Thatch is the build-up of organic debris that prevents air, water, and sunlight from reaching the soil in the lawn. Excessive build-up of thatch can stop grass from growing and may make the lawn more prone to insects and lawn disease. Power-raking or dethatching lawns removes the harmful build-up. Power-raking at least once annually will help prevent thatch from making the lawn root system vulnerable to extreme dehydration. A combination program of preventative core aeration and proactive dethatching will improve even the most neglected lawn.