Why is my lawn full of moss?

Moss does not outcompete healthy grass; it colonises space where grass is struggling. Killing the moss without addressing why the grass is weak guarantees it returns, usually within a season.

Four conditions favour moss, and most affected lawns have more than one.

Shade is the most common. Grass needs substantial direct light, and areas under trees or on the north side of buildings will never sustain dense turf. Thinning overhanging branches helps; where that is not possible, a shade-tolerant seed mix or accepting a different ground cover is more realistic than fighting it.

Poor drainage and compaction come next. Waterlogged or compacted soil suffocates grass roots while suiting moss. Aerating, by removing cores rather than merely spiking, relieves compaction and is one of the more effective interventions.

Acidic or nutrient-poor soil favours moss. A cheap soil test kit gives a pH reading; grass generally prefers slightly acidic to neutral, and lime can raise a pH that is genuinely too low. Testing first matters, because liming an already suitable soil causes its own problems.

Mowing too short weakens grass considerably, reducing its ability to photosynthesise and recover. Raising the cutting height is free and often makes a visible difference within weeks.

The practical sequence is to treat the moss, scarify to rake out the dead material, aerate, then overseed and feed so grass fills the space before moss returns. Doing this in autumn or spring, when conditions favour establishment, matters. Treating without scarifying leaves a dead mat that blocks new growth.