OK...
Marijuana goes through four stages in one life cycle (from seed to harvest):
1. Germination
2. Seedling
3. Vegetative
4. Flowering
Since I assume you are already familiar with the germination and seedling stages, you need to know about vegetative growth and flowering, or bud production
Vegetative Stage
After your plants have been growing for roughly one week, they graduate from seedling stage to vegetative stage. This stage is when rapid leaf and node (branch) growth occurs. Outdoors, the vegetative stage is march through august. During this season the days are longer and the plants receive 18 hours of light a day. The plant will continue to grow in this fashion until the light cycle changes.
Flowering stage
Once the plants have reached your desired height, you can stimulate them into producing buds by changing the lights to accommodate the shorter days outdoor growers use when planting outdoors. The lights for flowering are 12 hours on and 12 hours off. It is important that the plants do not receive light during the 12 hours of darkness. A plant will go into shock if it suddenly thinks the sun has come out at 2 in the morning!
After the plants have received this light cycle for a few days they will begin to show what sex they are. Marijuana is an interesting plant because it is one of the only plants on earth in which a male marijuana plant looks different than a female marijuana plant. When you plant cannabis, it can either grow into a male or female plant. Male plants do not have bud, but they have sacs of pollen that pollinate female plants. Female plants are the most desirable because they produce bud, which we smoke!!! When you grow female plants and male plants together your female plants will produce buds with seeds and your male plants will not produce anything. When you grow female plants by themselves you get only bud (which is what you want). You can tell if your plant is a male when after a week or so of 12/12 your plant will begin to grow little green sacs at the base of the branches. Once you see it is a male, you have to remove it right away. It may be tempting to leave it in because you took so much time growing it, but it will ruin an otherwise good yield. If the plant is female you will see small white hairs coming from the base of the stem where the branches (nodes) begin.
Once you have removed your males, let the females continue to grow and produce bud until the right time to harvest, usually when all the hairs on the bud have turned brown. Once they are brown do not feed or water them and leave the lights off for two days (my method) then chop them down.
Now if the plants are stressed from either heat or cold or having the lights turn on during 12/12, they run the risk of growing into
hermaphrodites. Hermies, as they are usually referred, are a combination of both male and female genitalia. If not removed they will usually pollinate themselves and produce seed in your good bud
Plus, in case you didn't know, marijuana plants exhibit a lot of smell, especially when producing buds, so be prepared for that.