Uncut grass strips usually trace back to an unlevel deck, a dull or bent blade, or too little overlap between mowing passes. Here is how to find the cause.
Thin strips of standing grass left behind after mowing, often running parallel to the direction of travel, almost always point to one of a small number of mechanical causes. Because the pattern is consistent and repeatable, working through them in order usually finds the problem quickly.
An Unlevel Deck
The most common cause is a mower deck that is not level from side to side or front to back. If one side of the deck sits lower than the other, the blade on the higher side cuts a shade shorter than the blade on the low side, and the strip left uncut lines up with whichever side is riding high. Deck height is normally set with a tape measure from the ground to the blade tip at multiple points around the deck, front, back, left, and right, and all four measurements should match within about an eighth of an inch. On riding mowers, tire pressure differences between the left and right side can also tilt the deck slightly and produce the same symptom, so checking and equalizing tire pressure is worth doing before adjusting the deck itself.
A Dull or Damaged Blade
A blade that has lost its edge tears grass instead of slicing it, and in the worst cases a sufficiently dull section of the cutting edge will bend blades of grass over rather than cutting them, especially at the very tips of the blade where cutting speed is often not the limiting factor but sharpness is. A blade that is bent, even slightly, from hitting a rock or root will also ride at an inconsistent height as it spins, cutting cleanly through part of its rotation and missing grass during the rest. Pull the blade and check it against a straightedge, and inspect the cutting edge closely for rounded or nicked sections rather than a clean bevel.
A Blade Installed Upside Down or Backward
After sharpening or replacing a blade, it is easy to reinstall it with the cutting edge trailing instead of leading, or flipped so the lift vanes pull air the wrong direction. A backward blade will still spin and may even appear to cut, but it typically leaves ragged, uneven results including missed strips, since the sharpened edge is no longer meeting the grass first. Check the blade orientation against the manufacturer’s manual, since the correct side facing down is not always obvious by appearance alone.
Not Enough Overlap Between Passes
Every mower cuts a swath slightly narrower than the full deck width because the blade tips do not reach the very edges of the housing. If mowing passes are lined up edge to edge with no overlap, a thin strip along the seam between passes can be missed entirely. Overlapping each pass by two to three inches with the previous one removes this gap and is standard practice on any mower, walk behind or riding.
Worn Spindle or Bent Blade Adapter
On mowers with more hours of use, the spindle assembly that the blade mounts to can develop play or wear, allowing the blade to wobble slightly as it spins instead of tracking a perfectly flat circle. This wobble has the same effect as a bent blade, producing an inconsistent cutting height around the rotation. Grabbing the blade by the tips with the engine off and checking for side to side movement in the spindle is a simple way to test for this.
Grass Conditions Making It Worse
Wet grass or grass that has grown well past the recommended mowing height is more likely to fold over and lie flat instead of standing up into the blade path, which can leave missed strips even on a mower that is otherwise in good mechanical condition. If uncut strips only show up in tall or wet sections of the lawn and not on evenly mowed shorter grass, the deck and blade are probably fine and the issue is more about mowing conditions, covered in why lawn mowers bog down in thick grass.
Working Through the Fix
Start with the blade, since it is the fastest thing to inspect and the most common cause. If the blade is sharp, straight, and correctly installed, move to deck leveling next, then check spindle play if the deck measures level. Keeping blades on a regular sharpening schedule, as outlined in how often lawn mower blades should be sharpened, prevents most of the dullness related causes before they start affecting the cut. If the blade itself is nicked, cracked, or has been sharpened down repeatedly, it may need replacing rather than another sharpening, as covered in signs a lawn mower blade needs replacing. A mower with chronically worn spindles or a deck that will not hold level despite adjustment may be nearing the point where comparing current options, such as those in our best lawn mowers guide, makes more sense than continued repair.
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FAQs
A strip missed consistently on one side almost always means the deck is not level, with that side riding slightly higher than the other. Check blade height at all four corners of the deck and equalize tire pressure before making any other adjustments to the blade or spindle.
Yes, especially near the outer tips of the blade where a rounded or nicked edge is more likely to bend grass over rather than slice through it. A blade that is dull enough will fold rather than cut in the weakest sections of its edge.
Overlapping each pass by two to three inches with the previous one is standard practice, since the outer edge of the deck cuts less cleanly than the center and blade tips do not reach the very edge of the housing on most mowers.
Yes. A bent blade or worn spindle that causes visible wobble increases vibration, stresses the spindle bearing further, and raises the risk of the blade separating during operation. It should be corrected as a safety issue, not just a cutting quality one, before the mower is used again.