Big Root Cranesbill (Geranium macrrorhizum) in a border by the polytunnel
Grass makes a cheap and durable path but will creep into borders if they’re not well established or don’t have dense growth.
As an extra barrier, I often line grass paths with clumping boundary plants to stop ‘Grass Creep’.
Here are some suggested plants:
Big Root Cranesbill varities recommended by Penlan Perennials: