Lawn thatch, the layer of organic matter between grass blades and the soil surface, may be the result of infrequent, high mowing, poor soil aeration, frequent light watering, excess pesticide, soil compaction or excess nitrogen fertilizer application. When thatch becomes more than 1/2-inch thick, it makes a lawn more vulnerable to drought and disease. Rent a core aerifier to help remove yard thatch about six weeks before the soil freezes in the fall. Not only does aerification stimulate the microbes that can help decompose thatch, it also encourages healthier and deeper root systems and promotes better air and water penetration of the soil.
Fertilize the lawn in the fall, following package directions, about one week prior to aerification. This will promote grass growth to help fill in the holes you'll be creating.
Push the core aerifier back and forth over the entire surface of the lawn. Go over the lawn a second time in a direction perpendicular to your first pass. Go over the lawn a third time in a diagonal direction.
Allow the cores on the surface to dry for a few days, then rake them to break them up and fill in the holes somewhat.
Repeat the aerification process the following spring if thatch is thick or soil is rather compacted.