Got a Bag Full of Grass Clippings?
Grass clippings make decent free mulch — but only if you use them correctly. Applied too thickly, they turn into a slimy, smelly mess. Here’s how to get it right.
Quick Answer: Grass Clippings as Mulch
Grass clippings are one of the most readily available mulching materials in any UK garden. Every time you mow, you generate a supply of nitrogen-rich organic material that can be put straight to work suppressing weeds and feeding the soil. The limitation is longevity — clippings decompose within weeks, so they’re a temporary mulch rather than a season-long solution like bark or wood chip.
How to Use Grass Clippings as Mulch
The golden rule with grass clipping mulch: thin and frequent beats thick and occasional. Here’s the proper method:
- Spread thinly. Apply a layer no more than 3-5cm deep. Scatter the clippings loosely — don’t compact them into a dense mat
- Let it dry. Fresh clippings are wet and dense. After a day or two in the sun, they dry out, shrink and become a better mulch layer. If possible, spread clippings on a tarp to dry for 24 hours before applying
- Build up gradually. After each mowing, add another thin layer on top of the previous one. This creates a layered, well-aerated mulch that decomposes steadily rather than turning anaerobic
- Keep away from stems. Don’t pile clippings against plant stems or tree trunks. The heat generated during decomposition can damage soft stems, and the moisture encourages rot
What Happens When You Apply Too Much
The most common mistake with grass clipping mulch is dumping a thick pile straight from the mower. Here’s what goes wrong:
- Anaerobic decomposition: Thick layers exclude air. Without oxygen, anaerobic bacteria produce hydrogen sulphide (rotten egg smell) and organic acids that can damage plant roots
- Water-repellent crust: Compacted clippings dry into a papery mat that actually sheds water rather than letting it through to the soil — the opposite of what you want
- Heat build-up: Fresh clippings generate significant heat as they decompose. A thick pile can reach temperatures high enough to damage nearby plant stems
- Nitrogen robbery (temporary): As fresh clippings decompose rapidly, soil microorganisms consume nitrogen from the soil to fuel the breakdown process. This is temporary, but it can cause yellowing in shallow-rooted plants
If you’ve already got a thick, slimy layer, rake it up, spread it thinly somewhere to dry out, and reapply it properly. Or add it to the compost bin where the nitrogen content makes it an excellent activator.
Where Grass Clippings Work Best (and Worst)
Grass Clippings vs Other Mulch Types
How do grass clippings compare to other common mulching materials?
For permanent beds where you want lasting weed control, bark mulch is the better choice. But for vegetable gardens where beds are replanted each season, grass clippings are practical, free and genuinely beneficial for soil health. Many experienced vegetable gardeners use both — bark mulch on permanent paths and borders, grass clippings between crop rows.
Important: Don’t Use Treated Grass
If you’ve recently applied weed killer or lawn treatment to your lawn, don’t use the clippings as mulch for at least 3-4 mowings. Herbicide residues (particularly clopyralid) can persist in grass clippings and damage or kill the plants you’re mulching. This is especially critical in vegetable gardens where herbicide contamination can affect food crops, especially sensitive species like dandelions if you’re using them for salads.
The same applies to composting treated clippings. Clopyralid and aminopyralid can survive the composting process and contaminate your compost with herbicide residues that damage tomatoes, beans, peas and other sensitive plants. If your lawn has been treated with any selective herbicide, bin the first few mowings or use them only on non-food areas.
Frequently Asked Questions
Can I leave grass clippings on the lawn instead of collecting them?
Yes — this is called grasscycling and it’s actually beneficial. Short clippings (from regular mowing) decompose quickly on the lawn surface, returning nitrogen and other nutrients to the grass. Only collect clippings if they’re long enough to clump and smother the lawn, or if you want to use them as mulch elsewhere.
Do grass clippings attract slugs?
They can. The moist, decomposing layer provides exactly the cool, damp environment that slugs favour. In vegetable gardens where slug damage is a concern (especially around lettuce, hostas and seedlings), consider using a drier mulch or applying slug control around vulnerable plants.
Can I mix grass clippings with bark mulch?
Yes, this can work well. A thin layer of clippings under a bark mulch layer adds nitrogen that speeds up the beneficial decomposition process. The bark provides the lasting weed barrier while the clippings feed the soil underneath. Just don’t mix thick layers of fresh clippings into bark — apply them as a separate thin base layer.
How long do grass clippings last as mulch?
A 3-5cm layer typically decomposes within 4-8 weeks in warm weather, faster in wet conditions. In winter, decomposition slows significantly. You’ll need to top up regularly throughout the growing season to maintain any meaningful weed suppression.
Will grass clippings make my soil acidic?
No. Grass clippings are close to neutral pH (6.5-7.0) and don’t significantly alter soil acidity as they decompose. This makes them safe for use around virtually any garden plant without pH concerns.
Can I use grass clippings from a lawn with moss?
Yes. Moss in the clippings won’t establish itself in your garden beds — it needs very different conditions. However, if your lawn has significant moss, addressing the underlying cause (shade, compaction, poor drainage) will improve your lawn health and give you better-quality clippings.
Want a Healthier Lawn With Better Clippings?
A well-fed lawn produces stronger, healthier grass clippings for mulching. Our mulching guide covers all the options for keeping your garden weed-free.
