The thatch layer in lawns is the brown section of the grass which sits between the green leaf at the top of the grass and the runners and root system at the base of the turf. The thatch layer consists of both living and dead lawn runners, and decomposing leaf and organic matter.
Thatch is a natural part of turf and serves a vital purpose for all lawns, which is to insulate the soil against heat - thus reducing water evaporation from the soil, as well as cushioning and protecting the underlying crowns and root systems. Without the thatch layer, our lawns just wouldn't be as soft nor be usable in the ways we want them to be.
Thatch becomes a problem for lawns when it excessively increases it's thickness over a period of time. The result is that the lawn browns when mowed, becomes more difficult to mow, less nice to walk over or for children to play on, it traps excessive amounts of moisture which can increase the likelihood of disease, and reduces the amount of water which actually reaches the soil.
Lawns will become brown when lawn mowing cuts into the brown thatch layer at each service. Increasing lawn mowing heights may seem to fix the problem by removing less grass with each cut and thus not cutting into the thatch. Unfortunately the opposite is true - increasing mowing heights will neither repair the thatch problem, and will in fact cause the rapid increase of more thatch being created, this is because we're also leaving too much green leaf on the lawn - which increases photosynthesis, increasing growth rates.
All grass types will create a thatch layer as a normal part of it's growth and life cycle, this includes both warm and cool season grasses. The thatch of cool season lawns contains no runners while the thatch layer of warm season grasses will contain runners, which often makes grass types such as Buffalo to increase it's overall thickness as the thatch layer increases.
Once thatch has increased to undesirable levels, it will be necessary to remove it effectively, and this can be best achieved differently with the different grass types.
The main methods of de-thatching a lawn can include very low mowing which will not be suitable for all grass types and could even kill a Cool Season turf, and the other methods include lawn aeration via a lawn coring machine, and last and most effective is to have the lawn professionally de-thatched in a process often referred to as Vertimowing.