Mulch vs Side Discharge: Which Mowing Mode and When

Mulching feeds the lawn and saves bagging; side discharge handles long wet grass without clogging. Most lawns want mulching most of the time.

By Zakaria October 17, 2026 4 min read Save to Pinterest

Lawn mower grass cutting — Photo: Counselling, Pixabay Content License, via Pixabay

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Mulch by default. Discharge when conditions force it. And the fear that mulching causes thatch is the most durable myth in lawn care.

The short answer

Mulching chops clippings finely and returns them to the sward, where they decompose and supply a meaningful share of the lawn's annual nitrogen — commonly cited as a quarter to a third — along with potassium and phosphorus. Side discharge is better for long, wet or very heavy growth, where a mulching deck packs and clogs and leaves clumps that smother grass. Mulching does not cause thatch: thatch is undecomposed stems, roots and rhizomes, caused by over-fertilising, overwatering and compaction, and clippings are about 80 per cent water and break down in weeks.

Mulch by default — it returns real nutrients, free
Discharge for long, wet or heavy growth
Mulching does not cause thatch. Clippings are 80% water
Mulching blades plus a blocked chute — both, or neither works well
Bag only for disease, weed seed, or compost

What each mode actually does

Mulching closes the discharge chute with a plug and uses blade and deck geometry to keep clippings circulating underneath, cutting them several times before they drop into the base of the sward. The pieces end up small enough to fall between the blades and out of sight.

Side discharge throws clippings out of a chute in one pass, more or less at the length they were cut. They land on the surface in a windrow.

Bagging collects them and removes them entirely.

Where each one belongs

Situation	Best mode
Regular cut, dry, normal growth	Mulch
Long grass after a holiday	Side discharge
Wet grass	Side discharge
First spring cut	Side discharge
Fast spring growth flush	Discharge, or mulch twice
Leaves in autumn, moderate	Mulch — they feed the soil
Leaves in autumn, heavy	Discharge or collect
Fungal disease present	Bag, to limit spread
Annual weeds in seed	Bag
Want clippings for compost	Bag

The thatch myth, properly buried

This is worth being precise about because it drives a great deal of unnecessary bagging.

Thatch is not made of clippings. It is a layer of undecomposed stems, crowns, roots and rhizomes — the tough lignin-rich parts of the plant that resist decomposition.

Grass leaf clippings are roughly 80 per cent water and decompose within weeks in a functioning soil. They contribute essentially nothing to a thatch layer.

What actually causes thatch:

Excess nitrogen, driving growth faster than microbes can break down the residue
Overwatering, and shallow frequent watering in particular
Compaction, which suppresses the soil life that does the decomposing
Low soil pH or poor microbial activity
Certain grasses — creeping bentgrass and some bermudas thatch readily by nature

A lawn that is bagged every single cut can and does develop thatch. The two are not connected.

Grasscycling, and what it is worth

Returning clippings is commonly found to supply around a quarter to a third of a lawn’s annual nitrogen need, along with potassium and phosphorus.

That is a real reduction in fertiliser, achieved by doing less work, on every cut.

Getting mulching to work properly

Most complaints about mulching come from one of four setup problems:

Mow more often. The one-third rule matters more in mulching mode than any other — removing more than a third of the leaf gives clippings too long to process
Mow dry. Wet clippings pack under the deck and clump
Use mulching blades and a mulching plug. A mulching blade with an open chute throws material out; a plug with a standard blade clogs. The two work as a pair
Keep the blade sharp. A dull blade tears rather than cuts, and torn material mulches badly and browns at the tips
Clean under the deck. Caked clippings destroy the airflow the system depends on

Autumn leaves

Mulching moderate leaf fall straight into the lawn is a genuinely good practice — the leaves break down over winter, feed soil organisms and improve soil structure, and research on the practice has generally found no harm to turf at reasonable volumes.

The limits:

Moderate cover only. If leaves still blanket the grass after mowing, there were too many
Go over it twice if needed rather than once slowly
Dry leaves mulch far better than wet ones
Heavy fall under a mature oak or maple needs collecting, not mulching

The simple rule

Mulch unless something stops you. The exceptions — long grass, wet grass, disease, weed seed, heavy leaves, wanting the clippings for compost — are specific and obvious when they apply.

Everything else is a free fertiliser application you get for choosing a deck setting.

Common questions

Is mulching better than side discharge?

For most lawns most of the time, yes. Mulching chops clippings finely and drives them down into the sward where they decompose and return nitrogen, potassium and phosphorus to the soil — research consistently finds this supplies a meaningful share of a lawn's annual nitrogen requirement, commonly cited as around a quarter to a third. Side discharge is the better choice for long, wet or very heavy growth, where a mulching deck clogs and leaves clumps. The practical answer is to mulch by default and switch to discharge when conditions demand it.

Does mulching grass clippings cause thatch?

No, and this is the most persistent myth in lawn care. Thatch is a layer of undecomposed stems, roots and rhizomes — the tough lignin-rich tissue — not leaf clippings. Clippings are roughly 80 per cent water and break down within weeks. Thatch is caused by over-fertilising with nitrogen, overwatering, compaction and soil conditions that suppress microbial activity, and lawns that are bagged religiously develop thatch just as readily as lawns that are mulched.

When should you use side discharge instead of mulching?

When the grass is long, wet, or growing so fast that a mulching deck cannot process it. A mulching deck recirculates clippings under the deck to cut them repeatedly, and that only works when there is room for air and material to move — in wet or overgrown grass it packs, clogs, and leaves clumps that smother the lawn beneath. Side discharge is also better on the first spring cut, on long grass after a holiday, and when clearing leaves in volume.

Do you need special mulching blades?

They help significantly, though a standard blade will mulch after a fashion. Mulching blades have a longer curved cutting edge and more aggressive lift, which keeps clippings circulating under the deck to be cut several times rather than thrown straight out. Used with a mulching plug that blocks the discharge chute, they produce noticeably finer clippings. Using a mulching blade without blocking the chute, or a standard blade with the chute blocked, both give worse results than either proper setup.

Should I ever bag my grass clippings?

Occasionally and for specific reasons, rather than as a routine. Bagging makes sense when the lawn has a fungal disease you do not want to spread, when seed heads of annual weeds are present, when the grass has become far too long to mulch, or when you want the clippings for compost. Otherwise bagging removes nutrients you would otherwise get free, costs time, and fills green bins for no benefit. The default should be returning clippings to the lawn.

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A cylinder mower throwing a spray of cut grass clippings as it cuts a green lawn

Lawn & Soil · Oct 17, 2026