Heavily fertilized Kentucky bluegrass often develops thatch. The thatch layer becomes a waterproof blanket on the soil surface. The roots and rhizomes of bluegrass grow in the thatch layer rather than in the soil where they belong. Rake and remove the material you cut with these machines. May and September are good times to remove thatch.
Research shows thatch buildup is related to earthworms and night crawlers in the lawn.
Thatch tends to develop in lawns where there are a few, if any, earthworms. Lawns do not require dethatching every year. Many lawns never require thatch removal. If a layer of thatch makes it difficult to water through to the soil, you should probably remove the thatch. Topdressing the lawn with one-fourth inch soil speeds natural thatch decomposition.