How to Mulch Leaves with a Lawn Mower

The upturned end of a rotary blade, the fan tip, creates lift and a vacuum that pulls leaves up into the deck. The airflow holds them there and the blade hits them again and again until they are small. Every tip on this page follows from that one mechanism.

Anything that kills airflow kills leaf mulching: a worn or dull blade whose fan tip has rounded off, a wet deck, a plugged discharge chute, a full bag, too low a deck in tall material, or too fast a ground speed. The practical consequence is that a blade that still cuts grass acceptably can mulch leaves badly. If the mower throws leaves forward instead of lifting them, suspect the blade first. Sharpen every 20 to 25 hours of runtime, which works out to 2 to 3 times a growing season for most homeowners.

A mulching mower works best because its deck recirculates clippings, but a regular rotary mower will do the job. The difference is how well the deck keeps the leaves in the airflow, not whether it can chop them.

1. Pick a dry day, and dry leaves. Wet leaves mat to the ground, do not chop fine, clog the deck and strain the motor. If the leaves are damp, wait.
2. Clear the lawn first. Sticks, limbs, rocks and toys become projectiles. This is a rotary mower, not a chipper.
3. Set the deck high, at the top setting. If a check stripe shows the leaves are not breaking up, drop a notch or two. Never scalp down into a thick leaf layer.
4. Take the bag off and fit the mulch plug if the mower has one. On a side-discharge deck without a plug, discharge toward the middle of the lawn and keep recutting the windrow until the pieces are dime-size, then spread the result back out.
5. Mow slowly, and make two or three passes, the second at about 90 degrees to the first. The crisscross catches what the first pass threw out.
6. Check the result. Aim for dime-size pieces or smaller, about 1/2 inch. You should still see grass through the mulch, and no more than about 1/2 inch of leaf litter should be left on the turf. If you cannot see grass, rake or bag some off before the next pass.

Start when the first leaves fall and mow weekly through the drop, twice a week after a windy spell. Waiting for every leaf to come down and then trying to deal with the whole crop at once is what turns this into a miserable job. It is real work either way, and many people mulch the back lawn and keep the front tidier. That hybrid is a legitimate answer.

• Up to 6 inches of dry leaves per pass for a capable mower, less for a small push mower.
• No more than about 20% of the lawn covered by leaves after a pass, with 10 to 20% the working range.
• Dime-size or smaller particles, a 1/2 inch maximum residual leaf layer, and grass visible through it.
• Two to three passes, the second at 90 degrees.
• Weekly mowing through fall.

A 2-inch layer of leaves mulched in over a fall is worth roughly 3.4 lb nitrogen, 0.5 lb phosphorus and 1.5 lb potassium per 1,000 sq ft. Leaves run about 2% nitrogen by weight, which is why the total adds up to a real feeding.

Repeated fall mulching cut dandelion counts by up to about 84% with sugar maple leaves and up to about 72% with red maple over three years, with a similar drop in crabgrass. Grinding also shrinks the pile dramatically: a mature shade tree's drop that fills two and a half 30-gallon bags whole fits, ground up, at roughly a fifth of the volume.

Every rotary deck mulches leaves to some degree, but the machine you own sets how much leaf litter it can take and what to watch for.

Corded electric

Light and cheap, and fine for a small lawn's leaf drop. The cord limits reach, so plan the passes around it and keep the cord out of the leaf pile. The motor bogs in heavy litter, so slow down rather than force it.

Battery and cordless

Good for light to moderate leaf loads. Expect reduced runtime in heavy leaf litter compared with grass, and expect a thermal cutout if you push it through damp or heavy material. Let it cool and restart rather than repeatedly forcing it, and keep a spare pack charged on a big leaf day.

Gas push

The workhorse. More torque for thick leaves than most electric walk-behinds, and no cord or battery to manage.

Self-propelled

The best walk-behind choice for a big leaf drop or a sloping lawn. You are making many slow passes, and the drive does the pushing.

Riding mower and lawn tractor

The fastest way through a large lot's leaves. Whether it mulches or collects depends on the deck setup: a mulch plug returns the leaves, a bagger collects them. Watch for chute clogs in heavy going.

Zero-turn

The highest throughput of all. The same mulch-kit-versus-bagger trade-off applies, and the machine's speed is wasted if you go too fast for the leaves to be recut.

Reel and manual reel

Not a leaf tool. There is no deck and no vacuum to lift and recirculate leaves, and leaves jam the bedknife. Use a rotary mower or a rake for the leaf drop and keep the reel for the grass.

Robotic

Handles light, dry leaf fall if it is running often. It will clog or stall on a thick or wet layer, so treat it as a maintainer, not a leaf-clearer.

More than about 6 inches of leaves, or a drop so heavy you cannot see grass after a pass, calls for raking or bagging some off first and mulching the rest. If the lawn sits under a heavy canopy, mulch-mowing alone will not keep up and a thick mat can smother the turf, so collect or remove part of the load.

Wet leaves are a no: wait, or bag them. Pine needles resist breakdown even chopped, so do not plan on mulching them into the lawn. If the trees had a serious foliar disease, remove those leaves rather than mulching them back. Black walnut leaves carry juglone, so keep them out of beds and compost.

Species matter. Oak and cottonwood are high-tannin and slow to break down, so chop them well and use them in smaller quantities. Ash and maple are harder to chop than oak. Legume trees such as honey locust add nitrogen. Whole leaf litter in beds and under shrubs is valuable overwintering habitat for insects and other wildlife, so mulch the lawn and leave the beds alone.

Mulched leaves do not cause thatch. Thatch is turf stems, rhizomes and stolons, not leaf litter. They do not acidify the soil in any meaningful way, oak leaves included. They do not increase disease or weeds, and the research points the other way.

Heavy leaf application has been shown to have no effect on turf quality, color, growth, thatch, soil pH, weeds or disease. A thick layer of chopped leaves is food and mulch, not a problem waiting to happen.

Leaves are hard on an edge, and a dull blade with a rounded fan tip mulches badly next fall. Sharpen the blade at the end of the season, on the same 20 to 25 hour schedule you use for grass.

Clean the underside of the deck before and after leaf season, because caked grass and leaf residue is what clogs a deck. On a gas mower, check the air filter after a dusty leaf season. Store battery packs indoors, off the machine, at the charge level the manual specifies.

If any leaf cover is left in spring, one early mow cleans it up and lets the turf green up sooner.

• Wear eye protection and a dust mask. Leaf dust and mold are real, and the mower throws debris.
• Clear sticks, rocks and limbs before mowing.
• Never clear a clog with the engine running or the battery in.
• Mow across a slope, not up and down, and remember that wet leaves are slippery.
• Keep children and pets clear of the discharge side.
• Do not use the mower as a chipper.