Vegetation, or the vegetative state occurs after your seedlings have shown their first true leaves, or your clones have taken root. During this phase of growth it is important to supply plenty of light (18-24 hours a day), feed and water your plants regularly, and supply them plenty of CO² via the use of fans (moving air helps strengthen your plants, and supplemental CO² can be added via timed release from tanks, or a simple CO² generator.) You can also utilize various growing “tricks” during this phase to maximize for grow space and ultimately your harvest.
For vegetation flourescent light is recommended, you could use Metal Halide lights but they are expensive and consume a lot of energy. You can also bring flourescent bulbs much closer to your plants as they produce much less heat. When purchasing your bulbs you want ones that produce light at 6300K to produce a nice blue spectrum for your plants.
It’s important to know that TOO MUCH water can cause stem rot, and limits the amount of oxygen available to your plants’ roots. Cannabis cannot tolerate soggy, saturated soil so good drainage is a must. Water your plants from the top when the top couple of inches of soil become dry, if you wait too long your plants will tell you by wilting. In that case be careful not to under water, as overly dry soil has a habit of not absorbing the water, and letting it drain right down the side of the pot. With a little experience you should get the hang of your plants’ need for watering, and most find that twice a week or so is appropriate. This depends on a number of variables (pot size, plant size, humidity of the grow area…etc.) and each situation is unique. As your plants get bigger their water intake will increase, so be aware of the soil and you will figure out your plants’ needs.
During this phase a water-soluble or liquid fertilizer that is high in nitrogen will feed your plants nicely, but you should be careful not to overuse your nutrients. About once every 3 waterings should probably suffice. Be careful with your youngest plants (clones/seedlings) and use a very diluted mixture so that you don’t burn the roots and kill your plants, a dilution to about 1/4 of the normal recipe should do.
Good airflow is important to cannabis plants as it helps to deliver fresh CO² and strengthens the stems and branches. Fans should never be directly aimed at your plants, and ones that oscillate are recommended. You should observe just a little bit of motion of the plants leaves and branches to know that you have your fans set correctly, any major swinging motion could damage your plant, or inhibit growth.