To switch the light cycle on your LED grow lights to the 12/12 of light/dark for flower is often referred to as “flipping” the plants into flower. Once you make this change to 12 hours of darkness you need to be careful to ensure the dark period is uninterrupted with no light getting to the plants. If your plants are exposed to light during the dark period they will become stressed and your yields will be affected, or you will have none at all.
Once you flip into flower the plants will generally grow rapidly for a period of time dependent on the strain you are growing. This is called “stretching” and it will happen until the plant diverts all energy to growing flowers. The spectrum in well-designed LED grow lights will minimize this stretching by creating a hormonal response using specific wavelengths (colors) in specific ratios.
Part of moving to flower is increasing their feeding through nutrients and also changing the nutrients to be specific to the flowering cycle. Select a reputable nutrient company and follow their instructions before branching out and mixing and matching different nutrients. Some say that LED grow lights cause the plants to use less nutrients but this is not based in any science. In fact the nutrient needs can go up with LED grow lights if the temperatures are correct and the light is the proper power for the size of garden.
If you are flowering these plants for the first time and not going from clone you will need to watch the plants as they develop flowers to determine the sex of the plant. As soon as you can determine the sex you should pull any male plants before they can pollinate the females in your garden, unless of course you are trying to breed your plants.
The flower cycle can take anywhere from 8 to 12 weeks for the flowers to reach maturity and be ready for harvest. Most strains have information on how long the plants will take to reach maturity. It is good to note some plants will finish a few days early under LED grow lights due to the optimized spectrum.