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Force Flowering: Outdoor Light Deprivation Technique Explained

If you have ever grown cannabis plants outside, you may have felt like you were in a bitter race to see your plants reach their finish stage before the sun really starts to fade away, or even worse, the rain and morning dew arrive causing that fatal bud rot. Northern Californian farmers who have been working with cannabis plants on a large scale for three or more generations have come up with some fantastic ways to get their plants to finish and ready to chop before these very real problems set in.

Pic: SappFire Seeds in NorCal with light deprivation in a polytunnel.

The thought of losing a crop to mould is enough to put even the keenest of gardeners off growing some of their favourite herb. The disappointment diminishes so much of the excitement you had throughout the three or even four months of growing such a glorious plant when you realise you’ve got a rotten jar of flowers.

So how do we hack the summer sun to ensure we get top quality cannabis without the end of summer risks? 

Northern Europe still has 16 hour days in the middle of June and start of July but 12 hours of light and 12 hours of darkness is the photoperiod that your plant reads as a signal to start flowering and produce seeds to ensure the survival of the species. We don’t want seeds in our cannabis but we do want to initiate that flowering process earlier than the start of August because it takes at least 8 weeks of that 12/12 light to reach full fruiting maturity. 

So, we need to force our plants to flower earlier than they would naturally do so. There are a couple of ways to do this. It does require a little bit more effort than just going out and watering your plant every day, so make sure that you have the time and dedication to do this every single day. 

How do we create 12 hours of darkness?

The first thing to do is pick the rough date you want to harvest. Let’s say August 31st for example. We then count back 8 weeks from this date (or our recommended flowering time) and just to be safe incase we need to give it any extra time, we add on a week putting us on 29th of May. So by 29th of May you need to have a plant that is as big as you would like to start flowering. But how big should that be? 

The size of your plant needs to match the size of the cover you are going to use to trick your plant into thinking it’s flower time. You will need at least 4 weeks for your cannabis plant to be big enough before you think of starting your forced flower process. Your plant could stretch 1 – 3 times the size during flower so take this into consideration. 

If you are growing a very small plant or a few small plants you might find that a dustbin is perfectly good for putting over the top of each plant for 12 hours at your chosen time. The risk you have here is if the plant over grows the size of the bin either in width or height. There’s also not much ventilation for your plants to respire and expire which will make the moisture build up inside. As you can see this isn’t that ideal for anything that big, although it does prove the concept if you just want to give it a try. But we need to be a bit smarter for something to get any yield. 

If you are growing something a little bigger, or a few different plants, erecting a gazebo and making it light proof from the inside and moving it over the plants each night at the time you have chosen can work. 

Pic: @FireKeeperFarms with a medium scale light deprivation using covers manually.

You may already be growing in a polytunnel or greenhouse? If this is the case making a covering out of light blocking material such as dark tarpaulin can work great. Just be sure to check for light leaks if you’re using multiple tarps together. 

If you don’t have a polytunnel or greenhouse, you can also pick the plants up and move them into the gazebo, or a shed/garage at the chosen time. The risk you have here is damaging the plants if they become too big in flower. Just remember to be careful at all times, not just for the plants but you don’t want to injure yourself lugging heavy pots around. 

How do I know it’s working? 

Between 7-10 days you should start seeing your first flowers shaping up in the form of the white hairs also known as pistils. Remember to be patient and let nature take its time. And don’t touch the buds! 

You may find that some or all of the pistils turn pink and this can be from a combination of colder temperatures and genetic factors – there’s nothing to worry about though and it looks pretty cool. 

Must Do’s

Once you pick your light dark cycle – stick to it! This is a daily task not to be slack about. Your plant’s day/night rhythm is not to be messed with or else you could risk not getting any flowers OR ending up your plants turning hermaphrodite and seeding your crop. If you pick 6am-6pm at the start, that’s when you will finish so make sure it’s something you are disciplined enough to stick to. You are acting like the timer for your light if you were growing indoors. 

Make sure your dark area is as sealed from light as possible. Check regularly for light leaks by getting inside and looking for light getting in and seal it up appropriately. 

Inspect regularly for mould and pest infestations. Outside these can happen over night (or day) and escalate very quickly into a “is my plant still ok” situation. 

If you’re using a gazebo make sure it doesn’t blow away over night! Peg it down. 

If you are growing seeds that recommend a longer flowering time make sure you calculate that rather than just the standard 8-9 weeks as suggested. 

Always remember to stay safe and respect your neighbours. 

Cultivation information, and media is given for those of our clients who live in countries where cannabis cultivation is decriminalised or legal, or to those that operate within a licensed model. We encourage all readers to be aware of their local laws and to ensure they do not break them.

This post is also available in: French


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