While the seedling stage discussed above is still considered a part of the vegetative growth, I mention them separately because what the plant is doing and what the plant needs during each is drastically different.
Now the plant has started to grow taller and wider, creating new sets of leaves with each day that goes by. It will start taking in more nutrients and can handle more and more light. Usually during this stage is where I start lowering my lights closer to the plant to increase growth.
During the vegetative phase the plants prefer a blue or “cool” light color. If you look at the box the lights are in at the store, whether they are a CFL (compact fluorescent) or HID (high intensity discharge) bulb, they will have a kelvin rating or “K” on the packaging. This tells you what color the light is and whether or not the bulb is the best choice for your particular growth phase.
Note that HID bulbs come in two different types, high pressure sodium (HPS) or metal-halide (MH)
For ideal veg growth, you will want a bulb around 4000K. More than likely this is going to be an MH bulb because an HPS is an orange light and falls at the lower end of the color spectrum (2600K). A little more or less than 4000K will be ok. You just don’t want to go outside 3600K-4400K in my opinion for the most ideal veg growth.
In reality, you can use any color bulb between 2400K and 6000K, but the range I specified is the best for healthy veg growth. When using an HPS bulb (2600K), I’ve noticed that the plants stretch upwards more towards the light. You want your plants to stay short and stocky to help it cope with the insane weight of the buds later in the flowering stage!