Why Sharp Mower Blades Matter for Your Lawn (and How Often to Sharpen)

A sharp mower blade slices each grass leaf clean. A dull blade rips it. The torn tips dry out, turn brown and give the whole lawn a whitish look. Torn grass also picks up disease more easily. Sharpen at least once a season. Purdue University says a busy lawn may need it four to six times a year. Always pull the spark plug wire before you touch the blade.

What does a dull mower blade do to grass?

Think of cutting paper with good scissors, then with a butter knife. The scissors make a clean line. The knife tears and leaves a fuzzy edge. Grass works the same way.

Purdue University says dull blades rip grass instead of cutting it. That leaves ragged tips. From across the yard, those tips give the lawn a whitish cast.

The University of California explains what happens next. Soon after mowing, the lawn looks a bit grey. The torn tips turn yellow, then brown, and die. Grass hurt this way handles drought and disease worse than grass cut clean.

Two Canadian cities say the same thing in plain words:

• Greater Sudbury says to sharpen blades every year. It says tearing from a dull blade makes grass more open to disease and pests.
• The City of Brantford says to sharpen blades so grass doesn't tear. It says torn grass is more prone to disease.

Does a dull blade cost you more to mow?

Yes, in a small way. Cornell University says dull blades raise fuel use by up to 20%. The engine works harder to rip grass than to slice it. A dull blade also tends to leave strips of uncut grass, so you end up going over spots twice.

The bigger cost is the lawn itself. A brown, ragged lawn looks tired even when it's freshly cut. You did the work, but it doesn't show.

How can you tell if your mower blade is dull?

You don't need to flip the mower to find out. Look at the grass first.

• Check the tips. Pull a few blades of grass a day after you mow. Clean tips mean a sharp blade. Frayed, chewed tips mean a dull one.
• Look across the lawn. A grey or whitish haze after mowing is a sign of torn tips.
• Watch for brown tips. Cornell says to sharpen when you see ragged brown tips on the grass.
• Notice missed strips. Tufts of grass left standing behind the mower can point to a dull or damaged blade.
• Listen for a hit. If the blade struck a rock, root or sidewalk edge, check it before you mow again.

The University of California says to look at the grass leaves often after mowing. It's the easiest test there is.

How often should you sharpen a lawn mower blade?

The short answer: at least once a season, and more if the lawn tells you to. Here's what the official sources say:

Source	How often
Greater Sudbury	Every year
Cornell University	At least once a season, or when you see ragged brown tips
Purdue University	Check a few times each season; may need it 4 to 6 times a year

Cornell suggests a tune-up and blade sharpening at the start of the season. That makes spring the easy time to do it. Then check the grass through the summer.

A big yard, sandy soil or lots of sticks and stones wears a blade faster. So does mowing every week through a long, wet spring. A small city lot with soft grass may be fine with once a year.

How do you sharpen a mower blade safely?

Safety first, because the blade is attached to the engine. The Canadian Centre for Occupational Health and Safety (CCOHS) says to disconnect the spark plug wire before you sharpen, replace or clean the blade.

Before you tip the mower

• Shut the mower off and wait for the blade to stop. CCOHS says to do this before you tip it.
• Pull the spark plug wire on a gas mower.
• Unplug an electric mower. On a battery mower, CCOHS says to remove the safety key.
• Wear heavy work gloves. Even a dull blade has sharp corners.
• Tip the mower so the air filter and carburetor face up. Your owner's manual shows which side.

Filing the edge

Purdue University gives these steps:

1. Take the blade off and clamp it in a vise.
2. File along the cutting edge at about a 45 degree angle.
3. Make the same number of passes on each side, with the same pressure.
4. Check the balance on a plastic blade balancer.
5. If one side drops, file the heavier side a bit more.

Balance matters. A blade that's heavier on one end shakes the mower. Before you take the blade off, mark the side that faces the grass. That way it goes back on the right way up.

Not handy with a file? A small engine shop can sharpen it for you. Many people keep a spare blade so they can swap one in and sharpen the other later.

When should you replace the blade instead?

Sharpening only works if there's enough metal left. CCOHS says to replace thin or worn blades. Before each mow, it says to make certain the blade is sharp and secured.

Replace the blade if you see:

• Deep nicks or chunks missing from the cutting edge.
• A bend or twist, even a small one.
• Cracks anywhere on the blade.
• A thin, worn edge after years of filing.

CCOHS also says to stop the mower right away if the blade hits a hard object. Inspect the blade and fix it before you mow again. It says not to run the mower over stones, pipes, rocks or sidewalk edges. Walking the yard for sticks and stones before each mow saves your blade.

What else helps a sharp blade do its job?

A sharp blade works best when the rest of your mowing habits line up.

• Mow tall. Greater Sudbury says to cut grass to 6 to 8 cm (2.5 to 3 inches). Brantford says to set the blade at 3 inches (about 7.5 cm).
• Take a third or less. Sudbury says never cut off more than one third of the grass at once.
• Mow dry grass. Brantford says to make sure grass is dry. The University of California says to skip mowing wet grass or in a drought.
• Leave the clippings. Brantford says to leave them on the lawn.

Not sure how often to mow? Our guide to how often to mow in Canada covers the rhythm through the season. For fall, see our post on the last mow of the season.

Can a local crew handle the mowing for you?

Yes. Golden Oak Lawn Care matches you with a vetted, insured local partner crew. They do the work. We stand behind it with the Golden Oak Guarantee.

Hiring any lawn service? Ask how often they sharpen their blades. It's a fair question, and a good crew has a quick answer. With Golden Oak, the same crew mows your lawn every visit and texts you a photo when it's done. If you see torn or ragged grass, text us a photo within 24 hours. Your crew re-cuts it free within 48 hours.

The Worry-Free Lawn Season plan books the whole season up front, and the first cut is free. We price every lawn by its size from satellite images, so no one needs to visit. Lawn size, how often you want a cut, slopes, gates and obstacles all change the price.

Want it handled? Get your price in 60 minutes, or call 1-866-904-LAWN.

Sharp Mower Blade Questions

Sources: Purdue University sharpen mower blades, University of California IPM dull mower blades, Cornell University make adjustments, CCOHS rotary power lawn mowers, Greater Sudbury green lawn care, City of Brantford yard waste and grasscycling.