Put a black heavyduty garbage bag over them for 12 hours. NO LESS must be at least 12 hours, or they wont flower. a LITTLE longer then 12 doesnt hurt too much.
I would suggest using 2 bags, cutting the bottom off of one and tapeing the other bag to it. Other then that I guess you would have to move them inside or use multiple bags, slide 1 on each branch.
I have the same thing outside and asked the same question, actually made abox out of thin plywood(light) and lined the inside with black plastic bag ,then asked the question and a few people told me to not do that if outside and let nature take its course,if its possable to keep outside,that the resilts should be great ,im in LA,dont know how weather is where ur at,if it makes a diffrence,just thought id put that out there?
You don't have to put the bag on it for 12 hours straight. Say the sun rises at 6 AM, you put the bag on at 6 PM and later on that night (when it's fully dark), take the bag off.
I'm surprised nobody is asking why he even wants to force a 12/12 outside. Maybe he thinks he has to to grow? And is he in position to actually do it every day at exactly the same time? RASCALONE might be right, let nature do it.