While it is possible to flower with the veg unit - you will have issues. Plant support is a problem, but more importantly, res access becomes an issue. It's hard to get into the bottom res to check your P's, res level, and then to add nutes and PH, etc... If you want to use just one unit - build the MiniStink Flower Unit. I just built one for my sister and it is the bomb! Drill it with 3 holes per 2' post - use as many sites as you want (just 1 would be cool!)
To produce a pound from one plant, you will need to veg for several weeks (months) (as Matty pointed out). The exact time will depend upon the strain and the environment it is grown in. Generally - to get a pound off one plant, it will need to be 6' minimum and even bigger around. Most people can not grow such monsters indoors due to space constraints. The reason the Stink system has 7" spacing on the plant sites is for efficiency. Using more, smaller, closely spaced plants, we are able to reduce our veg time to a few weeks or less (some go zero veg!). Not only can we produce that pound of bud more quickly than veging out a single plant for an extra month, but the quality of the bud is generally better. There is a direct relationship between the productivity (and usually size) of roots to plant mass. In short, 10 small plants with a large main cola each, will have a higher ratio of productive roots to plant mass, than a single plant that has been topped and grown to produce 10 colas - hence the potential increase in quality. In the preceeding scenario, we would save approx 3 - 4 weeks of veg time using 10 small plants vs 1 large plant.
Having said that - there is a growing trend towards large plants indoors with lower plant counts (likely due to MMJ allowable plant counts, etc). Growing larger plants certainly has advantages too! You can move your op quickly, you have fewer plants to tend to, and growing trees is just plain cool! Many contend that more mature plants have more mature flavors too... I am not sure about this. I think that the makeup of the final dried product is better when the internal branching of the plant is smaller.
Currently, there are only 2 real options for gassing: Use CO2 from a tank, or generate CO2 by burning gas or propane. Search CO2 generators to see options to tanked CO2.
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