A plant stays in vegetative state if your provide it with more than 14 hours of light a day. Aproximately. I think.

When you decrease the light cycle, the plant starts to flower, because this is how it happens in nature. As winter approaches, days get shorter (more darktime), and plants bloom.

There's debate as to wether it's the "less than 14 hours of light" that causes flowering, or is it "increasing dark-periods" that cause the flowering. I'm a newb, so I don't know. But, 12/12 = flowering.

Best,

-turtle420
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