Those are rough guides for good growth and such, sure you could use less or more than that, but it will effect your yield. the sun puts out like 100,000+ a square foot on a sunny day in direct lighting. the plant can for sure use more that just 7.5 thousand per square foot easily.
That is not what books on the subject say. They list 7.5 as the upper limit of plant utilization (marijuana growers bible).
Its easier to deal with watts/foot or some other measure as there allot of factors that go into the footprint of your bulb. Chiefly, your hood design. All hood are not created equal. Therefore, people often state a size of a grow area instead of lumens. I wish people would stop talking in lumens and just talk lux because lux is the only thing you can measure, and when you get down to it,is the more important of the two. Your 9.5k bulb will fade to about 7.5k in about 6-8 inches.
Yeah, your bulb gives off X amount of lumens.. how many of those lumens are actually reaching your plant though? That's the really important part. Lux is the common measure of light intensity that you can actually measure. 1k lux equals ROUGHLY 10k lumen. I probably spent 3 hours with my lux meter and a grid finding the best height for my hoods and the size of my SCROG net.
When dealing with these issues it is best to consult the published books on marijuana cultivation and setting up grow rooms. If you pick through forum responses you will inevitably find a piece of information that is not really correct, or partially correct..thus making your decision making process more difficult.
Based on my literature review..
Optimum grow space per bulb
400 watt system 3x3
600 watt 3.5 x 3.5
1000 watt 4 x 4
Most efficient duration for veg growth = 18 hours a day. After this point, the ability of the plant to process light and produce sugars fades rapidly. 18 hours is the point of diminishing returns. You can piss away your money on electricity but those extra 6 hours are getting you less and less return as you go past 18.
Most efficient Co2 = 1300 PPM. The rate of photosynthetic rate increase with increased co2 saturation is not linear. It increases rapidly at first, levels off, then increases again with temperature. The rate of increase pretty much flat lines at 1300. From 1300 to 1500 there is a TINY increase but to me (because I have an advanced ppm controller) that extra 200 is kind of wasted.
just my 2 cents. Some people want to provide the ultimate growing environment and some people are growing plants in a computer case with a single CFL bulb. It's all good in my book.
