Our climate provides the perfect set of growing conditions for moss to grow. Lawns that maintain more than 4 hours of shade during the day are prime candidates as moss prefers shade and moisture. During the late summer, if you choose not to water your lawn that is planted in full sun, the grass will begin to go dormant or die off creating another prime candidate for moss growing. This dying out of the grass allows for pockets of empty space as fall arrives.
The cool temperatures and rain allow the moss to grow into the areas where the grass is sparse and continues to thrive over the winter. As soon as the temperatures in the spring begin to rise the moss begins to turn yellow and dies off in too much sun or drought. Yet only the surface is actually dying off. The roots are alive and waiting for the fall and winter growing conditions to arrive. The other result of letting your sunny lawn go dormant in late summer, is that these empty patches are havens for weed seeds which can grow well in moss and without water. So after a few seasons you have a weed and moss infested lawn.
Eliminating moss in a mostly shady area requires continuous use of iron (Moss Killer) and re-seeding at 3-week intervals. Grass will only grow in the shade if its 90 degrees or above and it will still be sparse and leggy. Many people choose to remove lawns in shady areas and replant with beautiful shade tolerant plants.
Eliminating moss in sunny areas requires using Moss Killer or a fertilizer that is high in iron in the spring and reseeding as soon as the moss is gone. It is essential to fill the mossy areas with lots of seed. Repeating this every 6-8 weeks during the growing season will promote thick grass that will prevent moss from creeping into your lawn. Iron also enhances the rich green color in grass. Continuing a water program throughout the late summer will prevents those spots from drying out.