The tangled mat of organic matter around the base of the grass plant (above the soil) is called thatch. In small amounts it is good for the lawn. It acts as a natural mulch to retain moisture, lower soil temperature, cushions plants crowns, and discourages germination of weed seeds. The decaying plants add nutrients back into the soil.
Too much thatch can cause shallow roots, nutrient imbalance, and an invasion of insects and disease. When mowing turf with a heavy thatch, scalping may occur because the wheels sink into spongy turf. Grass which thick thatch is less tolerant to stress because of poor root structure.
Excess thatch results when the organic matter accumulates faster than the microorganisms can break it down. The could result from over-fertilization, infrequent watering, or leaving long clippings on the lawn.
Two mechanical procedures that help keep thatch under control are aeration (removing grass plugs of soil from the lawn) and de-thatching or power-raking (removing accumulated materials around the crown of the grass). These methods allow water, air, and nutrients through to the soil.
De-thatching involves cutting through the thatch layer either with a cavex rake or power rake. When to de-thatch depends on the type of grass and time of the year. Power raking can cause damage to the lawn and can even remove large amounts of living grass. Shallow power raking may be more beneficial. Power raking should be done prior to grass greening up.
Aeration involves the use of hollow tines, which penetrate the lawn and remove a sod core. The benefit of core aeration includes the relief of compaction and improved root growth. The removed cores should be left on the surface to be allowed to break down in time.
Most home lawns should be core aerated at least once a year. The best times are either spring or fall. Use an aerator that will remove a 2-3 inch long plug. Water lightly the day before so that the tines can penetrate the soil and remove the plugs easily. Flag all sprinkler heads. If the turf is thick, aerate twice. The second one should be at right angles to the first.
A certain amount of thatch is normal - 1/2" or less. The secret is to maintain that delicate balance of micro-organisms necessary for the turf environment.