labifleanswered grow question 6y ago Hey gardener! To know the sex, you need to have branches, and you have to block the light to one of those branches so that, being in the dark, it starts to flower. It's an old trick that my former roommate gave me, but I always found it useless since in 12/12 you just have to pay attention and remove the males. However, it's up to you to find a way to stop the light on the branch. Good luck, and I wish you lots of females; we love females!