Moss is a common problem and is frequently blamed on low pH, but this is only part of the problem. Even though mosses are often found on soils with low pH values, they may also become established in areas of low fertility, shade, improper drainage, and/or packed soil. Often, two or more of these factors combine to create conditions unsuitable for vigorous turf growth.
A soil test will determine the pH and fertility level of the soil and recommend the necessary lime and fertilizer. The other factors can be determined by visual inspection.
Site conditions need to be addressed to eliminate the moss completely. Compacted soils can be loosened manually or mechanically, and poor drainage may sometimes be improved by minor regrading.
Trees can reduce the amount and quality of light your lawn area receives, and tree roots compete with the lawn for water and nutrients. Where the tree cover isn’t especially dense, removal of a few carefully selected lower branches may improve light conditions enough for moderate grass growth. Another option is mulching or growing plants more suitable to the site or planting a shade tolerant type of grass. There are many types of ground cover that will thrive where grass has a tough time.
Once the pH and other conditions are resolved, the moss needs to be physically removed before reseeding. It should come up easily with a rake as the moss does not have deep roots or anchors. Grass won’t grow unless the seed can make physical contact with the soil. There are some commercial moss killers on the market that contain ferrous sulfate or ferrous ammonium sulfate, but they’re not always useful and they can stain concrete walks or patios. Sometimes it’s easier to just skip the killer and rake it off. Conditions still need to be improved or moss will return, and the moss killers leave behind dead moss which still needs to be removed.
On the flipside, moss transplants easily and can make a great groundcover in rock gardens or out of the way places. It doesn’t stand up well to traffic and isn’t good for walkways.