the preflwers are very tiny immature flowers, usually 2 at each of the lower nodes of the plant that at first can be hard to see or tell between male and female. I've never read anything that really differentiates between flowering and preflowering, but I'd have to say that when vertical growth slows down and the flowers start to be larger and appear moremature. On the female flower if you can clearly see the 2 stigmas or "hairs" protruding from the calyx and they don't just look like tiny fuzz, they're flowering. Probaby 1-2 weeks after you see preflowers. Mine just began flowering this week. 18 plants, 9 bagseed, 9 are blueberry x ak-47 ( 7 out of 9 female so far). I'm terribly impatient, lol.