Marijuana and weed plants experience lush growth in foliage during their vegetative state. Using lights, you can set the veg phase by giving your plants 18 hours of light and you can choose when these plants flower and produce buds by dropping the light cycle down to 12 hours. Knowing when to switch your light cycle is crucial and always dependent on your setup. I use the height of my grow tent as a reference to when to flip and I always assume that the plant will double in size in its flowering phase.
Basically, the formula for flipping or switching is as follows:
Take the height of your tent in inches and subtract 18 inches for the light. Then subtract the height of your pot(the distance from the floor to where the stem of your plant starts). Now take this number and divide it by two and when your plant reaches this height, you want to flip or switch the light cycle from vegetative to bloom.
All it takes to switch the light cycle is dropping the hours of light from 18 hours to 12 hours. I use a timer to get minute by minute accuracy and after about two weeks, you could start transitioning in bloom nutrients and transitioning out vegetative nutrients. It’s important to make sure that when the lights are off during the bloom phase, it is complete darkness. Any interruptions with light may cause them to stay in vegetative phase or worse, turn to hermaphrodites.