this debate has gone one for ages on multiple sites. and even after all these years, the same faulty logic and personal bias shows its head.
There are no absolutes. so anytime someone says something like 'its a fact, you can yield more with a vertical/horizontal bulb', i shake my head.
vertical bulbs give you a much LARGER canopy area (potentially) than a horizontal bulb would. This IS a fact that cannot be argued with.
However, larger surface area does NOT necesarily mean MORE yield. If yield were a purely a function of increased canopy size, then the best grows would be 20x20 400watt bulb grows with 1000 plants. Obviously, this would never work because the INTENSITY of the light would be greatly diminished. this is why, many jedi growers continue to kill it with horizontal as opposed to vertical. Horizontal lights aim MORE intense light at plants than a vertical setup. This also cannot be disputed. Which yields more? It depends...
Large yields and efficiency often come down canopy management and environmental control. There is a balance between canopy AREA and light intensity that greatly effect yields. This can be messed up or done well with both with vert, horizontal, or a combo of both. Just because someone doesn't yield as well with Horizontal as they do with vertical DOES NOT make this true for everyone.
I read people in this thread and claiming how 'if grower x turned his bulb the other way, he would increase his yield'. That to me, is a very naive assertion as there are MANY growers neck and neck with top yields from BOTH methods.
Heath robinson, the internet legend, and now seed vender (not lending much to his credibility anyway) has journaled BOTH flat and vertical grows. If there were a clear cut 'Vert is better than horizontal' fact, why would he even bother with flat grows?
Why do jedi growers continue to kill it with both methods if there is ONE superior method? Why is it that all over the internet on the major sites, many who have jumped to vert grows cannot live up to the hype despite elaborate setups that cost a ton, and require a ton of maintenance
On another site, there was a journal side by side from a good grower with a horizontal vs vert setup. The results were NOT conclusive as to which method did better but it tells me alot that his next grow he opted for horizontal.
Hopefully, we as a community can keep an open mind and try to discover NOT which method is superior. But instead, we should be fact finding to see what factors can maximize the yields for our situation.