Most people already know how to use a string trimmer in the sense that they can start one and wave it at long grass. What separates that from a clean edge is knowing which way the head spins. Everything else, the scalped patches, the grass thrown onto your face instead of away from it, the ragged line along the drive, follows from getting that one detail wrong.
Quick answer: To use a string trimmer, hold it at hip height with the head tilted about 5 to 10 degrees so only the tip of the line cuts, and work in a slow sweeping motion. Most trimmers spin counterclockwise, which means they cut on the left side and throw debris to the left, so move right to left and keep the discharge away from you. Never trim against a tree trunk: the line strips bark and can kill the tree. Wear eye protection every time.
Key takeaways
- Know your rotation. Counterclockwise heads cut on the left and discharge left, so you work right to left. Get this backwards and everything fights you.
- Only the tip cuts. Burying the head in grass does not cut faster, it just breaks line and bogs the motor.
- Tilt about 5 to 10 degrees and take the top off in layers rather than trying to cut tall growth in one pass.
- Never touch a tree trunk with the line. It strips bark, and repeated damage around the trunk girdles and kills the tree.
- Eye protection is not optional. A trimmer throws stones and wire at head height, straight at you.
- Line thickness must match the machine. Too thick and you overload the motor; too thin and it snaps constantly.
Which way does the head spin, and why does it matter?
This is the single most useful thing to know, and almost nobody is told it. The vast majority of string trimmers spin the head counterclockwise when viewed from above. That has two consequences.
First, the cutting happens on the left side of the head. The line is moving forward into uncut grass there, so that side does the work. The right side is dragging the line backward out of the cut, which just beats grass down and throws it around.
Second, debris is thrown to the left and slightly back. So if you walk left to right, you are throwing clippings, dust, and gravel onto the ground you have not cut yet and, worse, across your own shins.
The fix is simply to work right to left, sweeping the head from right to left through the grass and stepping left as you go. The cut lands on the correct side of the head and the debris flies away from you into the area you have already finished. If your trimmer spins clockwise, and a few do, reverse all of the above. Look at the head with the machine off: the guard is always on the discharge side.
What line should you use?
Line is consumable and it is where most trimmer frustration lives. Two things matter: diameter, which must match what the machine is built for, and shape, which decides how it cuts.
| Diameter | Suits | Use for |
|---|---|---|
| 0.065 to 0.080 in | Light electric and battery | Lawn edges, light grass |
| 0.085 to 0.105 in | Mid size battery and small gas | General yard work, thicker grass |
| 0.110 to 0.130 in | Larger gas | Heavy weeds, brush, commercial use |
Never go thicker than the manual allows. Heavier line means more drag, and on a battery trimmer that means a hot motor and a flat pack in twenty minutes. The best string trimmer line guide covers which shapes actually last: round line is durable and cheap, twisted line is quieter, and square or star shaped line cuts noticeably better in tough weeds but wears faster. For the two most common sizes we have specific roundups of the best .080 trimmer line and the best .105 trimmer line.
One tip that costs nothing: line dries out on the shelf and snaps. Soak a spool in water for 24 hours before use, or store it in a sealed bag with a damp sponge. When it does run out, how to restring a string trimmer covers the reload.
How do you use a string trimmer step by step?
- Put the gear on. Safety glasses or a face shield, long trousers, closed shoes, and hearing protection for a gas machine. A trimmer flings gravel and old wire at eye level, and it is aimed at you.
- Walk the area. Look for rocks, hidden hose ends, sprinkler heads, dog toys, and buried wire. The line finds all of them and turns them into projectiles.
- Set the harness or strap. The head should sit level at about hip height with your arms relaxed. If you are holding the weight up with your arms, the strap is wrong and you will be exhausted in ten minutes.
- Trim after you mow, not before. Mow first so you can see the actual height you are matching, then trim the bits the mower could not reach. Cutting order is covered in how to mow a lawn.
- Tilt the head 5 to 10 degrees toward the direction of the cut so only the last inch or so of line touches grass. The tip is moving fastest and does nearly all of the cutting.
- Sweep right to left, slowly. Move the head through the grass in a steady arc, step left, repeat. Let the tip do the work rather than pushing the head into the growth.
- Take tall growth in layers. Knock the top third off on the first pass, then lower the head and go again. Diving into 2 foot weeds in one pass snaps line and stalls the motor.
- Keep the head off the dirt. The moment the line touches soil it wears out in seconds and you scalp a bare patch that will grow weeds.
- Feed line as needed. Bump the head lightly on firm ground for a bump feed, or wait for automatic feed. If nothing comes out, the spool is either empty or welded together from heat.
How do you edge with a string trimmer?
Edging is trimming turned 90 degrees. Rotate the trimmer so the head is vertical and the line cuts down into the seam between lawn and pavement. Some trimmers have a rotating head or a dedicated edging guide; on the rest you simply turn your wrists and your body with it.
Walk slowly, keep the line just kissing the edge of the concrete rather than grinding on it, and follow the existing line rather than trying to invent a new one on the first pass. A crisp edge is built over a few cuts, not carved in one. If you edge every time you mow, a dedicated combo tool is easier on your wrists, and the best cordless string trimmer edger combos switch between the two jobs without you contorting. The best trimmer edger combos cover the corded and gas options.
One warning about pavement: line spinning against concrete wears out fast and throws chips of it. Ease off. You are cutting grass, not resurfacing the path.
Never trim against a tree trunk
This deserves its own section because it quietly kills more trees than any pest. Nylon line beating against bark strips it away, and the damage happens at the root flare where the trunk widens, which is exactly where the grass you are trying to cut grows.
Just under the bark are the vessels that move water and food, plus the cambium layer the tree grows from. Purdue Extension is direct about the consequence: any damage to this transport system can affect tree health and the tree could die. When repeated wounds join up all the way around the trunk, the tree is girdled and it dies. Young, smooth barked trees such as birches and maples are the most vulnerable, and the wounds also let decay fungi in.
The fix takes ten minutes per tree. Clear the turf from around the base and lay a mulch ring, 2 to 3 inches deep over the root zone, and importantly do not mound it up against the trunk like a volcano. Now there is no grass to trim, the mulch is a visual cue to keep the machine away, and you save yourself the job every week. Trunk guards work too where mulch is not practical.
Common string trimmer mistakes
- Working left to right. You throw debris at yourself and use the wrong side of the head.
- Burying the head in grass. Only the tip cuts. The rest is drag, heat, and broken line.
- Scalping. Holding the head flat and low takes the grass to soil and leaves bare patches that fill with weeds. Those patches are then a job for how to kill weeds in a lawn and reseeding, all self inflicted.
- Trimming around trees. Slow motion tree killing.
- Using line that is too thick. Overloads the motor and drains batteries.
- No eye protection. The one mistake with permanent consequences.
- Trimming wet grass. It clumps on the head, wraps the shaft, and cuts raggedly.
- Cutting at full throttle everywhere. Fine in heavy weeds, wasteful and messy on a light lawn edge.
If the machine itself is the problem, rather than the technique, that is worth knowing too. Battery trimmers have become genuinely capable for home lawns, and the best battery powered string trimmers cover the models with enough torque to hold speed in thick grass. For long boundaries, brush, and hours of continuous work, the best gas powered string trimmers still win on runtime, while the best electric string trimmers suit small yards near an outlet.
Frequently asked questions
Which way should you move a string trimmer?
Right to left for most trimmers, because the head spins counterclockwise. That means the line cuts on the left side and throws debris to the left, away from you and onto ground you have already finished. Working left to right throws clippings and stones at your legs and uses the wrong side of the head.
Should you trim before or after mowing?
Mow first, then trim. Mowing sets the height you are matching, so you can see exactly where to trim the edges and around obstacles to blend in. Trimming first means guessing the final height, which usually leaves the edges noticeably shorter or longer than the lawn.
Why does my trimmer line keep breaking?
Usually because you are burying the head in grass instead of cutting with the tip, or touching soil, fences, and concrete. Dried out line snaps too: soak a spool in water for 24 hours before use. Line that is too thin for the machine or too old also breaks constantly.
Can a string trimmer kill a tree?
Yes. The line strips bark at the root flare, damaging the vessels that carry water and food. Repeated wounds around the trunk girdle the tree and kill it, and the wounds let decay fungi in. Young smooth barked trees like birch and maple are most at risk. Lay a 2 to 3 inch mulch ring instead.
What size trimmer line do I need?
Match the machine, never exceed what the manual specifies. Roughly, 0.065 to 0.080 inch for light electric and battery trimmers, 0.085 to 0.105 for mid size machines and general yard work, and 0.110 and up for large gas trimmers cutting heavy weeds and brush.
How high should you hold a string trimmer?
About hip height, with the head level and your arms relaxed, supported by the harness or strap rather than your shoulders. Tilt the head 5 to 10 degrees so only the tip of the line contacts the grass. If your arms are carrying the weight, adjust the strap before you carry on.
Final thoughts
Using a string trimmer well comes down to three things: work right to left so the head cuts and throws the way it was designed to, cut with the tip of the line at a slight tilt rather than burying the head, and keep the line off tree trunks and soil. Add eye protection and a strap set at hip height and the job stops being a fight. If your trimmer runs out of power halfway down the fence line, the best battery powered string trimmers are the place to start looking.