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light is light, and you can use it efficiently whether it is vertical or horizontal imo - but with vertical it looks to me much more awkward to tend the grow.
i think what is important is that you believe in the grow method that you choose, and that it works for you.
i hope you guys dont mind me discussing this with you - othewise it would just be you vertical guys patting each other on the back lol.
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light is light, and arguably, yield too, using it the most efficient way possible is clearly going to pay.
Years ago I read Ed Rosenthal explain how Indoor crops were best arranged as "Stadiums" not "Domes" ... you want your shortest plants in the middle, tall ones on the outside, you get much better yields.
Now, how you achieve this Stadium, how extreme you take it, is up to you. Fundamentally though, going 3d really does seem to make a more efficient use of Light.
Ages ago I grew walls of tall Swazi clones around the 4 edges of a shorter Superskunk crop, the Swazi getting fat on side light that normally goes to waste. Growing in pots of Coco now, it is very easy to position the plant just how and where you want her.
I am now planning a homebrewed experiment in full 3d growing with 2x 600W vertical hung lights and a few different plant/system setups to try, it will be a real challenge, but I really believe it is worth the trouble.
VG, I think you are spot on about "much more awkward to tend the grow" .. that is one reason I am not really happy with anything I have seen on the market.