10-14 days after starting the 12/12 light cycle, female marijuana plants will start to develop pistils, they look like white hairs in this photo, at bud sites.
This closeup photograph shows the pistils on a flowering female marijuana plant.
If you grow marijuana, either hydroponically or with soil, the male plants must be removed before pollen is released.
If allowed to release pollen, a male marijuana plant will pollinate all the nearby female marijuana plants. Pollinated females will use energy making seeds, rather than producing THC.