The first seven years with a new home lawn has been called the "Honeymoon" period. Most of the reason behind that nickname is the absence of a harmful thatch layer that is over one-half inch thick.
Up to one-half inch of thatch depth is considered alright, with minimal thatch even being considered beneficial. However, when thatch exceeds one-half inch in depth, serious problems begin to occur. It really gets serious when thatch reaches one inch thick.
GREEN-UP AND GROW!
One of the main contributors to thatch development is ongoing heavy nitrogen fertilization. Nitrogen is the first number on a fertilizer label and the nutrient that pushes grass to green-up and grow.
Some varieties of grass, such as Kentucky bluegrass, tend to produce thatch due to the growth habit of their roots (rhizomes). Heavy clay soil can also contribute to more rooting on the lawn surface, instead of into the soil beneath. Aerating a lawn at least once annually helps to combat these poor rooting situations, as well as preventing thatch build-up.
MULCH YOUR GRASS CLIPPINGS?
Mulching clippings will not contribute to thatch in normal situations, where there isn't already a thatch problem. In heavily thatched lawns it will be beneficial to bag and compost clippings, since clippings are less likely to reach the soil surface where microbes can break them down naturally.
Core aeration done once or twice annually, involves "plugging" the lawn with a "hollow-spoon" aerator. SEE VIDEO BELOW.
De-thatching once every year or two, involves mechanically removing some of the dead debris from the lawn surface.
While the soil pH of a lawn can be in the proper range and not require lime, a light application of lime will benefit thatch decay.
In cases where thatch build-up has exceeded 1-inch, Bob recommends stripping the lawn down to the soil with a sod-cutter and starting from scratch with a new lawn. At this extreme thatch thickness, it would take several years to reduce the thatch to an acceptable level using normal means, with costs in the long run being about the same. Seeding a completely new lawn also allows for the introduction of improved turfgrass varieties which weren't available 50 years ago.