Forcing cannabis to flower: How to speed up the flowering of outdoor marijuana plants

For outdoor growers, the question of how temperature will affect the flowering time of their marijuana plants is a big one. Especially in some places where fall can come early, and September might bring temperatures in the 30s! The flowering phase doesn’t usually begin until mid-August (or later), meaning that the harvest won’t actually happen until mid-October.

This makes the temperature question a very relevant one: is a changing temperature going to make your buds mature more quickly and increase the production of resin glands? Or is it going to stunt growth and keep the buds and plants from reaching their full potential?

Use short-season strain

Marijuana plants fall under the “short-day plant” category, which means that their flowering period is initiated once they have received uninterrupted darkness for a long time on a daily basis. This also means that other factors like humidity or temperature changes do nothing to trigger this flowering phase in your marijuana plants.

short season marijuana strain fast flowering

That being said, humidity and temperature can have an effect on the overall flower growth that your plants are able to achieve. For instance, lower temperatures will slow growth, and a very dry environment is going to speed up the maturing process, therefore making buds that are smaller in size.

Download my free marijuana grow guide for more speed growing tips

When fall comes, the amount of sunlight decreases and the temperature drops, bud growth slows down. This is simply because, with less sunlight, photosynthesis cannot carry on at its normal efficiency. The lower temperature has an effect because it diminishes your plant’s respiration processes, making it more likely to get things like fungal infections. If your plant matures earlier, it will most likely have a high yield in the end – so you will probably want to take certain steps to hurry things along.

force flowering outdoor marijuana plants

Any kind of marijuana strain that is known for having a shorter season (Indica) will begin to mature far before the late-season varieties (Sativa). If you have your sights set on a future harvest, this will be the best option for you. You can also use autoflowering marijuana seeds. They grow from seed to bud in only 10 weeks! Read the article Best seeds for your climate for more information about strains per region.

Force flowering

This is a technique used by many growers for all sorts of marijuana strains. You can force the flowering phase to begin earlier by ensuring your plants receive no light for at least 12 straight hours. Cover them with a cover of some sort, such as a tarp that is lightproof. You could begin this process as soon as the end of July. Simply shade your plants in the early evening, then remove the cover every morning.

Autoflowering cannabis seeds

Buy autoflower seeds

  • Flower automatically
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If you do this every day, then it will trick your plants into flowering several weeks earlier than normal, meaning you could harvest as early as mid-September. You can stop covering these plants in late August since the amount of natural darkness is most likely long enough at this point in the season.

All marijuana seeds are not the same. If they are auto flowering seeds, they’re going to grow a lot faster than regular options.

Having your plants flower earlier is going to give you another advantage: the buds will be bigger and tighter. This is because they will start blooming during a period when there is stronger sunlight, meaning they have more light energy at their disposal to focus on growth. The more UV-B rays, the higher the THC levels. You won’t risk mold at this time of year, plus there is the added bonus of almost no one looking around for plants to steal when it’s this early in the season.

Our free little Harvest Guide will help you determine the best moment to cut your plants. 

Pollinate the flowers

This is an option you may not have thought of before, but will allow you to harvest far earlier than normal. If you pollinate the flowers, you can then harvest those plants just one week afterwards – right as your fertilized plants are beginning to make seed pods, but before the seeds are actually formed yet. This is done primarily by growers who are avoiding an early frost that could kill their plants.

pollinate marijuana flowers fast flowering

One advantage of this method is that you can make the decision to do it fairly last-minute. If the first frost happens at the end of September, for instance, and your plants are still 3-4 weeks out from reaching maturity when it’s already mid-September, glaze the flowers with pollen that you already have on hand.

Also read “Should you defoliate autoflowers?

Right when the female flowers have been pollinated, the plant will focus all its energy on seed production rather than growing new flowers. You will notice the pistils drying up and receding into the calyx, which is a sign that the ovary is turning into a seed. You will be harvesting the bud before the seed even gets a hard shell around it. Because the glands will be complete at this point, the resulting buds that you harvest are going to be ripe and firm. More information about how to pollinate in the article How to make marijuana seeds

FAQ about outdoor marijuana strains

What are the best outdoor fem marijuana seeds?

Here are top 5 outdoor feminized marijuana seeds: Girl Scout Cookies, Super Skunk, Jack Herer, Critical Kush and White Widow.

What are the best outdoor autoflower marijuana seeds?

Here are top 5 outdoor auto marijuana seeds: Gorilla Glue Auto, OG Kush Auto, Amnesia Haze Auto, Zkittlez Auto, Northern Lights Auto.

What are the highest yielding outdoor seeds?

Here are top 5 highest yielding outdoor seeds: Gold Leaf, Sour Diesel, Strawberry Kush, Super Silver Haze, Chocolope.

Thanks for reading. Please leave comments or questions below and don’t forget to download my free grow bible


PS – If you have a website on cannabis, like Cannabis Seeds NZ, then you can partner with us and earn money from our marijuana affiliate program

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Mr Robert Bergman PhD


Robert Bergman is an Amsterdam-based marijuana grow expert who has years of experience from small grows to massive operations ... See profile

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27 comments on “Forcing cannabis to flower: How to speed up the flowering of outdoor marijuana plants”

  1. Hi George,

    Thanks for your comment! That’s good to hear:)

    Keep us posted on the yield turns out!

    Kind regards,

  2. Ps I forced it into flowering by covering it with a very big tarp a couple of hours before sunset and removed it just before sunrise and this worked great.

  3. My plants are a whopping 12 1/2 ft tall 5 1/2 ft in diameter my colas on each plant are 40 colas on top and way over 100 colas all the way plant is a indica sativa hybrid and very beautiful I will post pics next week how many pounds should I get out of this?

  4. Don’t throw away your moldy bud. It can be used to make B.H.O. the mold spores do not transfer with the solvent and the finished product, once purged, is safe for consumption.

  5. My sprouts are ready to plant and are showing signs of flowering. Is there any way to stop this and revert to vegetative growth?

  6. So once the 12/12 forced flowering has been initiated and plants have stopped vegetating, forced flowering must continue until harvest?

  7. You can harvest now .10days wi just take aliltle thc away but not much if u let go u could loose it all .id pull it n hang it in a dry area .

  8. I think i got a autofower i planted it in late may n now have buds on it .can i let it go till first frost or later if i bring it inside n will the yield be more .

  9. Once again thank you for all the valuable information you pass along I very much appreciate it and consume every bit of it have a great day you guys are awesome

  10. What if you put your plants in complete darkness to trigger bloom? Will it make them bloom and produce bud or do they need at least some kind of light once a day?

  11. Hey I’m completely new at this, how can I give my plants 12 straight hours if they are outside and extremely to big to bring in. Please help me!

  12. What determine flower the white hairs or bud formation? How long after u see white hairs

  13. I live Southern Oregon, USA . In the last month we have had a lot of rain and little sun and I have already Suffered Significant amount of mold, especially on the Gelatos.. Thankfully i was able to harvest some, but Threw away a bunch. I’m starting to see more mold on the top kolas of LSD and Bruce Banner. I am at least 7 to 10 Days away from Harvest.. i shake them off gently, what else can i do at this point. They are too tall to put Plastic over.

  14. I’ve been growing outside 30+ years , and have experimented with tarping ( cutting daylight hours back below 12 hours per day ) ,, I currently am tarping ,I have used heavy 55 gallon plastic garbage bags the last 4 years and it works quite well just make sure you get them off first thing in morning before sun starts hitting them directly , the only problem is once you stop covering them they will revert back to vegative stage , which can ruin bud growth , so once my plants start flowering I’ll keep covering them for a couple weeks , then I’ll do it every 3rd day for 3 days start then 3 day break , it seems to keep them in flowering stage any other growers experience similarities?

  15. Shane,

    Once flowering has been induced, some growers claim that they revert to 14/10 and continue to flower. This is risky however. Every strain will react differently to photo period change. If you force flowering, then I recommend not changing the photo period until there are at least 11 hours of darkness to be on the safe side. This suggestion also depends on your geographical location; i.e. how long is your longest day. For further discussion; Join us at:

  16. Can i trick my plants into flowering in mid June and then stop the 12 and 12 once they start to flower?