Lawn thatch is the dead grass and root tissue between the green vegetation and the soil surface. In layers of 1/2 inch or thicker, thatch blocks water, air and nutrients from reaching the roots and provides a nesting place for insects and disease.
Many grass varieties in a traditional synthetic lawn care system tend to build thatch layers quickly. Excessive nitrogen pushes out excessive top growth, but it limits life in the soil and therefore slows decomposition of roots even more. The process of dethatching, either with a rake or a power machine, removes the thatch, which can then be gathered and composted.
Some thatch is common and acceptable in all lawns, but too much must be removed. The good news is that natural lawn systems that add life into the soil will rarely have issues with excessive thatch.
