Over the years we have come in contact with a number of landscapers and equipment suppliers that think July is a great time to aerate. The grass generally isn’t growing so why not go out and aerate. After all we need to do something! WRONG!!
The truth is July is not a good time to aerate. Generally the grass has or is attempting to go dormant. If you aerate now, you will put the grass under more stress than it is already experiencing. You will bring weed seeds to the surface or spread annual weeds like crabgrass over greater areas. Since you are also bringing soil cores to the surface you are providing better weed seed to soil contact, allowing greater light and warm soil to ensure germination.
A much better time is to wait until mid- to-late August. The cool season grasses are beginning to come out of dormancy, the warm season grassy weeds like crabgrass begin to die off with the cooler evening temperatures. Over-seeding at this time is especially beneficial. We found that if we broadcast grass seed after we aerated we would further thicken the grass.