Location and variety flower time frame. You would want to run them as close to 24 hr before placing them outside. The shock of 24hr to a 12 to 13 hr natural light will induce flowering. As spring goes on the day light will increase until the summer solstice June 21. Research your location with average day light hours, if you have a fast variety you might be able to do this. Or to stop the plant from revegetating because of the increase of daylight you can cover your plants to provide total darkness. Being careful not to harm the plants, use a frame or box to place the covering over.