Q.Planter Box Sizing
I plan on planting a cedar hedge in a planter box. The mature size of the hedge will be trimmed at 5′-6′ high by 16 inches wide. Would a 16-inch square box per running foot of hedge be large enough? Would freezing of the roots in winter be a issue? Any other advice would be helpful.
The most critical measurement would be the depth of the container. 16" seems very narrow for a 5-6' tree, but if that's what is demanded by your space requirements, you need a container at least 2' deep, I would guess. Here are a couple of articles that will give you some tips: https://www.gardeningknowhow.com/ornamental/trees/tgen/growing-trees-in-containers.htm