Aerating a Bluegrass lawn is a good way to encourage a healthy root system. In turn, your lawn will look better and withstand the heat of summer better. Aerating has two immediate effects. First, it allows air to move into the soil where it provides the oxygen needed for optimal root growth. Second, it creates passageways for water and nutrients to enter the soil. The best time of year to aerate a Kentucky bluegrass lawn is in early September before you fertilize. The second best time in late February/early March, just before fertilizing and applying pre-emergence weed control.
Remember not to aerate after you have applied pre-emergence weed control. It will disrupt the chemical barrier of the weed control and reduce its effectiveness. Also, do not aerate during the summer months when your lawn is weakened by heat and drought.
The most effective way to aerate a lawn is to use a core aerator. Core aerators are powered by an engine and remove cores of soil from the top layer of soil since most people do not own a core aerator, they rent one or have a lawn service aerate for them. Core aerators work best on moist soil so you will need to irrigate several days before you plan to aerate. Make one or two passes across your lawn so that there is a hole every 4-6 inches. Afterwards, there will be cores of soil deposited all over the lawn surface. If the cores bother you, you can rake the lawn and most will break and drop below the level of the grass such that they are no longer visible. Otherwise, they will naturally disintegrate in a couple weeks with the help of rain.