May is an optimum time to aerate and dethatch your lawn.
If your lawn is made up of perennial ryegrass or tall fescue, you likely don't have to worry about thatch, said Alec Kowalewski, a turfgrass expert for the Oregon State University Extension Service.
But if it's Kentucky bluegrass or creeping bentgrass, Kowalewski advised dethatching or aerating your lawn once or twice a year in the spring and fall.
Thatch is a layer of decaying roots and stems that build up between grass and the soil. If you can see thatch on the surface of the soil, you've likely got too much, he said.
"The best way to tell if your lawn needs dethatching is to take a shovel and dig out a little piece of soil and look at whether there is thatch layer accumulation," Kowalewski said.
Aerating your lawn is the best way to get rid of thatch, Kowalewski said. Aeration, or removing soil plugs the size of wine corks out of the lawn, not only gets rid of thatch but also keeps air and water moving easily through the soil.
Too much thatch can lead to diseases, reduced vigor, poor color and dry spots in the summer, according to the Extension guide "Maintain a Healthy Lawn in Western Oregon," which is available online at http://bit.ly/YLgygI.
If you live in areas of the state that get frequent snow, Kowalewski advised dethatching or aerating your lawn after the snow has melted.
Golf courses, athletic fields and lawns used heavily by kids, pets and sports activities are prone to soil compaction and need aeration, regardless of the type of turfgrass, Kowalewski said.
Rent a heavy dethatching machine, sometimes called a vertical mower, which you can pull behind a lawn tractor to tear up thatch, he said. The machine uses a row of blades or tines that are especially designed to extract thatch lodged between grass blades and soil.
You can also use a dethatching rake with sharpened tines intended to lift thatch from the soil manually. Pull the rake toward your body, loosening the layer of thatch.
To aerate, you can rent a core aerator machine, which has hollow tines that dig holes in the ground to pull out small fingers or cores of soil.
"As an alternative for getting rid of dead organic matter, you can mow down your lawn really low and scalp off the dead material, leaving the green stuff showing through," Kowalewski said.
Compost discarded thatch or mow it back into your lawn. The soil plugs left over from aeration can be mowed back into the lawn as well.