Core aeration is a type of lawn aeration whereby a machine (a lawn aerator) with hollow tines mechanically removes plugs or "cores" of soil and thatch from a lawn. Core aeration reduces soil compaction, creating a channel through which oxygen, water and nutrients can penetrate into the soil.
There are other ways to aerate a lawn. For example, some lawn aerators drive solid tines into the earth. But that method isn't considered as effective as core aeration, in which you're creating holes of a significant diameter: 1/2 inch to 3/4 inch. Likewise, you could plunge a pitchfork into the grass every X number of square inches, but that's not feasible for most people.
What's the Purpose of Core Aeration?
Picture the soil under a healthy lawn. It's a soil studded with networks of air pockets. Oxygen travels through these pockets, and that's important: Just because grass roots are underground, that doesn't mean they don't have to breathe! But it's more than just oxygen that must percolate down through the ground, as your grass roots also need water and whatever nutrients homeowners are supplying when fertilizing lawns. There's a veritable beehive of activity going on down there under a healthy lawn!
Now consider the soil under grass that is performing poorly. Instead of being fluffy enough for oxygen, water and nutrients to flow through it (down to the grass roots), it is compacted. It may also be crowned with a thick layer of thatch, rendering it even more impervious. This lawn is crying out for core aeration!
A Specific Example of Where Core Aeration Can Solve a Lawn Problem
Many people complain about having moss plants growing in their lawns, and they'll proceed to wage a campaign against the moss, thinking that all that is needed is to apply the correct herbicide. This is a misguided policy. Lawns that drain poorly due to compacted soil and/or thatch might just as well be displaying a sign that reads, "Moss Welcome." Compacted soil is a common problem for lawns subjected to excessive foot traffic (as when kids play on the lawn frequently).
What's the Best Time for Core Aeration?
For cool-season grasses, perform core aeration in early fall. For warm-season grasses, the time generally recommended for core aeration is mid-spring to early summer.