Lawn Mower Won't Mulch Properly? Causes and Fixes
When a lawn mower won't mulch and leaves clumps or whole clippings instead of a fine, invisible cut, the usual causes are a missing mulch plug, the wrong blade, a dirty deck, wet grass, or cutting too much at once. Fit the mulch plug and a mulching blade, keep the deck clean, and mow dry for clippings that break down into the lawn.
In This Guide
Mulching works by keeping clippings circulating under the deck so the blade cuts them again and again into tiny pieces that fall into the turf. When a lawn mower won't mulch and instead drops clumps or visible clippings, that recutting is not happening, usually because the deck is not closed off for mulching, the blade is wrong or dull, or the grass is too wet or long to recut cleanly. This guide works through the mulching setup and conditions so the clippings disappear into the lawn instead of lying on top of it.
How Mulching Works
To mulch, the deck has to hold clippings in so the blade can chop them repeatedly before they drop. That needs the discharge closed with a mulch plug or cover, and a mulching blade whose curved shape keeps grass circulating. If the chute is open, or a standard high-lift blade is fitted, clippings are thrown out whole instead of recut, so you see clumps. Understanding that mulching depends on a closed deck and the right blade tells you the first things to check are the mulch plug and the blade, not the engine.
Fit the Mulch Plug or Cover
Most mowers need a mulch plug, or a closed rear door on bagging mowers, to block the discharge so clippings stay under the deck to be recut. If the plug is missing or the discharge is open, the mower cannot mulch, it just side-discharges. Fit the correct mulch plug or insert for your mower and make sure the discharge is fully closed off. On mowers with a 3-in-1 deck, confirm it is set to mulch mode. Simply closing the deck for mulching with the right plug is often the whole fix when a mower will not mulch.
Use a Mulching Blade
Mulching needs a mulching blade, which has a more curved cutting edge and extra surface that keeps clippings circulating for repeated cutting, unlike a high-lift blade designed to throw grass out for bagging. If a standard or high-lift blade is fitted, or the mulching blade is dull, the clippings are not chopped fine. Fit a sharp mulching blade suited to your mower, and keep it sharp, since a dull mulching blade tears rather than finely cuts. The right, sharp blade paired with a closed deck is the core of clean mulching.
Mow Dry and Take Small Cuts
Mulching only works well on dry grass cut a little at a time. Wet clippings clump and stick instead of breaking down, and cutting too much length at once gives the blade more than it can recut, so both leave clumps. Mow when the grass is dry, and follow the one-third rule, removing no more than a third of the blade height per cut, mowing more often in fast growth rather than letting it get long. Dry grass taken in small amounts is chopped fine and vanishes into the lawn, which is what mulching should look like.
Clean the Deck and When to Call
A deck caked with old clippings leaves less room for grass to circulate and recut, so scrape the deck underside clean for good mulching. Keep the blade sharp and the deck clean through the season. If the mower still will not mulch cleanly with the right plug, a sharp mulching blade, dry grass, small cuts, and a clean deck, the deck design may simply mulch poorly in heavy growth, or a bagging setup may suit better. Wildwood can set the mower up for mulching with the right blade and plug and advise what your deck does best.
Mobile Mower Repair From Wildwood
Not sure whether to fix or replace? Wildwood comes to you, diagnoses the mower in your driveway, and quotes a flat rate before any work. More than 10,000 repairs since 2019, mower tune-ups from $38, house calls from $75. Call 888-594-5516.