Why does my lawn feel spongy underfoot?

A little thatch is useful — it cushions wear and holds moisture. It becomes a problem when it grows faster than soil organisms can break it down, which happens on compacted, acidic or waterlogged lawns and where clippings from long grass are left on the surface.

What you'll need

• Scarifier or springbok rake
• Hollow tine aerator or fork (optional)
• Sandy top dressing
• Grass seed

Step by step

1. Measure the thatch.Cut a wedge of turf out with a spade and look at the profile. Below 10mm of brown fibre is normal and beneficial. Above about 20mm the lawn will feel bouncy and shed water.
2. Work out why it built up.Compaction, acidic soil, poor drainage and leaving long clippings all slow decomposition. Removing thatch without changing those means it returns within two years.
3. Choose the right season.Early September is best — the grass recovers fast and there is rain. Late spring works. Never scarify hard in midsummer or midwinter; you strip the lawn and it cannot regrow.
4. Mow short first.Cut lower than usual and box the clippings so the scarifier reaches the thatch rather than the leaf.
5. Scarify in two directions.Once across the lawn, then again at right angles. The amount of material that comes out is always alarming; a healthy lawn regrows within four weeks in autumn.
6. Remove everything you pull out.Rake and bag it. Left on the surface it smothers the grass and adds straight back to the thatch layer.
7. Hollow-tine afterwards.A hollow tine removes a plug of soil and leaves a hole the surrounding compacted soil can expand into. A solid tine or a fork only pushes soil sideways and compresses the hole walls.
8. Brush the cores in or remove them.Let the plugs dry, then break them up and brush them back in as a free top dressing, or rake them off if the lawn is heavily mossy.
9. Top dress with sandy loam.A few millimetres brushed in fills the holes and improves the surface drainage that caused the problem.
10. Overseed and feed.Scarifying opens the sward. Overseed immediately so grass rather than moss fills the gaps, and feed with an autumn lawn feed.

Tips

• Cut a wedge out and look at it — guessing at thatch depth from the surface is unreliable.
• Hollow tining relieves compaction; solid tining mostly just lets surface water in.
• Do it in September and the lawn looks worse for three weeks and better for three years.

Common mistakes

• Scarifying hard in spring and then leaving the lawn to it — Spring scarifying opens the sward just as weed and moss spores are most active. If you must do it in spring, overseed and feed at the same time.
• Using a solid-tine roller and calling it aeration — Solid tines compress the sides of the hole they make, so on clay they can make compaction slightly worse. Hollow tines remove material and genuinely relieve it.

If it doesn't work

Questions people ask