Actually, the days start getting shorter after June 21, the summer solstice. It doesn't need to get to 12 hours of light and 12 hours of dark for the plants to start flowering outdoors. The days getting shorter tells the plant that it needs to flower. I am at the 44th parallel and my plants usually finish the first half of October.
I never really noticed what you would call a "growth spurt" with my outdoor plants
I an not in southern Cal, but have found that different strains start flowering at different times. As one would suspect, indicas will start flowering a bit sooner than sativas.