Moss in lawns: why it comes and how to keep it away

Moss is a symptom, so read the clues

Moss does not push grass out. It moves into gaps that thin, stressed or struggling grass has left. The RHS puts it plainly: there are no products to kill moss permanently, and it is very likely to come back if the growing conditions stay the same.

That is worth taking seriously before you buy anything. The RHS also points out that moss is not always unwelcome. If your conditions suit moss better than lawn grass, it makes low, evergreen ground cover that you can walk on without mowing. Some people choose to leave it in a shady corner and concentrate their effort elsewhere. If you want a grass lawn, though, you will need to deal with why the moss is there.

The usual causes

The RHS lists these as the common reasons moss thrives in a lawn, and Oregon State University names shade, poor drainage and low soil pH as the main drivers:

• Sparse, weak or patchy grass, often from drought stress, shade or mowing too low.
• Worn, bare patches where people or children play or walk a lot.
• Compacted soil, or poor soil preparation before the lawn was laid or sown.
• A damp climate, or waterlogged conditions.
• Acidic soil, which many mosses prefer. The RHS notes this, though it does not give a pH figure.
• Shade from trees, fences and buildings.

Why killing it alone does not last

Synthetic mosskillers work on what is already there. The RHS says they are not recommended, because control is temporary unless you change the growing conditions. Oregon State says much the same: chemical treatments are short-acting, the dead moss still has to be raked out, and the conditions that favoured moss remain.

Many mosskillers contain iron sulphate. The RHS says it kills moss but leaves black dead moss that you then have to rake out. It also warns that iron sulphate sold as lawn fertiliser should not be used for moss unless it is part of an HSE-approved lawn mosskiller, which comes with timing and rate instructions. Household products such as baking soda, vinegar, bleach and soap should not be used, because they can kill grass and are not approved.

The RHS gives another option: a bacteria-based moss remover. It describes this as digesting the moss, so you do not need to rake out the dead material, and says it does not remove all of it but usually makes a noticeable difference.

The sequence that lasts

Oregon State sets out a sequence that fits well with the RHS advice, and it is a sound way to think about the job. The order matters, because doing the later steps first wastes them.

• Decide whether it is worth acting. If the moss is in deep shade where grass will never thrive, accept it, or plan a different surface.
• Remove what is there. Rake it out with a spring-tined rake on a small area, or scarify. The RHS says to do this in October or early November while the grass is growing strongly, and notes spring is also a good time to rake it out. Westland suggests treating heavy moss first, because living moss can spread spores when disturbed.
• Fix the cause. Open up shade by pruning nearby trees or shrubs. Relieve compaction by aerating and top dressing, which the RHS recommends. Improve drainage where it is poor, though the RHS admits that installing drainage is a major job.
• Put grass back. Overseed the bare ground, using a shade-tolerant mix where light is poor. Oregon State says to cover bare soil with seed and keep it watered until it sprouts.
• Maintain it. Feed, mow and water sensibly. The RHS recommends a programme of spring and summer care plus autumn care to keep the lawn healthy.

Mowing height and feeding

Mowing too short is one of the causes the RHS names, and its mowing page says very close cutting weakens grass, encourages shallow roots and makes lawns more prone to drought damage, weeds and moss. The RHS gives 13-25mm (half to one inch) as the summer height for ordinary domestic lawns, and up to 40mm in spring and autumn, never taking off more than a third of the grass in one cut. Our guide to mowing height through the year goes into this.

Feeding is more nuanced. The RHS says most lawns recover from summer wear without feeding, and that you should use the minimum needed. A hungry lawn can thin out and let moss in, but Clemson names over-feeding as a cause of thatch, so more is not better. We cover that in the thatch guide.

A word on lime and drainage

Oregon State includes liming where soil pH is low. The RHS page we read lists acidic soil as a cause but does not set out a liming step, so we would test the pH before adding anything.

Drainage is more difficult. A compacted, waterlogged lawn, particularly on heavy ground, will keep inviting moss until the soil can drain. Hollow-tine aeration followed by top dressing is the standard route, and the RHS says to do it every three to four years on clay or waterlogged soil. See our aeration guide and lawns on sandy, chalk and clay soil in East Kent for how soil type changes the plan.

Putting it into a plan

Treat moss as a yearly cycle, not a one-off battle. Rake or scarify in autumn, repair and reseed, fix one underlying cause each year, and keep the mowing height sensible. Our lawn restoration service follows this same order, and the annual plan spreads the work across the seasons so the causes get dealt with, not just the symptoms.