The indoor grower will need to artificially induce flowering/budding in plants by changing the light schedule so the plant receives only 12 hours of light a day, and 12 hours of uninterrupted darkness.
Height: A general rule is that your marijuana plant will double or triple in size during the flowering stage from the point where you first change over the light schedule to 12/12. Some plants will grow more, some will grow less, but a good rule of thumb is to change your light schedule over to flowering when your plants have reached half of their final desired height. Bending, known as "LST" or "low stress training" can be used to control colas that get too tall. Simply bend too-tall colas down and away from the center of the plant. Some growers will even slightly break or "supercrop" branches to get them to bend at a 90 degree angle. For those growing in a small space, height may be the primary concern. However, there are many techniques available to grow a short, bushy weed plant or basically train your cannabis plant to grow into any shape you want.
Lingo: When a grower provides 18 hours of light a day and 6 hours of darkness, this is commonly known as the 18/6 light schedule. For 24 hours a day, this is referred to as the 24-0 light schedule.
So indoor growers have a choice to flower their plants whenever they want… When is the best t ime to start flowering your cannabis indoors?
Giving cannabis plants more time in the vegetative stage, and taking time to train them to fit your space, will give you the best final yields. However, if space is tight, then it's better to switch when the plant is half the final desired height, or even to just attempt to flower your cannabis plant straight from seed.
Most indoor growers provide 18-24 hours of light a day (known as 18-6 or 24-0 light schedules). Giving your cannabis plants more hours of light each day in the flowering stage will encourage faster growth.
The plant can receive as much as 24 hours of light a day while in the vegetative stage. Many indoor growers provide 18-24 hours of light a day (known as 18-6 or 24-0 light schedules) during the vegetative stage to encourage faster vegetative growth.
Source: Maximum Yield, May 31, 2019
Selecting the Wrong Light Spectrum
In order to grow efficiently, plants need different types of light for each growth stage. With full-spectrum LED lights this is easily achievable, but if you select the wrong setting, the plant will not develop well. You should also take into consideration the conditions in the room where you’re doing the growing. For example, plants benefit most from light in the blue spectrum when they’re in the vegetative stage of growth. That means that if you set the light to emit light in the red spectrum, you will only be hindering the plant’s growth.
Forgetting to Adjust the Lights
Many people forget to adjust the height of the lights as the plants grow bigger and end up with burnt leaves. Another important thing is to occasionally rotate the pots in order to prevent the plants from leaning towards the light too much.