Thatch is a layer of organic matter that accumulates on a lawn around the base of the grass plants. Thatch is a combination of living and dead plant matter.

Removal of thatch is an important part of good lawn maintenance.  If it is left to build up, eventually the lawn will weaken and become prone to moss and disease.

If your soil pH is low, applying lime to your turf will raise soil pH levels and encourage thatch decomposition. 

There are two types of thatch, surface and subsurface.  Scarify to remove surface thatch and aerate to remove sub-surface thatch.  If possible aim to prevent thatch from building up in the first place.  Carrying out core aeration in the autumn will reduce compaction and help to improve the air movement in the soil which will also improve microbe activity.

Fertiliser is good for turf health, although it is very important to apply the correct feed, the correct amount at the correct time, or you may inadvertently increase thatch build-up.  Should the thatch layer build up to an inch or more, you can rake it out if the area is not too large.  Hire a scarifying machine for bigger lawns.

If you plan to remove thatch do this when the grass will grow back quickly, it can be quite shocking if you have a lot of thatch as when you remove it you may be left without much grass.   Over seeding will be required.

Only consider de-thatching if you have a strong healthy lawn, weak damaged turf will not recover well.