We use cement cinder blocks stacked up. Doesn’t rot and doesn’t leach chemicals into your garden soil. Easy to put arches up and cover with plastic to warm beds up in Spring or Extend the season in the Fall.
The simplest and inexpensive way to build a Raised Bed Garden is with wood. The best wood to use is Cedar and rewood. You can make the wood life longer by painting it with an oil-based wood stain.
In this way you can make a life longer raised bed garden.