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How to sharpen lawn mower blades

Woman looking at lawn mower blades
(Image credit: Getty Images)

Mowing is essential if you want to keep your lawn healthy, but even the best gas lawn mowers (opens in new tab) with dull blades are worse than useless, so knowing how to sharpen lawn mower blades is vital. You can choose from all kinds of lawn mowers these days (if you’re looking for a new lawn mower read our guide to the best gas mowers here) - but beneath the glossy chassis and extra features, it’s the blade or blades that actually do the proper work - so knowing when and how to sharpen a lawn mower blade is the key to a healthy lawn.

Along with the height that you mow a lawn (opens in new tab), the lack of sharpness of your lawn mower blade will be the cause of stress and result in your lawn suffering from bad health. An unhealthy lawn will be prone to disease and less able to cope with extremes in weather conditions, such as dry spells or heavy rains. You are likely to end up with brown and patchy grass rather than a green, lush lawn.

Why should you sharpen your lawn mower blades?

The most important thing is that you must cut your grass. No matter whether you're using the best electric lawn mower (opens in new tab) or the best robot lawn mower (opens in new tab), if the blades are not sharp, instead of cutting the grass they will rip and tear it. These tears will in turn weaken the grass, and leave it vulnerable to disease and fungal growth. Another thing to consider is that a lawn mower with dull blades will actually make it harder for you to mow - making the task longer and more stressful. 

How to sharpen lawn mower blades

David Gower, chairman of the Lawn Association (opens in new tab), says: “Sharpening is something of a skillset we’ve lost. A tool to sharpen lawn mower blades (opens in new tab) could consist of a simple file (or angle grinder if you are feeling a little braver) will take minutes to create a new sharp edge. And remember, even new blades don’t always come sharpened. But a great idea is to buy a spare blade, which gives you all week to sharpen before the next mow!’’

What the expert says…

David Gower explains what happens when lawn mower blades are not sharpened: “Blunt mowing of grass comes down to many lawnmower manufacturers not being lawn specialists and often advise sharpening a blade with a service (once per year) is sufficient. It won’t be. When we cut or mow grass with a blunt blade it literally tears the grass blades causing the grass to turn dull at the end of the tips. Do this all over your lawn and you instantly lose the color in the lawn. It will then struggle to take in food and water and ultimately be unhealthy….’’

You can remove the blade to sharpen it, although it is possible to do it in situ. Whichever method you choose, ensure that the mower is unplugged if it is electric, or that there is no chance of the mower starting by accident (you may have to tape spark plugs for instance).

If you remove the blade, be sure to mark which is the top or bottom edge, so that you replace it the right way up. It won’t matter how sharp your blade is - it won’t cut if it’s upside down!

The best way to hold the blade is by clamping it into a vice before you start work.

Use your file to sharpen from the top edge of the cutting blade (and remember the file will only cut on the push stroke - do not run it backwards and forwards). It should take no more than 50 strokes to get the blade sharp again - it would be quicker with an angle grinder, but unless you are very confident, you run the risk of damaging or overheating the lawn mower blade.

The lawn mower blades will have come from the factory with a specific angle on their cutting edge - make sure you keep to that angle each time you sharpen - it is usually between 35 and 45 degrees.

When you replace the blades in your lawn mower, make sure that they are balanced correctly, or they may cause vibration and ultimately damage to your mower

If you have a cylinder mower, it is best to leave sharpening to a professional.

How often should you sharpen lawn mower blades?

David Gower says: “This is very much down to any individual and how nice a lawn you want. Grass obsessives should be doing this each week. For many, every two to four weeks will still be beneficial. It’s down to your own effort but never blame the tools when they’re your tools!’’

Generally, the consensus seems to be that blades should be sharpened at least after every 20-25 hours of mowing, but if it seems to be getting harder to mow your lawn, that’s a clue that your blades need some work.

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Naomi MacKay has been a freelance writer and editor for the past 20 years. She previously made the move from local newspapers and consumer technology magazines into the gardening press as Assistant Editor at Garden Answers magazine, and has also worked for the Royal Horticultural Society, and writes garden columns for a number of publications.

With contributions from