How to Force Flowering in Cannabis Plants

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. Whether you call it flower forcing, blooming, flowering, or photo-period manipulation, it is all the same thing. Some plants measure the dark periods and light periods of each day and change their growth patterns based on that information (i.e. switching from vegetative growth to flowering growth).

You might remember your mother, grandmother, or aunt keeping a poinsettia in a dark closet or spare room around Christmas time. For most plants the trigger to grow flowers, fruit, and seed is based on the length of the dark period each day.During the summer, when the day length is longer nearer the poles than the equator, a particular marijuana variety flowers much earlier in Florida than in Vancouver. It begins to flower when a critical dark period is reached. For most varieties this seems to be between 8 and 12 hours of uninterrupted darkness daily.

Why should I force-flower my 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.

When should I force-flower my cannabis?

Cannabis plants typically don’t start flowering until August/September. This is a result of the natural reduction in daylight hours. Cannabis plants will switch from the vegetative stage to the flowering stage when they start to receive 12 hours or more of uninterrupted darkness. Given that in the majority of northern European countries frost starts to set in around this time, a disaster for your crop could be just around the corner. Not to mention that rainy and humid conditions can cause your buds to mould.

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.

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

The biggest example is how plants will put their full energy into a reproductive effort in preparation for the coming winter. The plants know when to do this by measuring the amount of darkness in each day. The nights get longer and longer in late summer and heading into fall. At some point the plants instinctively know to begin their reproductive efforts. It is exactly this response to the dark period that will give you control over flower forcing.

You can find out a plant’s critical light level by observing plants outdoors to see when they start flowering. Check the length of the darkness period when you observe the first flowers. Indoors, you can start at 9 hours of darkness and increase the dark period by 15 minutes each week. Figure the critical period as 15 minutes shorter than the dark period length when you see the first flowers. Suppose a plant’s critical dark period is 10 hours. This means that it will flower under 14 hours of light per day. This is 2 hours longer than the 12 hour light period which is generally recommended. The two hours extra light each day is a 16 percent increase in light to the plant. This is critical because light = growth.

A shorter dark period may lengthen the flowering period a bit. To get the buds to ripen quickly, the dark period can be lengthened to 12 or even 14 hours.

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.

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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.

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.


By tricking your cannabis plants into flowering early, you can enhance the quality of your buds. All the daylight still prevalent in late July/August will now be focussed on bud production rather than plant growth. Richer, more potent buds can be harvested, all before any undesirable weather conditions start to set in. Increases in UV-B rays as a result of extra sunlight directly supports THC production. Maximising this window is vital.

With careful and strategic planning, forced flowering can enable outdoor growers to have multiple harvests. As many as 2-3 cycles a year. Clever planning means a grow started in the early spring can be forced to flower and ready for harvest in June, whilst another grow running alongside it could be ready in the autumn.

[Updated, originally published 1.8..2017]

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