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How to build a blackout shelter for outdoor plant to induce flowering


New Member
Looking for some advice for growing a single high quality Jack Herer (non-auto) in my greenhouse. If all goes well, I’m expecting this Sativa dominant plant to reach about 8+ feet in height, as it will be planted in the ground. And probably about 3 feet in diameter. (Does anyone here have any experience with Jack Herer’s grown in a greenhouse to confirm these sizes?)

The main issue I’m dealing with is the late 12/12 photoperiod here in Canada. Our longest day of the year is 16.5 hours of daylight on or about June 21. The natural 12/12 photoperiod doesn’t begin until Sept. 21. That would put harvest somewhere around Nov. 21 which is way too late this far North. I will need to induce flowering around mid July in order to be able to harvest by mid September (Jack Herer is given as 8-9 weeks flowering period under good conditions).

So I’m looking at constructing a removable DIY blackout shelter for the plant in the greenhouse. I would be covering the plant at most for 3-4 hours a day in July (<3 hours in August and maybe 1-2 hours in September), either before sunset, or just before sunrise. I understand that transpiration is very important for all plants, so I’m hoping that 2-4 hours per day of this type of restricted airflow/transpiration environment would not harm the plant. (Does anyone have any experience with this?)

The DIY idea I had was to construct a giant cover (9-10 feet tall, 3-4 foot diameter) out of heavy-duty puncture-resistant 4 foot tall XXL garbage bags, overlapping them and taping them at the seems. I wouldn’t mind spending $100-$150 for something already made, but I can’t afford to go much beyond this. (Are there any reasonably priced commercial products out there to serve as blackout shelters for something of this size?)

The giant patchwork garbage bag would be placed on a single pulley system (with taped hooks) suspended above the plant and attached to the roof of the greenhouse. This would be used only to raise the height of the giant cover as the plant grew. A secondary (triple) pulley system would then be used to lower and raise the edges of the the giant patchwork cover every day to fully enclose and reveal the plant.

Any tips, ideas, or comments would be welcome.


Grow Journal of the Year: 2017 - Grow Journal of the Month: Sept 2017
Hi! It's gonna work probably, but don't you think it would be much easier to cover whole greenhouse with heavy duty black plastic once a day? You could probably get it automated or something with the right technology ;)


New Member
Yes, easier, but way more expensive. Plus the greenhouse is also being used for other things (veggies and herbs) so don't really want to cover the whole thing. Also, I just found out about Panda film, white on one side black on the other and completely light proof. Comes in rolls of 10 feet by 25/50/100. So this will greatly simplify the construction of the blackout shelter.


New Member
Use one corner of your green house and corner your plant with black plastic from the ground all the way to the top of your green house ceiling. Use a black piece of tarp on top of your green house to cover your cornered off plant on the out side. You have to make sure no light is entering into your corner. Maybe use Velcro to secure your corner to use for access to your corner , to water and feed. Duck tape can be used to secure most of the plastic. For the tarp out side I would use a roll type of system like PVC on a swivel and pipe coming down to roll it up. Sort of like how a dump truck covers it's load using a tarp.

It is just some thing I was thinking of. IMO
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