There are several threads about this exact topic someplace here on TNDeer.
I saw some Tennessee Gardening show on public tv and they called these raised beds. They used round poles. To extend life, the beds had a waterproof liner safed from the wood by a 1/2 IIRC dirt-like-plaster covering. I do not recall how they dealt with excessive water. The ends were square cut, the planters were maybe 2 feet tall, 10 or 12 feet long four feet wide. The liner was nailed to the second-to-last pole and trimmed to be hidden from view Dirt was kept just at that level.
The pole touching the ground was sitting on a bed of gravel had holes drilled in it every n feet and they pounded rebar about 2 feet down. The poles above were each nailed to their neighbor. These are supposedly all over Nashville.
The claims for longevity were hard to believe: I think 16 years!! And no the wood was not treated but I think many were painted.