In general grass seeds will not germinate until they are below the surface of the ground and have enough moisture. Overseeding is a process of using a broadcast spreader to apply grass seed on the surface of the ground. To increase the percentage of germination, we recommend in conjunction with our aeration service. The cores created by aeration allow the seeds to drop below the surface of the ground for a better rate of germination. Slit seeding is a process of utilizing a machine with knife blades to create slits in your lawn, which allows grass seed to drop below the surface of the ground, maximizing the percentage of germination. As a general rule, if your lawn is at least 80% thick the process of aerating and overseeding is a good choice; any percentage less, slit seeding is a better choice. We have experienced lawns that were 50% thick and have undergone the process of overseeding each year and it has taken 3 years to completely fill in, but other lawns that were slit seeded only once and have completely filled in within the first year.