That's awesome, I was already set on 1 block spacing but that def seals it for me, sounds like I'll set up a pine farm in an area where bears spawn...
Yeah thankfully I figured out they stop growing pretty quickly into testing, our home sits directly on top of spawn and it was dead of winter so no wonder they weren't growing! I still needed to establish a source of firewood/charcoal so my solution was starting a path highway which goes 10,000 blocks south into a Mediterranean climate where I can grow Pine year round.
Didn't know about growing underground though, I thought with greenhouses being a mechanic that natural light was required for all things to grow. Now I just want to build an underground farm for fun.