Here’s a good article from the Editors of Landscape on dethatching to make your lawn beautiful.

Thatch is the term used in modern landscaping for the organic material buildup between living grass and the soil’s surface. This buildup is composed mainly of the roots and stems of lawn grasses, which tend to break down more slowly than they build up. While manageable in lower quantities, a thatch buildup of excessive depth (greater than ½ inch) will inhibit grass root development as well as provide a breeding ground for lawn pests. Excessive watering and fertilizing are major causes of thatch buildup due to the lawn’s speedy growth. New grass grows much more quickly than dead grass can be broken down by natural processes in such lawns. This can cause the new lawn grass to root into the thatch rather than the soil, depriving it of important nutrients and potentially causing drainage problems.

In order to combat the myriad of threats posed by thatch buildup, homeowners must first keep a close eye on their fertilization and watering habits. Remember that aggressive growth can also lead to aggressive thatch buildup, so try to strike a balance between lush coverage and thatch presence. If thatch reaches problematic levels—as evidenced by grass tearing up by the root quite easily—core aeration can help alleviate the problem. Core aerators are machines that pull up small cores of soil to leave behind plug-like holes. Cored soil can then be covered with a thin layer of organic material (topdressing) to assist in breaking down the excess thatch. Soil cores remaining on the surface of the lawn accelerate thatch breakdown. Core aeration also helps to correct drainage and compaction issues.

When considering whether to dethatch their landscape, homeowners need to be aware of a few principles. First, dethatching places a good deal of stress on the lawn. Core aeration and dethatching machines are aggressive practices. Lawns typically need a recovery period of thirty to fifty days of relatively mild weather in order to regenerate. Severe weather during this time can be extremely detrimental to the recovery process and may damage large swaths of the landscape. DIY landscapers should keep an eye on their weather reports to gather an idea of when to dethatch most effectively. Much also depends on the variety of grass used in the lawn—some varieties grow best cool weather while others prefer warmth. Homeowners need to prepare themselves by learning as much as possible about their particular variety of grass, preferably prior to the initial seeding. Choose a variety that will not produce much thatch. If DIY landscapers are working in a pre-existing lawn, ask questions at the local plant nursery or garden center regarding the correct thatching timeframe for your variety.