What do scarifying and aerating actually do to a lawn?

They address two different problems that are frequently confused, and doing the wrong one achieves nothing.

Scarifying removes thatch.

Thatch is a layer of dead grass stems, roots and organic debris accumulating at soil level between the green blades and the soil. A thin layer is beneficial — it insulates and retains moisture. Beyond about 1cm it becomes a problem:

It repels water, so rain and irrigation run off rather than reaching roots.

It holds moisture at the surface, encouraging shallow rooting and fungal disease.

It blocks air and nutrients.

It harbours moss, which thrives in the damp organic layer.

Scarifying uses blades or tines to rake vertically through the sward, tearing out thatch and moss. The result looks catastrophic — brown, torn and bare in patches — and this is normal. The lawn recovers within weeks, and the alarming appearance is why people stop halfway.

Aerating relieves compaction.

Compaction occurs when soil particles are pressed together by foot traffic, mowing, play and rain, reducing pore space. Roots need air as well as water, and compacted soil suffocates them and prevents drainage.

Hollow-tine aeration removes plugs of soil, physically creating space — the most effective method, and the cores can be left to break down or raked off.

Solid-tine or spiking pushes holes without removing soil. Easier, but it compresses the surrounding soil further, so it is a weaker fix.

Which do you need? Push a screwdriver into damp soil — if it resists, that is compaction. Part the grass and look at the base — a spongy brown layer over 1cm is thatch. Many lawns have both.

Timing matters. Both are stressful, so do them when the grass can recover strongly — early autumn is generally ideal, or spring. Avoid summer drought and winter waterlogging.

Follow with top dressing, overseeding and feeding.