Use this two-step process for a neatly edged garden bed that keeps both lawn and mulch in their proper place.
For a neat look when edging a garden bed, do the garden "two-step." All that's needed is a sharp flat-blade spade.
First, face the garden bed and slice into the turf repeatedly until you've outlined the entire bed. Do not remove sod at this point. This is your opportunity to reshape the bed, changing the contours to make room for plant growth or to simplify mowing.
Once the bed has been edged in one direction, it's time to finish the job. Face away from the bed and slice into the turf at an angle to sever the grass clods. Remove these clods and compost them.
The bed will now have a very clean edge with a V-shape trench. It not only looks good, it also slows the migration of grass roots into the bed and prevents any mulch you put down from washing into the lawn.