What is your question about this project?

Cedar does not last near as long as popular belief has it. Redwood will be hard to find and very expensive. Probably a special order item.

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I wouldn't use wood. You might consider something like poured concrete or brick... or landscape stone of some sort (bluestone slabs on edge, for instance).

But another option is to get the kind of marble or granite slab that is used for countertops - even the manufactured materials of that sort are good.

There is a marble/stone shop near me that always puts its off-cuts into the alley and a lot of my raised beds are sided with long narrow pieces I got there for free. But for that price they don't match, and I work with the sizes I get, or the odd time pay to have it cut (we've also done some cuts with a diamond blade).

If you wanted to pay for it, you could order pieces to size and pick a material with an optimal thickness. The regular thickness can be a bit fragile in some applications - and long thin pieces are that application. Obviously it needs to be held upright in place somehow too.

Finally, isn't there something on the market like a plastic wood? Trex or something? Can that be used in soil?

Karin L