Lawn Mower Cutting Unevenly

There’s nothing more frustrating than finishing a mow, stepping back, and seeing a striped, wavy mess. If your lawn mower cutting unevenly, you’re not alone. This common problem has a set of common causes, and the good news is that most are simple to fix yourself. Let’s walk through the diagnostic steps and solutions to get your lawn looking smooth and professional again.

This main heading describes your exact problem. An uneven cut, often called scalping or a washboard effect, means your mower is leaving patches of taller grass next to areas cut too short. It makes your lawn look messy and can harm the grass itself. We’ll start from the ground up to identify why.

The Usual Suspects: What Causes an Uneven Cut?

Before you adjust anything, it’s smart to understand the potential culprits. Often, it’s not just one thing but a combination of factors. Here are the most frequent reasons for patchy results:

Uneven Tire Pressure: This is the #1 cause. If one tire is low, that side of the mower deck dips down and cuts lower.
A Bent or Dull Mower Blade: A blade that’s lost its edge tears grass instead of slicing it, leaving a ragged, uneven finish. A bent blade throws the entire deck out of balance.
An Unlevel Mower Deck: The deck itself might be tilted forward, backward, or to one side, causing an inconsistent cutting height.
Mowing Too Fast: Speeding over bumps causes the mower deck to bounce, scalping high spots.
Clogged or Damaged Deck: Built-up grass clippings disrupt airflow, preventing grass from standing up straight for a clean cut.
Worn Deck Components: Spindles, mandrels, or deck hangers can wear out, allowing the blade to wobble.

Your Step-by-Step Diagnostic and Fix Guide

Follow this checklist in order. Start with the simplest fixes before moving to more involved adjustments.

Step 1: Check and Equalize Tire Pressure

This takes two minutes and often solves the problem. You’ll need a tire pressure gauge.

Consult your mower’s manual for the correct PSI (usually around 10-14 PSI for rear tires, slightly higher for front).
Check the pressure in all four tires (or two, for a push mower) when they are cold.
Inflate each tire to the exact same, recommended pressure. Don’t just eyeball it; use the gauge.
Re-mow a test strip. You might see immediate improvement.

Step 2: Inspect and Service the Mower Blade

A sharp, balanced blade is non-negotiable for a clean cut. You’ll need a wrench, a block of wood, and a blade balancer (or a simple nail on a wall).

Disconnect the Spark Plug! This is critical for safety. Remove the wire boot from the plug.
Tip the mower on its side (with the air filter and carburetor facing up to prevent oil leaks).
Use the block of wood to jam the blade so it doesn’t turn, then use the wrench to remove the bolt holding the blade on.
Inspect: Look for nicks, dents, or a obvious bend. Hold a ruler vertically against the wall; place the blade against it. If you see a gap, it’s bent.
Sharpen: If it’s just dull, sharpen it with a file or grinder, following the original cutting angle. Remove metal evenly from both ends.
Balance: This is crucial. Place the blade on a balancer. If one end dips, file a small amount of metal from the back edge (the heavy end) until it sits level. An unbalanced blade causes vibration and uneven cuts.
Reinstall the blade tightly, making sure the “star” or marked side faces toward the deck.

Step 3: Level the Mower Deck

This ensures the deck is parallel to the ground. You’ll need a tape measure and a flat surface, like a driveway.

Set your mower to its intended cutting height (e.g., 3 inches).
Park it on the level surface.
Measure from the ground straight up to the blade tip (or deck edge, as your manual specifies) on the left and right sides. Note the measurements.
Now, measure from the ground to the front of the deck (or blade tip) and the rear. The rear should be 1/8 to 1/4 inch HIGHER than the front. This “rear-high” setting prevents scalping.
If adjustments are needed, most mowers have lift rods or adjustment nuts near the wheels. Turn them equally to raise or lower each side. Re-measure until the deck is level side-to-side and slightly higher in the back.

Step 4: Clean the Undeck Thoroughly

A clean deck improves airflow and discharge. Do this after every few mows.

After disconnecting the spark plug, use a putty knife and hose to scrape away all caked-on grass and debris.
Check the discharge chute and opening for blockages.
Let it dry completely before next use to prevent rust.

Step 5: Adjust Your Mowing Technique

Your equipment might be perfect, but technique matters.

Slow Down: Mow at a steady, walking pace, especially over uneven terrain.
Overlap Passes: Overlap each pass by about 2-3 inches to ensure no strips are missed.
Change Directions: Alternate your mowing pattern each week (north-south, then east-west) to prevent grass from leaning in one direction.
Don’t Mow Wet Grass: Wet grass clumps, clogs the deck, and lays over instead of standing up, leading to a terrible cut.

When the Problem Might Be Your Lawn, Not Your Mower

Sometimes, the terrain itself is the issue. If you’ve fixed all the mower issues and still see problems, look down.

Bumpy or Uneven Lawn Surface

Over time, soil settles, creating high and low spots. A mower deck will scalp the high points.

Solution: Consider core aeration and topdressing. Fill low spots with a sandy topsoil mix, and gradually level the lawn over time.
For severe bumps, you may need professional grading.

Spongy or Thatch-Ridden Turf

A thick layer of thatch (dead organic matter) acts like a sponge, letting the mower wheels sink and the deck bounce.

Solution: Dethatch your lawn in the growing season using a dethatching rake or power dethatcher.

Advanced Checks: Spindle and Deck Integrity

If uneven cutting persists, the issue could be mechanical wear. This requires a more careful inspection.

Checking for Bent Spindles or a Worn Deck

With the spark plug disconnected and the blade removed, try to wiggle the spindle pulley by hand. There should be very little to no side-to-side play. If it wobbles, the spindle bearing is likely worn.
Spin the spindle by hand. It should spin smoothly and quietly. Grinding noises mean bad bearings.
Visually inspect the deck itself for cracks, especially around the spindle housings or where the hangers attach. A cracked deck can warp under vibration.

Replacing spindles or welding a deck are more advanced repairs. Consult a repair manual or consider a professional service if you’re not comfortable.

Preventative Maintenance Schedule

Stop problems before they start. A little regular care prevents most uneven cutting issues.

Before Each Mow: Check tire pressure. Clear the deck of debris. Look for obvious blade damage.
Every 8-10 Hours of Use: Sharpen and balance the blade. Clean the deck thoroughly.
Twice a Season: Check and adjust deck level. Lubricate all fittings (zerks). Inspect belts for wear.
End of Season: Perform a full service: change oil, replace air filter, stabilize fuel or drain the tank, and store with a clean, sharp blade installed.

FAQ: Common Questions About Patchy Lawn Results

Q: Why does my mower leave strips of uncut grass?
A: This is usually due to a dull blade, mowing too fast, or not overlapping your passes. It can also happen if the deck is clogged, disrupting the airflow that stands the grass up.

Q: My mower scalps the ground on every turn. What gives?
A: Scalping on turns is classic sign of mowing too fast or having the deck set too low for your lawn’s unevenness. Slow down on turns, and consider raising your cutting height slightly.

Q: I leveled my deck, but it still cuts lower on one side. Why?
A> Double-check that your tire pressures are absolutely identical. If they are, you may have a worn wheel mount or a slightly bent deck hanger affecting that side’s geometry. Also, re-check your blade balance.

Q: Is it better to bag or mulch to avoid an uneven look?
A: Mulching can sometimes hide minor imperfections because fine clippings settle into the turf. However, for a truly even cut, the underlying issue (blade, deck level) must be fixed regardless of your collection method.

Q: Can a new mower blade cause uneven cutting?
A: Yes, if it’s not properly balanced from the factory. Always check the balance of a new blade before installing it. It’s rare, but it does happen.

Getting a smooth, even cut is a matter of methodical troubleshooting. Start with tire pressure and blade sharpness—you’ll be surprised how often that’s the solution. By following this guide, you can diagnose the root cause and implement the fix, turning your patchy, struggling lawn back into a striped, green carpet you can be proud of. Remember, consistent maintenance is the key to avoiding these problems in the first place, saving you time and frustration down the road.