The layer of dead grass and debris that frequently builds up under the surface of a lawn is called thatch; a thick layer of thatch deprives the grass of air, water and nutrients that it needs to stay green and grow vigorously. Lawns are aerated to control the buildup of thatch and to reduce the compaction of the underlying soil that often occurs when the lawn is heavily used.
A lawn should usually be aerated twice a year if it is growing in clay soil. It should be aerated if thatch is more than ½ inch deep, if it is heavily used or if pools of water form after it rains or the lawn is irrigated. If the lawn turns brown after a stretch of hot weather, it may also need to be aerated. Core aeration is best for these needs; spike aeration is less effective.
A core aerator, the most often recommended method of aeration, uses hollow tines to remove cores of soil from the soil. These cores are usually 5/8 inch wide. They're about 4 to 6 inches from one another and penetrate 2 to 3 inches deep.
The aerator deposits these slender columns from the soil beneath the grass and deposits them on top. For best results, the cores should be extracted in one direction then again at a right angle. Each square foot of turf should have at least 12 holes in it.
A spike aerator punches holes in soil by dragging a rolling bar of spikes across the grass. Spike aerators are usually cheaper to own or rent than hollow-tine aerators, but they tend to increase soil compaction, not reduce it, especially in heavy, clay soils.The soil becomes more compact on both sides of the spikes and underneath them. This makes it more difficult for water and nutrients to reach the plant roots.
The aerator plugs or cores deposited on top of the soil will eventually dissolve back into the soil when it rains or the lawn is irrigated. If the plugs are objectionable, drag a 5-foot by 5-foot piece of carpet across them to break them up.
Dr. Robert Carrow, turf expert at the University of Georgia, studied the effectiveness of core and spike aerators. He concluded that core aeration resulted in a 25 percent increase of water penetration and deep rooting. Aerating with spikes did not affect grass health.