can someone explain why u use different times for lights?

IIRC, the cannabis plant (and most, if not all plants) produces a particular hormone at all times, the name of which escapes me ATM. During periods of light, this hormone breaks down quickly, in a period of a couple minutes. During periods of dark, the hormone is allowed to build up in the plant. When the quantity of the hormone builds up enough, passing a certain threshold, the plant begins the flowering stage. A photoperiod with more than 14 hours of light per day is enough to keep the plant in vegetative stage and prevent the buildup of this hormone. With the light schedule, you're mimicking what the plant would normally receive if it were in an outdoor environment.

 

Whether you're starting from clone or seed, you can begin the light cycle with 12/12 and go straight into flowering. If the height or width of your grow space is an issue, just LST or FIM the plant as necessary. If you're not sure how to do that stuff, there's plenty of info on this site and around the rest of the internet.

 

also, when you switch the lights, the plant will grow RAPIDLY creating new flower spots (between the fan leaf and stem) Sativa's are known to Shoot up like rockets (very thin leafed plants) indicas will shoot up, but not as rapidly as sativas

 

There are basicly 2 stages of the plant the vegetative and the Flowering
the veg stage is when the plant grows from seed to the height you want.
the second is the flowering stage this is when the Weed wants to reproduce and make flowers for the male version of the plant to pollenate.

Veg plants want lots of light you can go 18/6 20/4 and 24/0 with this much light the plant will not flower.

Flowering plants want 12 hours of light and 12 hours of darkness thats when you see people refer to time as 12/12

So Veg plant the more light the faster and more it will grow i go 24/00 and they grow like monsters. I am in flowering stage now so i have switched to 12/12. and all my plants are rocken!!

 

I use MG tomato fert for Veg, and Fox Farm Beasty Blommz for flowering, or FF Big Bloom.

 

I still don't think you've been given the whole picture. Yes, the plant reacts to different lengths of the uninterrupted dark period, but why is what you asked. Because it mimics the sun/seasons, which is what the plant is reacting to.

First off, MJ is an annual plant, meaning that in Nature it is born from seed every Spring and then flowers and dies in the Fall. In Summer the days are long and the sun is more overhead, and the plant has adapted to take this as a signal that it is vegging season, time to get itself big and become ready for flowering. To trigger the plant's built-in hormonal reactions we give at least 16 hours of light and up to a full 24 hours of light. We also use a more blue-spectrum light because that is the more like the sun's light in Summer (directly overhead it filters through less of the atmosphere).

Then in Fall the days grow shorter, the nights longer, and the sun moves towards the horizon. This is when the plant flowers, and to trigger that hormonal reaction we give 12 hours of uninterrupted darkness each day, and we also use a more red-spectrum light because the sun's light, coming at a lower angle in the Fall, filters through more of the earth's atmosphere and filters the color to more of a red (Ever notice how a nice bright sun when overhead turns red/orange when it is setting on the horizon later that same day? The color shift is due to the light coming through more atmosphere).

So the reason we use different light cycles with indoor grows is to provide the triggers that the plant has adapted to in order for it to veg out robustly and to trigger flowering.

And yes you should use different nutes for each stage.