glass bottle edging. I've seen edging bottles buried half way in the ground, but I think this depth is nicer. Remove bottle labels, and bury of the bottle. Fill the bottles with pretty rocks or colored sand prior to placement for a more decorative look.
Best4Garden No-Dig Recycled Lawn Edging Economy - Green 60mm - Easy installation with pegs supplied - Straight Shape, also can be shaped to any curves - Safe and flexible plastic edging - Creates a crisp defined separation - Can be used to edge turf, flower beds, brick paths, trees, Gravel, Bricks, Slate or any aggregates driveways and pathways - Available in range of packs sizes. (1, No extra nails) Best4garden http://www.amazon.co.uk/dp/B0119NLCFI/ref=cm_sw_r_pi_dp_wC5kwb17HC85K