Moss can be a symptom of several problems in the turfgrass and garden:
Poor drainage. Aerate the soil yearly if necessary.
Improper soil pH. Check level and modify accordingly. The soil pH level should be between 6.0 and 6.5 for most turfgrass species.
Shade. Moss can not grow in sunny conditions. Trimming lower branches orprofessionally thinning the tree's canopy may limit moss development.
Too much water. Avoid over irrigation. Unfortunately, you can't prevent rain.
There are no chemicals currently available to homeowners that will limit or prevent moss growth.