Why Dethatch?

Thatch can keep water and air from reaching the soil and, if left untreated, can create an environment that harbors pests and diseases. Dethatching removes those layers of dead grass, roots and debris matted between the soil and the growing grass, keeping the grass greener and healthier while minimizing the chance of disease.

WHEN TO DETHATCH A LAWN

Depending on where you live in the country:   warm-season grass, you should dethatch in the spring after the grass has begun to green.

If you are planning to overseed a lawn, you should plan to dethatch before seeding.

Source: Why Dethatch? | RYAN® Turf Renovation Equipment (ryanturf.com) 

For more information:  Controlling Thatch in Lawns | Home & Garden Information Center (clemson.edu)