You can start flowering when the plants are a minimum of about 6 inches high and have at least 4 sets of leaves. This will take 2 weeks to a month of vegetative growth in most hydroponic gardens. When you want to start flowering, cut the light back to 12-13 hours a day.
The height of the plant will continue to increase for about two to four weeks after flowering has been started, after that all the plants energy will be used for flower production.
Limiting the height of a marijuana plant has always been a difficult task for me, so I just raise the height of the light as the plant grows. There are methods of limiting the height of a plant, but none of them have worked for me, so I'm not going to recommend one, the Marijuana Growers Guide has the most comprehensive descriptions I have seen on limiting the height of weed plants.
The flowering cycle lasts about two months, sometimes a little longer, depending on the type of seed. So the total length of time to raise a crop should be 3 to 4 months in a hydroponic garden.
During flowering the dark period must be perfectly dark. No room light, sun light, or any other light should reach the plant in the 12 hours of darkness that the plant must get everyday. The strongest light that should reach the plant during flowering would equal that of moonlight.
Stronger light than this will delay flowering, and if it continues there is a chance that the plant will not flower, but stay in the vegetative phase.
This is more important in the fist 6 weeks of flowering. When the plants have started to flower for over a month, small amounts of light for short periods of time are not as harmful.
You will know the plants are flowering when you see what look like little white hairs developing at bud sites. They should be visible after about two weeks of the flowering light cycle. These will eventually be the red hairs that you see on good bud that you have probably smoked in the past or are smoking now.
