and Afghanistan, where summers are generally hot and bright and winters cool and dark. Thus, they have adapted to fully mature and flower by the time the winter cold sets in, and rely heavily on the dramatic change in day length to provide the trigger to induce flowering.

Sativa and ruderalis strains

Cannabis which is predominantly “sativa” or “ruderalis” may flower according to somewhat different rules. The cold-adapted ruderalis flowers automatically, in response to a set of genetic triggers activated according to the maturity of the plant rather than photoperiod (hours of daylight). Tropical sativas may also flower automatically, although the genetic mechanisms behind this phenomenon are complex and not fully understood.

In the case of ruderalis, the plants have a short window in which to complete their life cycle, as temperatures and light intensity are simply too low for survival throughout much of the year. They must grow and flower quickly in the course of one season (late spring/summer). Thus, relying on seasonal change to trigger flowering does not work as an evolutionary strategy. Plants that can flower automatically and produce seeds before dying with the onset of the cold northern autumn are more ‘fit’, and over the millennia have become established in such environments.

With tropical sativas, the lack of climatic variation means the plant has an almost year-round growing season, and environmental triggers such as changes in photoperiod are much less dramatic—although they do exist, and their existence helps cannabis to maintain a roughly annual life cycle in such environments. Therefore, sativas in their natural habitat have many months in which to leisurely vegetate before flowering; when they do flower, it is partially related to environmental cues and partially dependent on the maturity of the plant.

Flowering indoor cannabis plants

If growing indoors, flowering can usually be controlled very precisely by simply changing the lighting regime. Indoor cannabis growers typically give their plants 18-24 hours of light per day during the vegetative growth period, and drop down to 12 hours to trigger flowering. Some growers will gradually increase hours of darkness over a period of 2-3 weeks to more closely mimic the outdoor cycle.

A cannabis plant with pistils
Certain varieties flourish without external stimulus

Some growers will even give plants a full, uninterrupted 24-48 hours of darkness in order to trigger flowering, prior to commencing the 12/12 cycle. This is thought to assist in achieving a rapid transition to flowering, as the build-up of florigen (“the flower hormone”) is thought to occur mostly at night. As is the case with so many widely-accepted growers’ folk tales, this appears to have little scientific basis. However, studies on inductive dark periods of up to 16 hours for short-day plants have been conducted, and have found no adverse effects.

Flowering indoor sativa and ruderalis plants

If growing ruderalis-dominant varieties indoors, plants may be kept at a constant 18-24 hours of daylight throughout their growing cycle, and will begin to flower once the plant reaches around 50cm in height (some “super-autos” may reach 90-100cm).

If growing sativa-dominant varieties, growers will usually stick to the 18/6 and 12/12 system, and will sometimes give plants just 10 hours of light and as much as 14 hours of darkness per day. Anecdotal reports suggest that the extra hours of darkness increase the rate of flowering and reduce overall flowering time. If opting for a 10/14 cycle, it may be advisable to begin flowering at 12/12 and reduce to 10/14 over a period of approximately one month.

A cannabis plant growing outside with mountains in the background
Usually the season dictates when a cannabis plant blooms

It is also possible to stick to a 12/12 (or thereabouts) cycle for the duration of the plant’s life cycle—typically, sativas will flower under this regime after 3-4 months of vegetative growth. However, giving around 18 hours of light during the vegetative period may increase the plant’s growth rate and speed its maturation, and plants may then be ready to flower earlier when the lighting is switched to 12 or more hours of darkness.

Temperature and humidity during flowering

When cannabis plants are in the vegetative stage of growth, they can generally tolerate higher temperatures than in the flowering stage. While in vegetative growth, maintaining a daytime temperature range of 24-32°C (75.2-89.6°F) is considered optimal; in flower, plants should ideally not be subjected to temperatures above 28°C (82.4°F)—although some strains may be able to tolerate higher temperatures, such as those whose lineage originates in the tropics.

The difference between daytime and night-time temperatures also comes into play during flowering. It is thought that a larger difference is beneficial for induction of flowering, as it mimics the still-warm days but significantly-colder nights that generally characterise the onset of autumn. The optimum difference is thought to be 8-10°C (14.4-18°F); thus, if maintaining plants at 28°C (82.4°F) during the daytime, the temperature should be 18-20°C (64.4-68°F) at night.

Cannabis plants in growing plant with ventilation
Environmental conditions are crucial for successful cultivation

Humidity should be slightly reduced during flowering, especially during the latter stages; as flowers become denser, they become more susceptible to mould growth. In the vegetative period, humidity should be maintained at around 60-70%, and in flower it should be reduced to 40-50%. Again, tropical strains that have light, airy flower structures can often handle much higher humidity during flowering, of up to 70%.

Watering little and often during flowering is generally advisable. If the plant is given more water than its root system can immediately utilise, the growing medium will stay damp and increase the rate of evaporation. Therefore, controlling water intake helps to control humidity, as well as avoiding other issues such as pythium (root rot) and botrytis (grey mould). However, this depends on the medium used, as some media are better at retaining moisture than others, and can be watered less frequently with larger quantities of water.

  • Disclaimer:
    Laws and regulations regarding cannabis cultivation differ from country to country. Sensi Seeds therefore strongly advises you to check your local laws and regulations. Do not act in conflict with the law.


17 thoughts on “Flowering Stage: How to Flower a Cannabis Plant”

  1. love the info , got a group of ladies coming along quite nicely my self love to read about these plants , anyway thanks again


    1. Mark Smith - Sensi Seeds

      Good afternoon Jeff,

      Thanks for your comment, I hope you’re having an excellent day.
      I’m glad to hear your ladies are in great health, I wish you the best of luck!
      If you’re looking for some inspiration, head on over to our grow reports. 🙂

      Thanks again, and I hope you continue to enjoy the blog.


      1. Can I force my plants into flower mode?
        Guess I should give some background here.. this is my first season growing and they’re 9 months under the light. Their both as tall as me (6’3”) and nearly as heavy. They’ve endured days without light in the past and kept growing anyway. Both in veg. Bugs and spiders have come and gone and I attribute their health to my changing of soil often. Both are in great shape but I can’t get them to flower simply by changing their light.
        What can I do?

      2. Mark Smith - Sensi Seeds

        Good afternoon Sean,

        I hope you’re having a great day, thanks for the question!
        Fortunately, we have an outdoor tutorial on force-flowering here 🙂
        I hope this helps!

        Have a great day!


    1. Mark Smith - Sensi Seeds

      Good morning Bonnie,

      Thanks for the comment. I’m so glad you are enjoying the Sensi blog!
      If you’re looking for more growing information head over to our archive. 🙂

      Thanks again,


  2. I have my plant growing in my back yard (Nevada) legal.
    My plant grew about 6 feet but not very leafy. The weather is starting to get cooler will that help fill in and start the budding cycle or is it to late now. End of September

    1. Mark - Sensi Seeds

      Good morning Lauren,

      Thanks for getting in contact with us,
      Nevada sounds great, looks like you’ve got a real (legal) monster on your hands!
      Of course, It is possible that a dramatic change in light or environmental conditions could help to initiate the flowering cycle.
      This article on Outdoor tutorial: force-flowering might also be of use!

      Good luck, and I hope you continue to enjoy the blog.


      1. Jerry combs

        How often do you recommend changing the soil? Is it a complete transplant when changing the soil? What happens when my plant out grows my grow tent?

      2. Tim - Sensi Seeds

        I recommend 1x from seedling to final pot. Still You can do it more to get a more compact root system. Up to you. Yes you fully transplant. When your plant outgrows your tent then you either get a bigger tent or you train the plant.

    2. Oklahoma patient so im legal here,only option is indoor (apartment) im looking into grow tent and light,is my best option to go with auto or a clone for my grow ?thanks JP

  3. Alright so it’s not really a question about the topic at hand here but… how long does it take you to put together these kind of posts? Is it easy? Like did you have to research all this stuff? I’ve been wanting to start a blog myself, so just curious. Sorry not totally relevant but figured I’d ask. Thanks in advance


    1. Scarlet Palmer - Sensi Seeds

      Hi Brian,

      Thanks for your comment.

      To answer your questions: A lot depends on what the post is about. Some writers already know a lot about a topic. Some have to research everything. New information and research about cannabis is constantly being published, so even if you think you know something, you still need to check it!

      All our posts that cover legal and medicinal topics are fact-checked by a lawyer and a medical doctor respectively, which takes extra time but is essential for the quality of our blog, and our obligation to our readers. Then we source images, and finally have everything translated into the five languages this blog is available in.

      My best advice to you as a writer is to blog about something you’re passionate about. Good luck!

      With best wishes,


    1. Scarlet Palmer - Sensi Seeds

      Hi Dave,

      Thanks for your comment 🙂 Unfortunately, legal restrictions mean we can’t answer grow related questions or give grow advice on this blog. However, we do have the Sensi Seeds Forum where you can browse through questions and share the experiences of a thriving community of cannabis and gardening enthusiasts.

      Also, other readers of this blog will often answer questions like yours. Sorry I can’t be of more help, and I hope you continue to enjoy the blog!

      With best wishes,


  4. i think i have a natural florigen spray that works anytime, any light etc……interested?

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    Sensi Seeds

    The Sensi Seeds Editorial team has been built throughout our more than 30 years of existence. Our writers and editors include botanists, medical and legal experts as well as renown activists the world over including Lester Grinspoon, Micha Knodt, Robert Connell Clarke, Maurice Veldman, Sebastian Maríncolo, James Burton and Seshata.
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