Be sure that your light cycle is synced up with the natural light cycle before you put them outside. Having them at 18 hours and then suddenly moving them outside when you only have 15-16 hours of light will cause them to start to flower most of the time. You can adjust them at MOST one hour per week.
So first, pick your planting date, figure out how much light you will have at that point and that will let you know when to start reducing their light cycle. For example, if you have 15 hours of light at the end of May, run them for a week at 17 hours, then a week at 16 hours, then a week at 15 hours, then put them outside.
This is a VERY important step that is for some reason almost always over looked by most growers.