Outdoor tutorial: force-flowering

A cannabis plant with a blackout material and a garden in the background

Growing outdoors can be tricky in terms of timing, and if you get it wrong–or if your location simply does not allow you the leisure of letting nature take its course–then your harvest may be drastically reduced, if not non-existent. Here, we provide a few helpful tips on how to ensure you get that harvest, wherever you are.

Why should you force-flower cannabis?

Force-flowering cannabis is one simple way to ensure that your outdoor or greenhouse crop begins to flower with enough time to complete its growth cycle before the winter cold sets in and sunlight becomes weak and limited in duration.

In the northern, cold-temperate zones of the northern hemisphere (and the corresponding southern zones in the southern hemisphere), the time between hours of daylight dropping low enough for flowering to begin and winter frost setting in can be insufficient for flowering to complete; thus, artificially reducing hours of daylight prematurely can supply the extra short days needed to achieve harvest.

Force-flowering can also be useful for growers in more favourable climates who wish to produce multiple crops per year (a “perpetual harvest”). Some growers are able to harvest two or more crops per year by strategically depriving plants of light. For example, plants can be started in February or March; some can be harvested by June using light-deprivation techniques, while others can be left to flower naturally in late summer and be ready to harvest by mid to late autumn.

A silhouette of cannabis plant and a moon in the background

For truly perpetual harvests, a few plants can be induced to flower every week or two (once they have undergone sufficient vegetative growth) so that harvests are similarly staggered.

When to force-flower cannabis?

Your window of opportunity when attempting to force-flower cannabis outdoors depends greatly on your location. In warm-temperate and tropical regions, there may only be a very short period each year in which cannabis cannot be grown, so ensuring that flower cycles end before that cold or wet season sets in is the only real requirement.

In cool-temperate climates such as the UK and the Netherlands, autumn conditions usually become too cold and damp to sustain healthy flower growth by late September to mid-October (although there are reports of outdoor crops flourishing until late November or even early December in some areas, as seasonal climate patterns shift and winters become milder).

Thus, to ensure that your crop finishes in time, it is advisable to commence light-deprivation in mid to late July, depending on the flowering time of the strain.

Naturally, your plants may not begin to flower until September, which can be a wet and windy month in many parts of the UK. In some cases, it may be advisable to commence force-flowering in mid-June or early July, which should provide your plants with enough time to grow vegetatively and complete flowering too.

Starting plants indoors is always advisable, particularly in regions that experience cool springtime temperatures.

How to force-flower cannabis?

Depending on various factors such as the size and number of plants, whether you are growing fully outdoors or in a greenhouse, and how much you are prepared to spend on your set-up, there are several ways to go about depriving your cannabis plants of light.

The basic concept is to cover your plants with material that cannot be penetrated by light, but that allows fresh air to circulate around. In some cases, use of fans may be advisable if possible, in order to ensure that airflow is sufficient.

If you are simply growing a few plants on your balcony or back garden, you can cheaply and easily construct lightweight frames from wooden poles and cover them with opaque, preferably breathable material; these frames can then be placed over the plants every night so that they are sure to get twelve hours of darkness each night.

Lightproof “sensory” tents designed for children with autism and related conditions are also commercially available for relatively low prices, and can be adapted into covers for your plants.

Four-layer blackout material

For growers conducting larger-scale or greenhouse grows, different methods may be required. Outdoor growers may place their plants in sheds to block out light prior to natural nightfall (if you think you will need to move your outdoor plants at night to supply them with sufficient light, you will need to grow them in portable containers rather than directly in open soil) and greenhouse growers may use blackout curtains that can be drawn in a matter of seconds and protect an entire crop.

Other things to consider when force-flowering

Of course, once natural hours of darkness have increased to twelve hours or more per night, the need to cover your plants disappears and they can be left to the elements. You may wish to continue covering your plants if your crop is affected by light pollution (such as by streetlights), or if unexpectedly cold temperatures or windy conditions necessitate some protection against the elements.

Another thing to bear in mind is that if you begin force-flowering, it is crucial to stick to the lighting regime and not miss even a single day, so that plants are not confused into reverting back to vegetative growth. If budget allows, investing in automation schemes may be a possibility.

A silver blackout tent against the white background

If you time it right, force-flowering cannabis can not only make the difference between a successful or failed harvest, but should also allow for increased yields.

In cool climates, the difference in temperature and light intensity between late summer and early autumn (when plants naturally begin to flower) and early- to mid-summer (when your plants will flower if forced to) can be so great that your buds will be noticeably larger, tighter and more resinous if grown under a force-flowering regime.

  • 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.


19 thoughts on “Outdoor tutorial: force-flowering”

  1. I would have thought moving them into a window-less garden shed for the requisite 12 hours would also work

  2. I have my plants in very moveable 5 gallon buckets. Would it work to keep them in the shade as part of the 12 light 12 darkness or does it need to be darker than that? I live in the north east and the 12/12 time is in late September, soon to frost and I think waiting until then is risky. Thoughts and help please?

    1. Complete dark is better. In tropical regions I have been successful rising shade cloth on the est side every day a bit trying to replicate the late setting of the ☀️ sun of autumn even if I was in spring. Also covered the plants with a green(green impede chlorophyll operation) thick shade cloth that permit air circulation

    2. Green shade cloth can be used at the right location and season. Like tropical South Hemisphere spring

  3. Flowering in summer is not a good idea as the high temps will cause your buds to be light and airy! Also they won’t be as potent and may cause foxtailing

  4. Who can tell me. what kind fertilizer to use force cannabis to flower soon?

    Help me please. Write down the name of fertilizer to force cannabis flower early…

    1. Scarlet Palmer - Sensi Seeds

      Hi there,

      Thank you for your comment ? Unfortunately, for legal reasons we cannot respond to grow questions on the blog, but we do have the Sensi Seeds Forum where you can ask a thriving community of gardening fans for advice, share your experiences, and see if your question has already been covered. I hope this helps.

      With best wishes


    2. Fertilizer has nothing to do with your plants flowering.Its how many hours they get light that count.When you want to flower them, give them 12 hours of light, and 12 hours of uninterupted darknes.

  5. I’m growing 3 in greenhouse I need to force them how many day do I do this for 1 week 2 weeks

    1. glen
      you must learn about the type of plant you are growing.if yours are 8 weeks in flower you must cover and uncover for the full 8 weeks.

    2. I was told that I could shut the lights off for three days and it would force my plants or cover them for three days. Have you ever herd that.


  6. Shelley brasington

    I’m disappointed I cant get my question answered they just keep sending me all over the fucking internet. My question is what are safe coverings to use for the 12/12 lighting system for my outdoor plants

    1. Scarlet Palmer - Sensi Seeds

      Hello Shelley,

      Thank you for your comment ? Unfortunately we cannot respond to grow questions on the blog, but we do have the Sensi Seeds Forum where you can ask a thriving community of gardening fans for advice, share your experiences, and see if your question has already been covered.

      Also this quote from the post: “Lightproof “sensory” tents designed for children with autism and related conditions are also commercially available for relatively low prices, and can be adapted into covers for your plants” may point you in the right direction. I hope this helps.

      With best wishes


    2. I think that the main problem is to understand your question.I read just the german translation of your question and their is no thens.

    3. use good quality landscape fabric. That would be your best option, mind you make sure there is no light leaks.

    4. VulcansKush

      Do the research like every other grower has. No ones going to spoon feed you such simple questions.
      Spend the time learning and you wont regret it!

Leave a Comment

Your email address will not be published. Required fields are marked *


  • Profile-image

    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.
    More about this author
Scroll to Top