by Seshata on 04/06/2015 | Cultivation

How to force outdoor cannabis to flower

Cultivation Depending on location, outdoor growers may need to force their cannabis plants to flower. Forcing cannabis plants to flower is particularly useful to growers outside the warm-temperate zones. In the warm-temperate zones, forcing is rarely necessary, as the natural seasonal light cycles are optimal for cannabis growth.


Depending on location, outdoor growers may need to force their cannabis plants to flower. Forcing cannabis plants to flower is particularly useful to growers outside the warm-temperate zones. In the warm-temperate zones, forcing is rarely necessary, as the natural seasonal light cycles are optimal for cannabis growth.

Why do extreme-climate growers force-flower cannabis?

Growers in the extreme northerly and southerly regions of the planet (such as Alaska, Scandinavia, southern Chile and Argentina) may force their plants to begin flowering early. Allowing plants in these areas to naturally commence flowering (when day length decreases to around twelve hours) will not give them enough time to complete flowering by the time the winter cold sets in and plants die off.

Growers in the equatorial zones may also force their plants to flower, in order to make them complete their life cycle more rapidly. In some cases, this can allow for multiple crops to be harvested in one year, rather than just one. Equatorial cannabis varieties will naturally go through a long period of vegetative growth before flowering, and can reach giant proportions. Thus, forcing flowering can not only speed up the harvest, but can also ensure plants remain smaller and easier to conceal.

Why do temperate-zone growers force-flower cannabis?

Providing a sufficiently long period of total darkness is crucial to making cannabis flower
Providing a sufficiently long period of total darkness is crucial to making cannabis flower

In the temperate zones, forcing plants to flower early is generally unnecessary. However, it may be highly advantageous for growers closer to the tropics—in the warmest reaches of the temperate zones, temperatures and growing season may allow for two or even three harvests per year. This is the case in many parts of the Southern USA, as well as in some Mediterranean regions and parts of Central Asia.

Growers in the colder reaches of the temperate zones may be advised to force flowering a little early, as often the growing season in such regions falls two or three weeks short of being ideal. Much of northern Europe, including the UK and the Netherlands, falls into this zone.

Urban growers may also need to force flowering to commence if there is a lack of total darkness during the night-time hours. People growing on balconies on well-lit streets are particularly prone to light pollution in the night-time hours.

How can outdoor cannabis plants be forced to flower?

To force outdoor cannabis plants to flower, they need to be given at least twelve hours of total darkness per day. This is usually achieved by “capping” plants with lightproof material of some kind—Commando cloth or even simple cardboard boxes are common choices. The plants are “capped” at the same time every evening, and “uncapped” at the same time each morning.

A blackout tent used for camping may be repurposed as a cap for multiple cannabis plants
A blackout tent used for camping may be repurposed as a cap for multiple cannabis plants

If possible, using one large cap which contains the entire crop is advisable, rather than capping each plant individually. This makes the job far easier, allows for greater circulation of air, and makes it possible to position fans or dehumidifiers inside the cap. It may also make it possible for the process to be mechanised. Some growers will repurpose blackout tents typically used for photography or camping in order to cap multiple plants at once, but it is also possible and perhaps cheaper to construct a cap by fixing blackout fabric to a simple frame.

When should cannabis plants be “capped” and “uncapped”?

In most locations, providing twelve solid hours of darkness is sufficient to force cannabis plants to flower. However, force-flowering in the equatorial regions may require a longer period of darkness (up to thirteen or fourteen hours) to be successful, as night length is typically around twelve hours the whole year round.

Most growers will uncap at dawn, but in warm temperate zones this may not be advisable—if dawn occurs at 5am, and plants are recapped at 5pm, there may still be several hours of intense sunlight hitting the outside of the cap and increasing temperatures to dangerous levels. Thus, uncapping and capping later in the day (around 10am-10pm) takes advantage of the typically-cooler morning, while roughly corresponding on the onset of night-time. Capping times may need to be adjusted as the season progresses.

Potential disadvantages of forcing cannabis to flower

Capping plants with heavy black material during the daytime can send temperatures within the cap soaring—to get around this it is advisable to use pale-coloured fabrics that reflect heat. Some white and beige blackout fabrics provide up to 94% opacity, which should be sufficient to force flowering while also reflecting significant amounts of thermal energy. It is also possible to use two layers of fabric, black on the inside and white on the outside, to ensure near-total blackout while also reflecting heat. If the grow site is supplied with electricity (balcony grows, for example) then use of fans may also prevent serious overheating.

This four-layer breathable blackout material is expensive but highly effective
This four-layer breathable blackout material is expensive but highly effective

Another issue caused by capping plants is mould. Capping plants severely reduces the airflow around the leaves and soil, and good airflow is crucial for carrying away excess moisture—when left to condense and settle on the leaves and soil, excess moisture can provide the ideal environment for mould growth, particularly as night-time temperatures drop. To minimise this risk, fitting dehumidifiers if possible is a good solution. Choosing a blackout material that is as breathable as possible may also slightly reduce mould growth. If budget allows, specialised breathable blackout fabrics are available.

Another downside of forcing cannabis to flower early may be decreased plant size and a corresponding reduction in yield. This only really applies to warm-temperate and tropical growers—indeed, not forcing plants to flower in the extreme north and south will be ultimately more damaging to yield, and may even jeopardise the chance of getting any harvest at all. But for growers weighing up the pros and cons of multiple harvests per year, this becomes a serious consideration.

As with most aspects of cannabis cultivation, trial and error—and the benefit of experience—provide valuable insights into the requirements of a particular strain, and how best to successfully flower it in a given climate zone.

 

Comment Section

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Kimo

Newbie here at growing. Have plants that grew very close together in my patio dirt. I counted 15 plants and it looks like 7 have all shot up with pistols. From watching videos on YouTube it looks like I've would of been able to tell if it was male or female. Sorry my questions is, are the plants to close and is it still early to tell if it's a male or female?

28/07/2015

Stefanie

Hi Kimo,
Thank you very much for reaching out to us. For these kind of questions I'd like to refer you to our forum: http://forum.sensiseeds.com/.

Best -
Stefanie

28/07/2015

Patio

Weed became legal in my state in July. I am growing a female plant and it's begun to flower, buds are about the size of a dime. Do I need to put some kind of bloom fertilizer on it now? It's a healthy plant and seems to be doing well on it's own. What will happen if I don't use a bloom fertilizer? Thank you!

01/09/2015

Joe dirtyyyyyy

Yo patio... Use the boosters. Use more phosphorus less nitrogen. .. don't forget to flush it out couple weeks before their done. Just water. And you'll be good. Do some LST (LowStressTest) and they gonna bloom alright. I wish I still grew. It was a great part of my life. Gave it up cuz of a house fire. But boy did it smoke. Lol

07/09/2017

brian

Kemo I use miracle grow bloom booster works great

06/10/2015

Tony

DO NOT USE MIRICLE GROW ON YOUR CANNABIS PLANTS!!

06/08/2017

brian

Patio that was meant for u

06/10/2015

brian

This is my recipe for success with fertilizer regular miracle grow through spring and summer every 2 weeks and I would leaf feed with a hand sprayer spray them with the same mixture that u would normally make.when the first sign of budding switch to bloom booster than last 2 weeks fresh water only from the roots make sure to spray at night and till it runs off the plant soak it good

06/10/2015

Nagesh

what about growing with 40°c to 45°c?

11/02/2016

Gaze

Hi, I have just started my second outdoor grow of CBD Skunk Haze and I forced my last ones by bringing them in to a dark cupboard at 7pm each night and putting them back out at 7am. they turned out perfect but it was a lot of hard work as they got very heavy but I managed to keep them low and bushy by kind of scrogging, supercropping ang fimming!! but after having spoke to other growers I am getting conflicting advice as to when the best time to start forcing them was and I am also wondering when capping and uncapping. Do I have to do this right up until harvest or just until they start to flower as I am told that I only have to cap and uncap until flowering then I could leave them out completley. could you let me know if this is possible and also could I start to force them at 7 nodes as another grower has advised, as I need quality rather than quantity right now
I would really appreciate any advice .

07/04/2016

Scarlet Palmer

Hi Gaze,

Thank you for your comment :) Unfortunately we cannot give grow advice or answer questions about it on the blog, but we do have extensive forums full of information and dedicated people where this kind of issue can be freely discussed. You can find the forums here, and if you do not find an answer to your question, please submit it!

With best wishes,

Scarlet

08/04/2016

Bluefin

I would like to know if gaze got an answer to there question?

12/06/2016

Glenn

Yes wen u force them do u have to keep at it by putting them in the dark for 12 hours or as soon as they flower u can leave them alone

07/07/2016

Gunbar

Use of pīpes in both ends of cower will reduce mold risk via natural air flow. Tested.

04/07/2017

jay

I'm having problem getting my plant to flower...it started off outside but I moved it inside for 12 hr nights and take it back outside for light...been 2 weeks now and nothing yet...and it's not a perfect schedule lol but I'm trying

08/07/2017

James

So ive seen these lil growths aroun the inner nodes where the pistols were I was wondering if that was an early sine of flowering

16/07/2017

Scarlet Palmer

Hi James,

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

Scarlet

17/07/2017

Chacho

I know it's just comments but, I have a question:
I have a revegetated plant, and I got it to actually produce a bunch of branches and fig leaves, will it Bud normally like any other plant? And, by the way, I'm using guano in a t form, and I'm also using Alaskan fish fertilizer. I feed them every other day, sometimes I skip a cycle. What kind of fertilizer do you recommend? for when I see that it's starting to Bud?
Thanks

17/07/2017

Scarlet Palmer

Hi Chacho,

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

Scarlet

17/07/2017

butterface

I started my 2 plants from seed may 28th and they barely 1 foot tall. I know 1 is female for sure but not sure yet about the other. When are they going to start budding!!? What's the 1st signs of budding? They have been outside whole time. 12 hrs light and 12 hours complete dark. Idk what to do.... What am I doing wrong? To impatient?? Idk. Help please.

17/07/2017

Scarlet Palmer

Hi Butterface,

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

Scarlet

17/07/2017

cameron

Do i Have to cap and uncap everyday during the flowering cycle or just the first couple weeks?

20/07/2017

Scarlet Palmer

Hi Cameron,

Well done on your grow :) unfortunately, for legal reasons we can't give any growing advice in blog comments, but you can check the Sensi Seeds Forum where a thriving community of cannabis fans exchange information. Good luck!

With best wishes,

Scarlet

24/07/2017

Tezza

Hi I put seed to pot around 8 weeks ago for outdoor grow. These are growing quite big. Leaves everywhere. Feeding them can A and B. The weather gone rather cold now and not sure if they will make bud if this weather continues. Any advice

30/08/2017

bigbang

Hey,i have some female plants which are in pre-flowering stage for 3 or 4 weeks.They are too young,some of them have 5 months but most of them have 3 months.They are tall enough and each one has more than 12 branches.I want them to flower sooner but we have 3 month left to winter.what should i do? Top them,give them more darkness or what??? Thanks for your help.

10/09/2017

Scarlet Palmer

Hi bigbang

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

Scarlet

13/09/2017

BigD

I have a question. Not sure if this the right place to ask but here's my question. I have an out door grow and live in Michigan. My plants have grown about 12 feet tall. Half of my plants have started to Flower and half hasn't. I can't get the really tall ones to start flowering. It's middle September and I have 5 plants that haven't started flowering. After a lot of thinking about it the only thing I can come up with is the plants that haven't started flowering also are the plants that are close to an outdoor motion light and I noticed that "Before" I addressed the light coming on "I turned the light off" the light" the plants were getting light when the light would trigger. So Do you think that the reason my plants haven't flowered is because of the light coming on they out the night? The plants that were blocked from the light seemed to have flowered.

13/09/2017

Scarlet Palmer

Hi BigD

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

Scarlet

14/09/2017

Westside

My outdoor plants are 10 feet tall thick stocks perfect lighting flowering but no buds yet it's almost the end of September in Ontario Canada what should I do? Is it's just a late strain?

22/09/2017

Scarlet Palmer

Hi Westside,

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

Scarlet

22/09/2017

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