I made some concrete planting rings before. If I remember correctly, I basically made two circular forms out of green fiberglass landscape edging. I made 4 quarter circle bracing forms out of 2x4s to hold the shape of the inner ring, then cut notches out of scrap pieces of composite decking to clip around both rings which held the rings’ shape when filled with concrete. I don’t exactly remember all the details, but here is a link with a picture. Looking for an inexpensive tree ring or edging material - #13 by ztom
I made several in my garage and they worked ok. Some stayed in one piece, but some broke in half, probably because I took the form off too soon. They still worked though, I just put the pieces together in the field. Eventually I made the notches a little wider in order to make the ring thicker. Also, I eventually just took the forms outside and poured in place. This worked a lot better.
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