I'm building a plantar off the front of the garage that will have a poured concrete interior with a stone veneer on the outside. The highest point will be 30" and the the narrowest point will be 24" across including the stone (2-2.5" thick stone). I'm trying to achieve the widest opening possible while maintaining a strong wall. The cap will be a poured concrete form (stained and sealed). What's the thinnest I can get away with for the vertical concrete?
4" would be the absolute minimum for any concrete application. I would prob go at least that to be safe. Plenty of rebar as well.
But since this is a planter, why dont you just pour a 8" footer, use 4" concrete block to build the walls, stick the stone right on the block and call it a day. Way less work then what your describing.