I would like to know the same.

I'm a Landscape consultant in Ontario, CAN, and while I can't speak to what was done for this project, one of our recent projects used a stainless steel planter hidden inside the wooden frame. I also know others that use a copper insert. Hope this helps.

An insert works great for smaller plants, here we use bichathane liner so the clumping bamboo root balls can get large enough and it's less expensive than a larger insert. But we also use inserts when needed as my colleague has pointed out.