Can I mulch with grass clippings?

Grass clippings are the largest free source of nitrogen-rich organic matter most gardens produce, and most of it goes into a green bin. Used properly they suppress weeds, hold moisture and feed the soil. Used badly — piled on thick and wet — they turn into a fermenting mat that repels water and smells appalling, which is why the material has a mixed reputation.

Safety

What you'll need

• Grass clippings
• A rake
• A tarpaulin or a wheelbarrow to dry them on (optional)

Step by step

1. Check what has been on the lawn first.Weed and feed and selective lawn herbicides contain hormone-type compounds that damage broad-leaved plants. Residues persist through several cuts. If the lawn has been treated this season, compost the clippings rather than mulching with them, and keep them off food crops entirely.
2. Let them wilt for a day.Spread them on a tarpaulin or leave them in a thin layer in the barrow for a day. Half-dried clippings mat far less than fresh wet ones and are much easier to spread evenly.
3. Apply no more than 25mm at a time.This is the rule the whole material stands on. A thin layer dries, breaks down and disappears within two or three weeks. A 75mm layer of fresh clippings ferments anaerobically, forms a sealed water-repellent mat, and smells of silage.
4. Reapply after each cut rather than stockpiling.Little and often is the correct pattern, and it matches how the lawn produces them. Repeated thin layers through the summer add up to a substantial amount of organic matter.
5. Put them where the nitrogen is wanted.Around brassicas, courgettes, sweetcorn, leafy crops, soft fruit and hungry ornamentals. They are a nitrogen source as much as a mulch — roughly one part nitrogen to twenty parts carbon, which is rich by mulch standards.
6. Keep them off the stems.A collar of hot, fermenting clippings against a stem causes rot. Leave 50mm of clear space around every stem, as with any mulch.
7. Do not use them around plants that want lean conditions.Lavender, rosemary, Mediterranean shrubs, alpines and anything grown for flowers rather than leaves get soft and floppy on a nitrogen-rich mulch. Gravel or nothing is better there.
8. Watch for weed seed.Clippings from a lawn that has gone to seed, or that contains a lot of annual meadow grass, carry viable seed onto your beds. Cut before seed heads form, or compost those cuts instead.

Tips

• Mulching a lawn — leaving the clippings on the grass with a mulching mower — returns most of the nitrogen and is the best use of the material if you have nowhere else for it.
• Clippings under a straw or bark mulch work well: the clippings supply the nitrogen the coarser material would otherwise draw from the soil.
• Around potatoes, a thin clipping mulch through the season suppresses weeds and keeps the soil cool and moist, and it is a traditional and very effective use.
• In a compost heap, clippings are the classic 'green' and need mixing with two or three times their volume of brown material or they slump into a slimy mass for exactly the same reason.
• A thin clipping mulch dries pale and looks like hay within a couple of days. That is normal and it is doing its job.

Common mistakes

• Emptying the whole mower box onto one bed — A thick layer of fresh clippings goes anaerobic within a day, heats, seals the surface against water and smells strongly. It also kills anything small underneath it.
• Using clippings from a recently treated lawn on vegetables — Selective lawn herbicide residues distort and kill broad-leaved plants. Tomatoes, beans and salad crops are especially sensitive, and the symptoms look exactly like manure contamination.
• Mulching seedlings with clippings — They generate heat as they break down and they smother anything small. Wait until plants are established and well clear of the mulch layer.