Definitely keep the lights on 24hrs during veg. I know that seems to go against nature but we aren't growing in nature, we are artificially enacting nature within our grow environments, so the same rules don't apply all the time. Experience has shown all who grow MJ indoors that it doesn't need dark time during veg and actually grows better and tighter nodes under 24hrs of light. 18/6 won't hurt but it won't help either, and it will cause the plants to stretch out in search of the light.
The reason that this happens and sooo many people just don't understand this, is because MJ is a high energy plant. It needs a lot of light energy to do its thing. Just like any fruiting trees or plants that produce fruits need a lot of direct sunlight. MJ creates complex compounds within its leaves(THC,CBD) and these compounds take a lot of energy to produce. So the plants have evolved the need for lots-o-light.
If they aren't getting enough light in nature, they will stretch themselves out to get above the competition to get to the light source. If you place several plants together under 1 light source and 1-2 of them are weaker than the others, the stronger ones will choke out the weaker ones by stretching over them to hog up the light. Its evolution, survival of the fittest.
The sun's light is sooo much more intense, encompassing, and invasive than what we can(reasonably) give the plants with artificial light sources. That is why we have to break the rules to allow the plants to get what they need to prevent evolution from taking control of our grows. That is why so many here preach "lumens, lumens!!" The sun puts out about 10,000 lumens per square foot, but in our grows we have found that running (at the very least) 3000 per square foot for 24hrs will suffice for vegging. raising it to about 5000 per sqft for vegging is best(most cost effective) in my opinion for getting the nice rich growth.
Once you are ready to switch to flowering, the plants change gears and really go into chemical production mode and need even more light energy. Now you have a plant that has to have 12hrs of darkness everyday to switch over to flowering but needs more energy than during veg. That is why I bump my lighting up to at least 7000 lumens per sqft. of floor space.
The flip side of all this light that is needed is; just like people who are charging up their daily activities and need more air to breath and cool off, so do the plants. The more light they get the more activity they do, the more "fresh air" they need. I have learned that while CO2 can be very beneficial for larger grows that are very tightly managing the atmosphere of the grow space, for smaller grows, it only adds a complication to the grow that is unneccessary. You have to have good exausting of the heat that builds up from the intense light or everything from plants to equipment suffers. There is plenty of CO2 in the outer atmosphere that you need to bring in to cool the space, so trying to work in extra co2 really doesn't add much benefit to the small grow.
Sorry to go off on a sunlight lecture here :doh: but I hope that it helps you to understand a little more about this really cool hobby that we all so enjoy. There is so much to learn and it is quite fascinating to learn it as we go. Just when you think you got it all down, you learn something new (or more truthfully, something old that you can apply in a new way).
