To force flowering you need to cut out all light for 12 hours a day.
Your going to have to cover them, depending on how low-key your operation is that could be difficult. Maybe hang some tarpaulin over/around them every evening?
you could do what the guy above me said or you could just wait for fall to come and nature will trigger flowering for you. I think that is your best bet
You definitely can wait or make a frame out of plastic and spray paint the inside black and cover them @ night for 12 hours. when i force flower mine i do it around 7:00p.m.-7:00a.m. for two weeks and let the sun finish them.
If you do cover them with bags or w/e, it'd be best to remove the bags once it gets dark. This way you don't have to wake up at 6 in the morning to get your plant its full sunlight.