Does Mulching Leaves Kill Grass? What the Research Says

Last updated: October 2026

No. Mulching leaves does not kill grass when the leaves are shredded small enough to settle between the grass blades. Whole leaves left in a thick, wet mat are what smother a lawn. Michigan State University researchers mulched up to 6 inches of leaves into turf every October for three years and reported a healthy lawn with far fewer weeds.

What actually kills grass under leaves?

Grass needs light and air. A layer of whole leaves blocks both. When that layer gets rained on, the leaves stick together and form a mat that stays wet for weeks. Under the mat the grass thins out, and by spring you can find bare or yellow patches where the pile sat.

Shredded leaves behave differently. Small pieces fall through the canopy and rest on the soil, where they break down. The grass blades stay in the light. So the question is not whether leaves are on the lawn. The question is whether you can still see the grass after you finish.

What did the Michigan State study find?

Michigan State University Extension describes a three-year study from the late 1990s. Researchers put down mixed tree leaves in three amounts: none, 3 inches and 6 inches. Each October they mulched the leaves into the turf with a rotary mower.

Their findings, as MSU Extension reports them:

• Up to 6 inches of leaves can be mulched at one time, depending on the mower.
• Lawns that received mulched leaves greened up faster in spring.
• After three years of mulching, dandelions and crabgrass fell by nearly 100%, because the shredded leaves covered the bare spots where weeds start.
• Mulched leaves add nutrients, help the soil hold moisture and loosen compaction.

That is a strong case for mulching over raking. It also comes with a condition: the mower has to be able to shred the layer you give it.

How thick is too thick?

Use your eyes. After one pass you should see mostly grass with small leaf pieces between the blades. If you still see a blanket of leaf bits covering the grass, the layer was too heavy for a single pass.

You have three ways to handle a heavy layer:

1. Go over it again. A second pass cuts the pieces smaller.
2. Mulch more often. Weekly passes during peak leaf fall keep each layer thin.
3. Bag the extra. Collect the surplus and use it on garden beds or the compost pile.

Our guide on mulching leaves vs bagging covers when to switch from one to the other.

Does the type of leaf matter?

It changes how much work the shredding takes. Large, leathery leaves such as oak break down more slowly than thin leaves such as maple or birch. The MSU study used mixed species, which is what most yards have. If your trees drop thick, tough leaves, plan on smaller pieces: a slower pass, a second pass, or a blade made for shredding.

What makes a mulching blade different?

A standard blade is built to cut once and throw the clipping out or into a bag. A mulching blade is shaped to keep the material moving under the deck so it gets cut again before it falls.

Mowrator makes a Mulching Blades 3-Pack for the S1 remote control mower. Per Mowrator, the design is dual-stage: leaves pass through the blade twice in one rotation, and the blades produced 931.55 CFM of upward airflow in Mowrator's test to lift leaves off the ground. The blades are 40MnB steel and fit both the S1 2WD and the S1 4WD.

Can you mulch and bag with the same machine?

Yes, and it is the most flexible setup for fall. With the mulching blade fitted, the Mowrator S1 2WD works in two modes, per Mowrator: it can mulch and return leaves to the lawn, or it can vacuum, shred and collect them in a 70 L bag. Shredded leaves pack much tighter than whole ones, so a bag holds more yard before it needs emptying.

The Auto Dumping Bag holds 70 L and empties without lifting the bag off the machine. It fits the S1 2WD and 4WD.

All figures are Mowrator's. The S1 is steered by remote control. It is not an autonomous robot.

When should you mulch, and when should you stop?

Start when leaves begin to cover the lawn, and repeat while they fall. Dry leaves shred into smaller pieces than wet ones, so pick a dry afternoon when you can. Keep going until the trees are bare. The MSU Extension article on this topic is titled "It's not too late to mulch fallen leaves into lawns", which tells you the window runs late into fall.

Avoid mulching leaves that are frozen to the ground or sitting in standing water. Wait for a dry day.

What about leaves on slopes and under trees?

Slopes are where raking is hardest and where leaves collect at the bottom. A walk-behind mower on a wet, leafy slope is a slipping risk. Mowrator rates the S1 2WD for 45% (24°) slopes and the S1 4WD for 75% (37°), and you steer from a remote while standing on level ground. Our guide to clearing leaves on a slope goes through the options.

Under trees the grass is often thin already. Go lighter there: mulch a thin layer and bag the rest, so the turf keeps as much light as it can get.

What mistakes should you avoid?

• Waiting for every leaf to fall. One deep layer at the end of the season is harder to shred than four thin ones.
• Mulching wet leaves. They clump under the deck and come out in wads.
• Leaving clumps behind. Spread any wads with a second pass.
• Using a dull blade. A dull edge tears leaves into large pieces that mat.

How do you check the lawn afterward?

Give it a week, then look closely at three places: the spot where leaves were deepest, the shade under your biggest tree, and any low area that stays damp.

• Grass is upright and green, with brown specks at soil level. That is what you want. The specks are leaf pieces breaking down.
• You see a brown layer and have to part it to find grass. Too much. Run the mower over it again or collect the surplus.
• The layer is slick and stuck together. It went down wet. Break it up with another pass on a dry day, and bag what is left.

Check once more before the first snow. Anything that is matted in late fall will still be matted in spring, with dead grass under it.

Does mulching leaves replace fertilizer?

Not fully, and no one should promise that it does. MSU Extension reports that mulched lawns greened up faster in spring and needed less fertilizer. Less is not none. Leaves return organic matter and some nutrients as they decay, and that happens slowly over months. Treat mulching as a free yearly top-up for the soil, and keep whatever feeding plan your lawn already does well on.

What does this mean for the time you spend?

Raking and bagging a yard is several jobs: gather, lift, carry, tie, haul. Mulching is one job that happens while the mower is already running. If you mow weekly until the grass stops growing, the leaves are handled in the same passes, and the only extra step is the mulching blade. For yards with slopes, the remote lets you do those passes from flat ground.

So should you mulch or rake?

For most lawns, mulch. The research supports it, it is faster, and the lawn gets the nutrients back. Rake or bag when the layer is too deep to shred in two passes, or when you want the leaves for beds and compost. A machine that can do both lets you decide yard by yard.

If you want to see how the S1 compares with a fully autonomous robot for fall work, read Yarbo vs Mowrator. Mowrator ships free on most products, usually in 7 to 10 business days.

Still not sure which fits your property?

Tell us about your lawn, trees and slopes and a Yard Automation Pro will reply by email with a recommendation, or tell you honestly if none of our machines fits. Book a free consultation, or call +1 423-927-3776, 9am to 5pm EST. You can also try the Find Your Robot quiz and cost calculator.