Two pegs and a string line.

Mulch is in place already - want to straighten out the lines and keep them nice - thinking edger?

I know that there are gas powered edgers commercially available at any big box store. The challenge is steering them in the right direction. Perhaps, similar to Ink's suggestion, a wood board with pegs on either end to act as a guide.

Alternatively, look into steel edging for crisp lines that would perhaps require less mechanical edging maintenance. May cost more upfront, but may save you more time in the long run.

- Audric