How often should you sharpen your mower blade?

Why bother

A dull blade tears the grass and leaves it ragged. A thorough-tier job. Skippable when life's busy, worth it when it isn't.

Why it's a range

Heavy mowing, warm-season grass, or gritty soil wants it every few months.

What the job involves

1. Get it safe before you go near the blade. Pull the spark plug wire (or the battery on a cordless), then tip the mower with the carburetor and air-filter side up so nothing leaks. Wedge a block of wood against the blade so it can't spin, and unbolt it with a wrench.
2. Clamp the blade in a vise and run a file or grinder along the cutting edge, following the angle that's already there, roughly 30 to 45 degrees. Only the top side of each edge gets sharpened. You're after a butter-knife edge, not a razor. A razor edge just chips off the first time it meets a stone.
3. Check the balance, because grinding never takes metal off evenly. Hang the blade on a nail or a cheap balancer. If one end dips, that end's heavy, so grind a touch more off it until it sits level. An off-balance blade shakes the whole mower and chews through the spindle bearings.
4. Bolt it back on the right way up (the curved wings, or the side stamped 'this side down', face the deck), torque it snug, and put the spark plug wire back. Mismount it and the mower cuts with the dull side.

Do it yourself, or pay someone?

Yourself, for most. It wants a file or angle grinder, a wrench, and some way to check balance, and a single push-mower blade is 15 to 20 minutes. Pay a pro mainly when you've not got the tools or the blade's badly nicked: drop-off sharpening at a hardware store or mower shop runs about $5 to $15 a blade, mobile on-site more like $40 to $75 a visit.

What skipping it costs

$25 to $1,750, depending on how long it's left. The cheap end is just buying a blade you needn't have: run one dull and bounce it off rocks and it wears past sharpening, so you pay $30 to $40 for a new one instead of $5 to $15 to put an edge back on the old one. The dear end is the lawn. A dull blade tears the grass instead of slicing it, leaving ragged, browning tips that lose water and let in fungal disease, and season after season the turf thins into patches. Reseeding that runs about $0.10 to $0.20 a square foot, $440 to $1,750 for a typical yard. A file and ten minutes is the cheap end of all that.

Signs it's overdue

• Grass tips look frayed, white, or brown a day or two after mowing instead of cleanly cut, and the lawn takes on a hazy, ragged cast.
• There are visible nicks or a rounded, shiny edge on the blade, or it's done a full season (roughly 20 to 25 hours) without a touch-up.
• The mower bogs, shakes, or leaves uncut streaks and clumps, and it's working harder, and burning more fuel, than it used to.

When to start thinking about it

A few days' warning does it. Grumpty grumbles 5 days out, then leaves you alone. Mark it done and Grumpty sets the next one, every 6 months, so you can go back to forgetting it exists.

Questions people ask