If I understand your question correctly, it is that you are outdoors, and wish to ‘force’ flowering when there is more than 12 hours of natural light. If so, I would think you could regulate your light by covering. My worry is that you might ‘cook’ your girls under black plastic at the end of the day. I would, given my assumptions, cover them at night, and leave them covered int he morning, until you reckon there is 12 hours of light left in a day. Ordinarily, outdoor growers are satisfied with letting the plants flower naturally in the fall. Another option, should they be in moveable pots, is to move them inside for the dark period, thereby elinating the chance of over heating. Most all strains finish flowering in approximately 2 months. There is massive info here and elsewherer to help you determine the correct time to harvest. Hope this helps!