A dull hoe, edger, or shovel isn't useless, but it does mean that you have to work a lot harder. Attach a rotary sharpener, such as a Multi-Sharp, to a drill, and put a keen edge back on these tools in a few minutes.
1. Secure the hoe or shovel to your workbench with bungee cords or clamps.
2. Attach the rotary sharpener to the drill, start it spinning, then lower it gently against the edge of the tool. Move the sharpener slowly back and forth along the edge until the desired sharpness is reached.
Alternatively, use a bastard file, the traditional tool used for sharpening shovels, edgers, and hoes. Use the flat side of the file, and keep it tilted at the same angle as the tool's beveled edge as you work. Grind the edge with long, sweeping strokes, always moving in only one direction, away from the edge. File shovels and edgers on both the front and back faces; file hoes on one side only.