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Quick Answer
Moss grows in lawns when the grass is weak. The four main causes are: shade, poor drainage, compaction, and acidic soil. Killing the moss with iron sulphate is the easy bit. The hard bit is fixing whatever caused it in the first place.
If your lawn is more than 50 percent moss, the cheapest long-term solution may be to start again. Strip the lot, improve the soil, and reseed with a grass mix suited to the conditions.
The Four Causes of Moss
1. Shade
Grass needs at least 4 hours of direct sun a day. Less than that and the grass thins, leaving bare patches that moss happily colonises. Shade can come from buildings, fences, hedges and trees. The most common cause in UK gardens is the north side of a house or fence.
Solutions for shade:
- Prune or thin trees: particularly evergreens and dense shrubs. Even removing lower branches can let in a lot of light.
- Reduce fence height: if practical, replace solid panels with trellis to let light through.
- Plant a shade-tolerant grass mix: fescues and bentgrass tolerate shade better than ryegrass. See RHS lawn seed guidance.
- Accept the moss: in heavy shade, moss is often the best ground cover. It is green, soft and needs no mowing.
2. Poor drainage
Waterlogged soil kills grass roots but moss loves it. If your lawn has standing water after rain, or stays squelchy for days, drainage is the problem. This is particularly common on heavy clay soils like those in Aylesbury and parts of Reading.
Solutions for poor drainage:
- Aerate the lawn: use a fork or hollow-tine aerator to create holes 10cm deep. Fill with sand to improve drainage.
- Top-dress with sand: apply a mix of 70 percent sand and 30 percent soil to the lawn after aeration. This improves surface drainage over time.
- Install land drains: for severe cases, a French drain around the lawn can carry water away. This is more expensive but solves the problem permanently.
- Re-level low spots: any dip in the lawn will collect water. Level with topsoil and reseed.
3. Compaction
Heavy foot traffic, children playing, and regular mowing with a heavy mower all compact the soil. Compacted soil has no air pockets, which kills grass roots. Moss tolerates compaction better than grass.
Solutions for compaction:
- Aerate annually: the single most effective thing you can do. Hollow-tine aeration removes plugs of soil, letting air and water into the root zone.
- Avoid walking on wet lawns: this is when most compaction happens.
- Install a path: if there is a regular route across the lawn, install stepping stones or a proper path.
- Use a lighter mower: heavy cylinder mowers compact the soil. Rotary mowers are lighter.
4. Acidic soil
Grass prefers a soil pH of 6.0 to 7.0. Below pH 5.5, grass struggles and moss thrives. This is less common on the chalky soils of the Chilterns but very common in other parts of the UK, especially where there are conifers or where lawn sand has been overused.
Solutions for acidic soil:
- Test the pH: simple kits cost 5 pounds and tell you the pH in minutes.
- Apply garden lime: if pH is below 6.0, apply ground limestone at 50g per square metre in autumn. Re-test the following year.
- Avoid over-use of ammonium sulphate: this common lawn fertiliser acidifies the soil.
Other Contributing Factors
Low mowing height
Cutting the lawn too short weakens the grass. Moss tolerates short cutting better than grass. Never cut more than one third of the grass height in one mowing. Set the mower to 30 to 40mm for family lawns, higher in dry weather.
Poor feeding
Under-fed grass is weak grass. Apply a balanced lawn fertiliser in spring and again in mid-summer. Avoid high-nitrogen feeds in autumn as they produce soft growth that is damaged by frost.
Thatch build-up
Thatch is the layer of dead grass and moss that builds up at the base of the lawn. A thin layer (under 1cm) is fine. More than that and water cannot reach the soil, which weakens grass. Scarify annually in autumn to remove thatch.
Tree roots
Trees near lawns compete for water and nutrients. Large trees like oak and beech can dry out a lawn completely in summer. The only solution is to water the lawn in dry spells or to extend the border around the tree so the lawn does not reach the trunk.
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How to Get Rid of Moss
Step 1: Kill the moss
Apply iron sulphate (also called ferrous sulphate or sulphate of iron) at 5g per square metre in spring or autumn. This kills the moss within 2 weeks and turns it black. The grass is unharmed. Iron sulphate also greens up the grass and slightly acidifies the surface, which most grasses tolerate.
Avoid moss killers that contain dichlorophen as these can damage grass in warm weather. Iron sulphate is safer, cheaper and more effective.
Step 2: Scarify
Two weeks after applying iron sulphate, scarify the lawn to remove the dead moss. For small lawns, use a spring-tine rake. For larger lawns, hire a powered scarifier. Scarifying removes a surprising amount of material. A 50 square metre lawn can produce 4 or 5 bin bags of dead moss and thatch.
Step 3: Aerate
After scarifying, aerate the lawn to improve drainage and reduce compaction. Use a garden fork or hire a hollow-tine aerator. Push the fork in 10cm deep and rock it back and forth to widen the holes. Fill the holes with a 70/30 sand and soil mix.
Step 4: Top-dress
Apply a top dressing of 70 percent sand and 30 percent loam to fill the aeration holes and level the surface. Work it in with the back of a rake. The top dressing should be no more than 1cm thick or it will smother the grass.
Step 5: Overseed
Where the moss has killed the grass, you will have bare patches. Overseed with a grass mix suited to the conditions. For shaded lawns use a shade-tolerant mix. For family lawns use a hard-wearing ryegrass mix. Sow at 25g per square metre and rake lightly into the top dressing.
Step 6: Feed
Apply a balanced lawn fertiliser 4 to 6 weeks after overseeding. This helps the new grass establish and strengthens the existing grass. Water well after feeding.
Step 7: Fix the underlying cause
If you do not fix the underlying cause, the moss will be back within a year. Prune trees for shade. Install drains for waterlogging. Aerate annually for compaction. Apply lime for acidity.
When to Replace the Lawn
If your lawn is more than 50 percent moss, or if the underlying cause cannot be fixed (such as permanent heavy shade from a building), it may be better to start again. The process is:
- Strip the existing lawn with a turf cutter (hire for 80 per day)
- Dig or rotavate the soil to 20cm deep
- Add compost and improve the soil
- Level and consolidate the soil
- Sow new seed or lay new turf
For a 50 square metre lawn, the whole job takes 2 to 3 days and costs 400 to 800 in materials and equipment hire. See our lawn recovery service for what we offer.
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Try Our Lawn Calculator
We have built a calculator that gives you an estimate range for lawn renovation based on lawn size, the problem you have and the finish you want. It takes 2 minutes and gives a realistic price range.
Moss Prevention Calendar
| Month | Job |
|---|---|
| March | First cut of the year, light feed with spring lawn fertiliser |
| April | Scarify light thatch, apply iron sulphate if moss is present |
| May | Aerate compacted areas, top-dress with sand |
| June to August | Regular mowing, watering in dry spells |
| September | Scarify heavier thatch, overseed bare patches |
| October | Aerate, apply autumn lawn feed (low nitrogen, high potash) |
| November | Last cut of the year, clear leaves |
| December to February | Keep off the lawn in frost and wet weather |
Common Myths About Moss
Myth: Moss kills grass
Moss does not kill grass. Moss grows where grass has already died. The grass dies for one of the four reasons above, and moss colonises the bare ground.
Myth: You can dig moss out
You can dig moss out, but it will come back unless you fix the cause. Moss produces thousands of spores and grows back from any fragment of stem left in the soil.
Myth: Moss killer is enough
Moss killer kills the moss that is there now. It does nothing to stop new moss growing. Without fixing the underlying cause, you will be applying moss killer every year.
Myth: Lawn sand is the best moss killer
Lawn sand is a mix of ammonium sulphate, iron sulphate and sand. It does kill moss but the ammonium sulphate acidifies the soil, which can encourage moss in the long run. Plain iron sulphate is better.
Final Thoughts
Moss in lawns is one of those problems that is easy to fix temporarily and hard to fix permanently. The temporary fix is iron sulphate and scarifying. The permanent fix is addressing whatever is weakening the grass.
If you have a mossy lawn and would like help, get in touch. We can assess the causes and give you a plan to fix the problem for good.
Further Reading
- How much does regular garden maintenance cost?
- How to restore an overgrown garden
- Garden tidy-up checklist before summer
On mature lawns, check for leatherjackets and chafer grubs before scarifying. Established turf recovers more slowly than newly seeded areas, so adjust the renovation intensity accordingly.
Put this advice into practice
Use the relevant CGM tool or follow a related guide, then send us a few photos if you would like a tailored plan.
Sources and citations
This article draws on the following sources. Specific claims are cited inline where relevant.
- CGM field records (2024-10. Site observations from gardens across Oxfordshire, Buckinghamshire, Berkshire and Bedfordshire.
- Royal Horticultural Society (2024). RHS Gardening Advice and Plant Finder. Available at rhs.org.uk.
- Met Office (2024). Regional climate averages, South East England. Available at metoffice.gov.uk.
- British Geological Survey (2024). Soil parent material data for the Chilterns and Thames Valley. Available at bgs.ac.uk.