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How to mulch mow a lawn

Updated: October 2026

The first warm week of a British spring is when a mulch plug disappoints people. You fit it on grass that has sat wet, and the lawn is left in clumps. This is how to mulch mow a lawn with a plug-style rotary: when the grass is ready, how high to leave it, and when the box goes back on. The sequence follows the Royal Horticultural Society and Warwickshire County Council, plus the limits on the mowers in our [mulch mower comparison](/hub/lawn-and-garden-equipment/mulch-mower). Peter F. did not trial these machines. A qualifying purchase can earn us a commission at no extra cost to you. Width and price stay on the [buying guide](/hub/lawn-and-garden-equipment/mulch-mower/sub/mulch-mower-buying-guide).

The first warm week of a British spring is when a mulch plug disappoints people. You fit it on grass that has sat wet, and the lawn is left in clumps.

Our top picks

Swift Swift EB122C

Best overall

Swift Swift EB122C
9.0/10about £134.99
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Hyundai Hyundai HYM3800E

Alternative

Hyundai Hyundai HYM3800E
8.4/10about £158.31
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Forte Forte MEB1637E

Best value

Forte Forte MEB1637E
8.2/10about £79.99
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On this page
  1. 1.Fit the plug only when the lawn is ready
  2. 2.A season, not one perfect cut
  3. 3.Plug or box for this cut
  4. 4.When to stop and take it back

Fit the plug only when the lawn is ready

Fit the mulch plug, or close the mulch mode, only after the grass is dry, free of frost, and clear of stones, toys and thick wet leaves. The RHS also tells you to stay off waterlogged soil. The plug keeps clippings under the deck so the blade can cut them again. Reviewers on the corded Hyundai HYM3800E split on whether that plug behaves as fine mulch. If the grass is about 6 cm, take it to about 4 cm. The Swift 22 cm steps from 30 to 50 mm, so it cannot copy a 20 mm cut. A saved review on the Greenworks 24 V describes clogging when the grass is wet.

A season, not one perfect cut

Plan on mowing from March to October, the RHS season, with a weekly cut in spring. Warwickshire says blades dull faster when you mulch, because the clippings stay in the deck. If the cut goes ragged, stop and let a dealer see the blade. The first cut of the year goes on the highest setting. In peak growth, The Best Mowers suggests every 4–7 days as their own line. The RHS says short, dry clippings under about 1 cm can be left. Warwickshire’s less-fertiliser line is their statement, not a measured saving. If you come home to grass well past one third, fit the box and take the height down in stages.
Swift Swift EB122C

Testsieger

Swift Swift EB122C
9.0/10about £134.99
View offer

Plug or box for this cut

Put the box on, or leave the plug off, when the grass is wet, over-long, diseased, or covered in leaves you do not want shredded back into the sward. The Best Mowers also says not to mulch a show lawn. Diseased clippings should come off the surface. Clear a blocked deck only with the machine stopped. On a cordless mower, remove the pack. On a corded mower, unplug it. The 10 m cables on the Forte and the Hyundai HYM3800E need an outdoor lead, fully unwound, a fused UK plug and an RCD, which the cordless or corded page sets out.

When to stop and take it back

Stop and take the mower back when the lead, plug or charger is hot or cracked, when the blade will not turn or will not stop, or when a guard is missing. One saved Swift review describes a trigger that failed and a blade that kept going until the pack came off. If the mower is faulty, the Consumer Rights Act 2015 gives a short-term right to reject, normally within 30 days of delivery. That is an outline, not advice on your contract. A blunt blade after a season of mulching is a job for someone equipped to do it.

Common questions

How do I fit a mulch plug?
Stop the mower, take the pack off or unplug the lead, and fit the plug so the path to the box is closed. Mow only if the grass is dry and you are removing about one third of the height. If your model uses a lever for mulch, collect or drop, set mulch and check that the box is off when the manual says the two cannot run together. The plug does not guarantee the clippings will vanish.
How short should the grass be before I mulch?
Short enough that one pass stays inside about one third of the height you find. A lawn at about 6 cm can come down to about 4 cm. Warwickshire’s guide says mow before about 8 cm if you plan to leave 4–5 cm. Grass well beyond that wants the box and staged cuts, not a single mulch pass.
Can I mulch wet grass?
Leave wet grass alone. Wet clippings clump, block the deck and lie in rows that yellow underneath. Mow when the surface is dry, and water afterwards if the lawn needs it. A saved review on the Greenworks 24 V in this comparison describes clogging in the wet. A cordless mower removes the lead and still leaves you with that poor cut.
What is the one-third rule in centimetres?
Take off no more than about one third of the height you find. At 6 cm, one third is about 2 cm, so you leave about 4 cm. At 7.5 cm, one third is about 2.5 cm, so you leave about 5 cm. The RHS summer target is about 2.5 cm, reached over later cuts. Your lowest step may be 30 mm, which cannot hit 20 mm whatever the arithmetic says.
When should I put the box back on?
Put it back for wet grass, for a lawn that has grown past one third, for diseased patches, and for a leaf layer you need to lift. Use the box, a drop mode or rear discharge if the listing has it. A mulch-only mower with no stated box leaves you mowing more often so the clippings never get long.
How often should I mow in a UK spring?
Weekly is the rhythm Warwickshire describes for spring, inside an RHS season from March to October. In peak growth, every 4–7 days is The Best Mowers’ line. The first cut stays on the highest setting. Little and often keeps clippings short and dry. That is not a measured fertiliser saving, and one wet pass does not feed the lawn.

Related pages

Best mulching mower

Mulch mower

Mulching lawn mower buying guide: plug, width, lead

Cordless or corded mulching mower: cable or battery

Mulching mower for a small lawn: 22 cm versus 33 cm

Sources

Public UK sources and the amazon.co.uk listings for this set. Prices from 19 October 2026. Retrieved 22 October 2026.

  1. RHS: how to mow a lawn (retrieved 22.10.2026)
  2. RHS: lawn mowing (retrieved 22.10.2026)
  3. Warwickshire County Council: mulch mowing, height (retrieved 22.10.2026)
  4. Warwickshire County Council: mulch mowing, deck and plug (retrieved 22.10.2026)
  5. Noise Emission in the Environment by Equipment for use Outdoors Regulations 2001, Schedule 3 (retrieved 22.10.2026)
  6. GOV.UK: Supply of Machinery (Safety) Regulations 2008 (retrieved 22.10.2026)
  7. Electrical Safety First: lawnmowers (retrieved 22.10.2026)
  8. Consumer Rights Act 2015, section 22 (retrieved 22.10.2026)