Yeah, the joy of growing in a box. That's what I was thinkin' too as far as the numbers.. 3 plants will be just right for the box. 5 plants after vegging under MH for 4 weeks pretty well fill it up; and requiring way too much trimming for the remainder of the grow. Based on 300g total with my 600 watt HPS, the yield last grow was .5g per watt (9 weeks flowering would be .25g per watt/month). My canopy height was varying too much too. I'm also going with the same strain for all 3 plants this go around and will be topping as necessary for consistent height.
Also built another veg box to speed things up a bit. But I'm hoping to be outta boxes within the next year, way to restrictive.
Peace, Farmer Rich
