Moss is so ubiquitous in central Oregon that many homeowners accept it as a fact of life. The cool, wet Oregon fall is the perfect breeding ground for moss. It starts spreading with the first fall rains, and by the following spring it can take over an unhealthy lawn. Moss only seems to disappear when summer brings heat and sun. Moss can survive long periods of drought to come back even stronger the next fall. To end the cycle and permanently kill moss in Central Oregon grass, the existing moss must be eliminated and the lawn strengthened to defend itself against subsequent invasions.
Remove the moss with a flail type dethatcher in early spring (March to April) when the moss is at its peak. This will remove roughly 75% of the moss.
Dust a dry fertilizer fortified with iron such as Scotts Granular Moss Control over the affected areas. Iron-fortified fertilizer will kill any of the remaining moss on contact and stimulate the grass to grow thicker, which will help fend off future moss invasions. Follow the manufacturer's recommendation for amount you should use but you will need approximately 35 grams--about 2 1/2 tbsp.--per square yard.
Troubleshoot your grass. Moss usually grows on grass that is thin, under fertilized, grown over highly acidic or compacted soil, under too much shade, too wet, or afflicted with a disease or insect infestation. Unless the underlying cause of the infestation is properly identified and corrected, the moss will continue to return.