Thatch is an organic matter that is found between the soil and lawn surface. This organic matter usually decomposes by the effects of air, water, and beneficial soil microbes. Cases where thatch decomposition slows down such as: Soil microbes are not active Lack of soil air Mowing is very high and infrequent Soil is acid Dethatch if your lawn thatch is more than 12 mm thick because thick thatch: Reduces water infiltration into the soil Dries out root mass and turns the lawn yellow in the summer Increases lawn diseases Decreases lawn tolerance to cold and heat Reduces soil air Reduces lawn nutrition Dethatching substantially improves lawn growth in the sense that your lawn becomes more beautiful, requires less water, becomes dense in a healthy environment with less weed invasion.