Creating a trench around your flower bed helps keep your garden bed looking neat, provides an easy-to-see border when you're mowing the lawn and makes it easier to keep mulch in the garden bed. Adding a trench to your garden bed is an easy and inexpensive way to edge your garden bed. A small flower bed can be edged in an afternoon; larger landscaping projects can take longer.
Lay a string along the area you intend to cut your trench. Step back and check the string, making any adjustments as needed. The trench can be as wide or as close to the garden bed as you desire, but should be uniform in width throughout.
Dig into the soil using a flat-edged spade 3 to 4 inches down. This will create a narrow straight segment. Pull back (or toward you) on the spade handle to loosen the sod. When the sod tears free from the earth, scatter any dirt in the garden bed and create a pile of grass and weeds beside you.
Move the spade over in the soil, digging into the earth again at the border of the first cut. When you've reached 3 to 4 inches down, angle the spade handle toward you to loosen the soil. Work in this manner until you have a rudimentary trench surrounding your flower bed.
Run a small garden rake through the trench to even out the soil. Toss any clumps of soil up into the garden bed. To neaten any rough trench edges, run one of the rake tines back and forth against the soil.
Remove the grass and weeds. Pick up and discard your string.