How to Sharpen Lawn Mower Blades

Lawn mower blade sharpening

So your lawn is starting to take off this spring, but when you finally got out there and mowed it, did that thick green grass start looking ragged and fringed with brown? A dull lawn mower blade is the likely suspect.

According to the experts, dull mower blades do worse than just make freshly cut rows look sloppy. They hurt your lawn and make it vulnerable to diseases and fungi. You’ll be much more satisfied with what sharp blades deliver: a clean-cut, healthy lawn.

Dull mower blades can cause the turf to brown or look ragged after mowing, because they’ll rip and tear the edge of the grass blades, leaving the grass more at risk for disease.

“With a dull blade, today’s rotary mower tears and shreds the grass rather than cutting it,” said Richard Hentschel, an extension educator with the University of Illinois Extension in DuPage, Kane and Kendall counties. Shredded grass blades will take on a brown, straw-like appearance after 24-48 hours. “Those shredded grass blades can’t really heal, exposing that tissue to disease pathogens.”

According to the University of Maryland Extension, “mowing injury” can be direct physical damage to the grass or lawn deterioration from subpar mowing practices.

Grass damaged by dull mower blades
Grass blade A was cut cleanly with a sharp mower blade, but B, C and D show dull-blade damage. Grass blade B was bent (arrow); C and D exhibit partial and severe shredding. Photo University of Arkansas Division of Agriculture / Extension and Research

From Colorado to Wisconsin to Georgia, extension services warn of what can befall a lawn cut with dull mower blades.

Ragged edges leave a better opening for multiple types of fungus as well as chronic problems, and once those problems are present, another round of mowing with the same dull blades can open up new wounds in the lawn, creating more infection sites and resulting in even more brown spots and an even more unsightly lawn.

Environmental Benefit of Sharp Mower Blades

There’s another reason to sharpen lawn mower blades, Hentschel pointed out: the environment.

Gas-powered lawn mowers — especially older ones — can pollute more per hour than several of today’s gas-powered cars, he said. Sharp mower blades make the job easier for the engine. It pollutes less when it doesn’t have to run as long or strain as hard. Hentschel also recommended owners perform other routine mower maintenance at the same time. Changing the air filter, cleaning the inside of the mower deck and oiling moving parts also lower pollution by making the mower run more smoothly and efficiently.

When to Sharpen

To see whether you should sharpen your mower blade, pay attention to the lawn and how the mower is cutting. If you start to see the grass getting pulled and torn instead of cleanly cut, it’s likely time for another blade sharpening. If the blade is warped or too far gone, it may be time to purchase a new blade.

There are plenty of factors that contribute to dulling lawnmower blades, Hentschel added, including having the blade too low to the ground, where it will kick up stones and dirt, which dull the blade faster.

It only takes about a month for lawnmower blades to go dull, he said, especially if you have seen clouds of dirt or sprays of rock shrapnel flying out from under the mower.

Step by Step: How to Sharpen Lawn Mower Blades

Tools you will need:

  • Gloves and eye safety equipment.
  • Hand metal file, angle grinder, mounted bench grinder, or dedicated lawn mower blade sharpener.
  • Wrench to remove and reattach blade.
  • Wood block.
  • Nail or screwdriver, or dedicated blade balancer.

Step 1: Prepare the Mower

To start, make absolutely sure the mower can’t start by accident. Human flesh and fast blades are a terrible combination

On gas-powered mowers, unscrew the spark plug. Most times, you should be able to leave it attached via the spark plug wire. Once the spark plug is disconnected, tilt the mower on its side, making sure the air filter and carburetor are facing up. That will prevent a belch of smoke on restarting. Tighten down the gas cap.

On electric mowers, disengage the cord or battery.

Mark the bottom of the blade with chalk or spray paint to make sure you don’t reinstall it upside-down — the most-common mistake DIYers make, the pros say. Some blades are also clearly stamped “bottom” to save you this step.

Step 2: Detach mower blades

To sharpen the blade, it will first have to be removed from the mower. Never sharpen the blade while it’s still mounted to the mower.

In general, blades are just bolted to the deck of the mower, but the precise way they are attached can differ. This would be a good time to head online to see if your lawn mower manufacturer has provided an online guide. Look up your mower and see if there are any quirks to detaching the blade.

The guide also will likely tell you the proper size wrench to remove the nut holding the blade in place.

If it’s stuck in too tight, try soaking with penetrating oil or using a long-handled wrench for more torque. Wedge in the block of wood to keep the blades from turning while you loosen the nut.

Step 3: Sharpen

Once you have the blade in hand, find the cutting edge on either side of the blade and inspect them to see how much sharpening they need. A newer blade or one that has been kept up well could just need a few passes with the hand file or sharpening stone, but one that has been used frequently since its last sharpening may be better served by an angle grinder or bench grinder. Follow the angle and grind already on each cutting edge on each end of the blade to get a sharp edge on both sides. Dedicated lawn mower blade sharpeners make this easy, using a guide for the edges of the blades so they get the correct angle from the grinding wheel.

Oregon Products, which sells a variety of lawn-mowing products, says that a dedicated blade grinder is the best way to give a mower blade the perfect edge — sharp, but not too sharp. “Mower blades should be aggressively sharp, but not as sharp as a razor’s edge,” the site says. “You should be able to touch the blade with your hand without getting cut.”

Something between butter knife-sharp and razor-sharp.

Step 4: Balance the Blade

An unbalanced blade can shake the mower and potentially cause damage, so balance the blade with a few extra passes on the sharpener. After the blade, balance it before Once the blade is sharpened, it’s important to make sure it’s balanced before you reattach it to the mower. This can be done by using a nail hammered into the wall of your garage or by using a screwdriver. Place the nail or screwdriver through the blade’s center hole and see if it’s heavier on one side. If it is, give it another quick pass with the hand file or angle grinder until it’s balanced.

Step 5: Reattach

With your lawn mower still on its side, make sure the blade is oriented correctly, checking with the spray paint or chalk mark from Step 1. Take this chance to add some oil and lubricant to the crankshaft of the lawnmower or clean the underside of the mower deck. Place the blade back onto the mower and tighten the hardware down to the recommended torque from your owner’s manual. Reattach the spark plug or battery and you’re ready to go.

That is, at least until those grass clippings start to look ragged and torn again.

When to Sharpen Again

Sharpen your lawnmower’s blade at least once a year to make sure you stay on top of your lawn-care game. The frequency of resharpening depends on the level of use, but for the average person mowing a home lawn, twice a year should do the trick.

Hentschel says he’d be thrilled if folks sharpened blades at least once a year, but he recommends twice per mowing season — starting off with fresh, sharp blades in the spring and then sharpening them again around the end of June or the beginning of July to finish off the season strong.

“Once a year is way better than never,” Hentschel said.

Derek Lacey

Derek Lacey

Formerly the agriculture writer for the Hendersonville Times-News, Derek Lacey’s articles have appeared in U.S. News & World Report, The Charlotte Observer, News & Observer, and The State. He has won 15 awards from the North Carolina Press Association and GateHouse Media, for pieces ranging from news features and investigative reporting to photography and multimedia projects.