Answer: I have made beds with landscape timbers on several occasions. The key is to not only anchor them to the ground but also to each other as they tend to warp over time.
First level the area so the timbers will lay flat on the soil surface. Lay out the timbers, overlapping each layer at the corners like a log cabin. This will help strengthen the beds. Next drive large 6" spike nails into the timbers to anchor the second level to first (this can be repeated for a third or forth level if you want beds that high). This can be a tough procedure as the long nails tend to bend easily. To make it easier, drill pilot holes, slightly smaller than the nails, into the timbers before driving in each nail. Place nails in 3 places along the timber, but not at the overlapping corners.
After bed timbers are nailed together, drill 3/8" holes downward through the corners of the timbers where they overlap. Drive 3/8" rebar rods (available from many hardware stores) cut long enough to go through the timbers and into the soil a foot deep. Use a sledge hammer and don't allow any rebar to extend above the top timber for safety reasons. Repeat the rebar anchoring along the length of the beds (once for beds one timber long and twice for beds two timbers long).
Then fill the beds with soil and enjoy. Thanks for the question!
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