Aeration, or you will sometimes see it as aerification, is the process of loosening the soil to reduce compaction and increase the movement of water and oxygen into the rootzone. Oxygen in the air is very important to the functioning of the roots and the health of the plant, and the lack of it in compacted soils is a limiting factor to root growth. Compaction, which compresses the soil, is a common problem in turf management. Turf areas are generally created for use, and the foot traffic on golf courses, sports fields, and lawns can easily compact soil and reduce the health of the grass. We also run mowers, golf carts, and other heavy equipment on turf areas. The more traffic an area gets, the more compaction will become a problem.

Tilling, of course, is an important method of aeration, but it can only be done before establishment. Turf areas can remain in use for decades after establishment, and for these older stands of turf we need other methods of reducing compaction. There are many ways to aerate the soil on existing stands of grass. It can be as simple as using a pitchfork to stab down into the soil and open channels for water movement. There are also many types of commercial equipment available for this purpose. This equipment ranges from small hand-held units to tractor mounted equipment that is used for the aeration of golf courses and sports fields.

There is both solid tine and hollow tine equipment available for use in turf aeration. Both can be useful, although clearly the best way to perform the process is to remove a core of the soil and lay it on top of the turf. That open tube that is formed by core aeration allows for loosening of the surrounding soil and provides an excellent channel for water and oxygen to get down into the rootzone. Solid tine aeration also opens a channel for air and water movement but does not reduce compaction to the same extent as the hollow time. Solid tine equipment is used because it is generally faster and cheaper to use, but hollow-tine equipment is preferred where possible.

Another reason for aeration is to reduce thatch build up on the soil surface. Thatch is a layer of undecomposed plant material and living roots and rhizomes that can form below the crown of the plant, but above the soil surface. A small amount of thatch is fine and can even be beneficial, but thatch can get out of hand and form a barrier to water and oxygen movement and restrict root growth. One inch or more is generally considered to be excessive. Thatch forms due to a lack of microbial activity, which is the best way to break down a thatch layer naturally. Aeration pokes holes in the thatch and allows moisture and oxygen to enter the thatch layer. This intern increases microbial activity and the thatch layer breaks down naturally.

There is equipment designed to remove thatch mechanically. This type of equipment is referred to as vertical mowers. The blades of these units are designed to vertically cut down into the thatch layer and remove some of the organic material. While this can effectively remove thatch, it does nothing for compaction. Core aeration is probably the best way to control a thatch problem on a lawn and it would be preferable to vertical mowing if a thatch problem develops, particularly if a compaction problem also exists.

So, when do you need aeration? On heavily used professional sites such as golf courses and sports fields, aeration is necessary management procedure to assure a quality turf stand. It is generally performed at least once per year and maybe twice per year on heavily trafficked areas. On home lawns, aeration may be necessary but is generally not a standard practice each year. I have personally gone years between aerations on my lawn if compaction from heavy use is not a problem. On lawns heavily used by kids and/or dogs, yearly aeration may be necessary. Grass grown on soils high in clay may also need aeration more often. On cool-season lawns, this is best done in late-summer or early fall. On warm-season lawns, the best time to aerate is spring, after green up, but before the heat of mid-summer.

Core aeration can also be very useful if overseeding is necessary. Overseeding is performed when the grass is thin, or there are small dead areas in the turf. The existing surface is not killed by herbicides and the existing living grass remains intact. Punching holes in the turf with an aerifier will open holes and allow seed to reach the soil where it can germinate and become established. The more holes the better within reason and you may want to go over the area with the aerifier more than once. The aerification holes provide a perfect environment for holding water and for germination. This process generally requires less water than germinating seed in a newly established seed bed and can be very effective in rejuvenating turf that has been neglected for years. When overseeding, you can use about one half of the of seed recommended for a newly established area.

It is not necessary to apply seed each time the lawn is aerated. A thick stand of Kentucky bluegrass or bermudagrass will generally not require over seeding. This is reserved for areas that are unusually thin and need additional plants to thicken the turf.

Figure 1. A two-inch thatch layer on a lawn.

Figure 2. A core of soil removed by core aeration.

Figure 3. Rooting in an aeration hole.

Figure 4. Core aeration on a golf course.

Nick Christians, University Professor of Horticulture, Emeritus, Iowa State University

books: http://www.amazon.com/Nick-Edward-Christians/e/B001H6SB7A/ref=ntt_dp_epwbk_0

turf blog: http://www.iaturf.blogspot.com/

Fundamentals of Turfgrass Mgt. 5^th ed http://www.wiley.com/WileyCDA/WileyTitle/productCd-1119204631.html

Web site: https://www.hort.iastate.edu/directory/nick-christians/

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