I think i have 13 beds, so I had to go with pine. I may replace them with cedar next time, or as they rot! I used 2x12’s too. One bed is 24 feet long.
I did too, as I needed it for the bare spots the dog kicked away elsewhere in my yard, else I would have left it. I did throw scrape pine, corn stalks, yard waste and pruning scraps on the bottom of many of the beds. A little hugelkultur bed.