Your lawn isn't absorbing water and the erosion from runoff water has left dry patches on the lawn where grass has stopped growing and all you have left is dirt. You know you need to aerate your lawn to try to get it to absorb water and nutrients again, but when is the best time to aerate? Well, it all depends on your soil type and your grass type.
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The type of soil you have dictates how many times you can aerate in a year. A clay soil compacts easily and needs to be aerated twice a year whereas a sandy soil should only be aerated once a year.
When to Aerate
The type of grass you have, or where you live, will dictate when you can aerate your lawn. For example, All About Lawns recommends that you aerate in late spring/early summer for warm-season grasses and in the late summer or early fall for cold-season grasses. Likewise, "Expert Guide to Lawns" also recommends aerating in the fall to avoid over-stressing a lawn.
When not to Aerate
Avoid aerating in the summer. Summers in most areas tend to be hot. And in the Western United States, the air is dry. If you aerate in summer, you risk allowing what little moisture is in the ground out and that could further stress a lawn. Aerating can also remove a protective layer of thatch that is beneficial to your lawn, as it protects it from too much sunlight and evaporation. Another time to avoid aeration when you have a weed problem. Aerating when you have a lot of weeds will spread their seeds throughout your lawn.