Core aerating is the process of removing vertical cores from the lawn; it should be done once a year. Aerating allows air into the soil, creates a proper thatch layer, and also helps develop a topsoil layer underneath the grass. Aerating is usually accomplished with a self-propelled aerating machine that a professional walks around the yard. The cores that are removed should be left in the yard. The lawn mower will break them up; this will help the lawn. Bluegrass and Fescue grasses are usually aerated in the spring or fall.