Aeration is a lawn care practice designed to create openings in lawn turf and the underlying soil structure in order to penetrate the root and thatch layer and allow essential water and air down into the soil where it can better reach the grassroots. The process is usually called "core aeration" after the practice of punching small plugs, or cores, into the lawn. Usually, the practice is done with a motorized machine that can be rented, though there are also hand tools that can be used and which may be practical on very small lawns. Lawn care companies also offer aeration among their repertoire of services.
Why You Should Aerate
Aeration is the antidote to the heavily compacted soil, which may be present on a lawn that gets very heavy foot traffic, or which is planted on a soil base that is heavy in clay content. It rarely is necessary where soils are on the sandy side. Core aeration is sometimes advertised as a solution to problems with thatch; however, it does not do much to relieve that problem if thatch is already present. Where thatch is a genuine problem, a true dethatching operation is a better solution. However, dethatching is a fairly violent action to a lawn, and an annual core aeration may well prevent thatch problems in the first place.
If you have the type of soil and conditions that require lawn aeration, you can do it once each year. It does not hurt your lawn, and in fact, will make it healthier and more attractive. Some people dislike the look of the small plugs of soil and turf that are pulled up and scattered over the lawn, and if so, you can rake them up. Left on the lawn, though, they will quickly break up and decompose.
When to Aerate Your Lawn
Spring is not the ideal time to aerate the lawn, but circumstances may require it. If the soil is so compacted that existing grass can't grow, it may be necessary to aerate in the spring. Generally, though, spring aerating is discouraged because the aeration holes provide a perfect spot for weed seeds to germinate. Weeds seeds (especially crabgrass) are the first seeds to germinate in the spring, and aerating the lawn just stirs them up and gives them an ideal home.
Late spring aerating may be beneficial, however, if the ground is extremely compacted, or if there is excessively thin turf. Later in the spring is best, when weeds have established but before they flower and go to seed. This occurs around Memorial Day in many climates.
But the very best time to aerate is in the fall when the temperatures have cooled off, when the weed pressure is minimal, and when grass is actively growing.