How to Use a String Trimmer to Edge and Trim Like a Pro

Learning how to use a string trimmer the right way is what separates a clean, professional-looking lawn edge from a scalped, uneven mess. This guide covers the technique, angle, and settings that get pro results.

An average lawn will have been mowed by a lawn mower and so finishing it off with a string trimmer is required. However, if used incorrectly a string trimmer can remove large sections of grass and leave them bare. It can also gouge the bark from trees and branches. Even small stones and objects can hit the shins when using a string trimmer. The two main uses of a string trimmer are trimming (horizontally close to the ground) grass and weeds, and edging (cutting a vertical border). Most people have no idea how to use a string trimmer for edging and thus do not.

Step-by-step

1. Gear up first — this tool is dangerous. The line spins up to ~10,000 rpm and throws rocks. Always wear safety goggles, ear protection, long pants, closed shoes, and gloves, keep the shield on, and make sure the line spins away from you.
2. Trim (horizontal) at the right height. Trim your lawn at the right height for your mower. Hold the head of your trimmer level and as close to the ground as possible with the tip of the line. Cut in very slowly from side to side. For really tall grass it is best to slowly acclimate to it rather than attempting to cut in too quickly.
3. Edge (vertical) by flipping it. Rotate the trimmer so the head is vertical and the line cuts a clean line where lawn meets pavement. Hold the side of the handle (not the top) for control and keep the head near 90° to the ground for the cleanest edge.
4. Mow around obstacles without scalping grass elsewhere on lawn. When mowing close to trees, fences and flower/vegetable beds use the tip of string and mow in such a way as to keep string as close as possible to lawn surface to avoid cutting too much grass and making bald patches. Be also very aware when mowing near young trees not to mow string too close to the bark as it can girdle trees.
5. Feed line and mind diameter. The feed line diameter is generally specified by the tree wacker manufacturer. For very hard woods a larger line, 1/4″, is recommended. As the line wears, try to advance it a little. If it is advanced too little then poor cuts will be made. If the line is advanced too much then the motor will bog.

Safety tips: never trim toward people, windows, or cars; scan for hidden rocks and wire first; and keep bystanders well back.

Pro Tips

• Raise the back of the trimmer so only the string tip contacts — that’s the pro trick for clean edges without gouging.
• Cut with the tip of the line, not the whole arc — it cuts cleaner and uses less line.
• Match line direction to your approach so debris flies away from you, not at your face.

Common Problems & Fixes

• Bald/scalped spots → the head is tilted into the soil. Keep it level and use the tip.
• Uneven edge → it’s not vertical/90°. Steady the head and hold the side handle.
• Line keeps snapping → wrong diameter or hitting hard edges. Use the correct line and ease off concrete.
• Damaged tree bark → string too close. Leave a mulch ring or hand-trim near trunks.

Edging vs. Trimming: How to Use a String Trimmer for Both

The key to knowing how to use a string trimmer well is understanding that edging and trimming are two different motions. For trimming, hold the head parallel to the ground and sweep slowly side to side, letting only the tips of the line do the cutting — plunging the whole head in bogs the motor and scalps the grass. For edging along a driveway or sidewalk, rotate the trimmer 90 degrees so the line spins in a vertical plane, then walk it steadily along the hard edge to cut a crisp vertical line.

A few habits protect both you and the tool. Always wear eye protection and closed shoes, keep bystanders well clear of thrown debris, and feed line a little at a time rather than running it too long. For broader guidance on operating outdoor power equipment safely, the Outdoor Power Equipment Institute is a solid reference. Master the two motions and your borders will look professionally finished every time.

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