How to determine sex

A friend of mine has a couple plants growing under T12 s, there on a 18/6 light schdule, and he asked me if he switched them to 12/12 would they show there sex before planting outside, he has about a month to go before the temperature will allow to plant outside

I would assume if you are determing sex, that the grow is from seed. Unlike clones, which are already sexually mature, seedlings dont become sexually mature until they reach a certain age. So if those plants are at least a month or two old, switching to 12/12, they would go into bud within the first 3 weeks I reckon, determing sex. The only problem is making plants bud, then reverting them back to veg mode takes a huge toll on the plant, and it will slow down the growth for several weeks. The best way to deterimate is take 2 cuttings off of each plant and put them in 12/12, just rembering which cuttings came from where, then you can match the males and throw them out. Me when I grow from seed, I take the extra effort and just grow my males with my females until they show sex naturally outside. hope that helps, happy toking.

Stoneman thank you so much for the information and that is a good suggestion, I wish I could have used clones from the beginning, but I did start from import seeds but not feminized seeds, I just dislike caring all season for a plant and have it turn out to be a male, and LEO grades it the same