Border timbers/logs? 

 I used old rail ties at my last house - they were neat 

 🤔 nice. gives me a few ideas 

 I took them away from my front flower beds and just use an edge tool and mulch, but I do see the appeal of having a solid border of some kind to block light because the grass is less likely to run under them into the beds. I suppose a low hedge row would work for the same reason. Or cardboard but it’s less pretty haha