I'm living in Southern California. I start indoors in early March and get them outside by April. Indoors my lights are 18/6 but outside daylight is only 13 hours. The plants do fine. They don't stress or start to flower just because there isn't enough day light hours. Each day the daylight gets a little longer and I believe the plants can detect this. Either way it's perfectly safe to plant them now. I've been doing it for years.
As for growing seasons for cannabis... there's one long veg season (spring through summer) and a short flower season (sept to november). The long veg season allows you to stagger your crop. For example, you can plant a few on April 1st, then a few more on April 15, then on May 1st, and again on May 15. You can actually keep putting them out all summer long, but it gets trickier. Young plants can't always take the high temperature of the summer heat. And of course the plants that are started really late don't yield as much, but hey, some is better then none right?
