Mosses are primitive, non-flowering, rootless green plants with simple stems and leaves. They produce a thread-like growth that forms a fine-textured mat on soil or lawn surfaces. The mat can become very thick, and it will often out-compete lawn grasses or other small plants to take over an area of ground. Mosses need specific soil and climatic conditions to thrive. Mosses are usually found where 1) drainage is poor, or 2) there is chronic excess moisture, 3) soils are compacted, 4) there is restricted air movement, so the soil surface stays moist for all periods, 5) there is a buildup of thatch, 6) there is very low soil fertility, 7) the soils are acidic – low pH, and 8) there is heavy shade.
The best way to control moss growth is to eliminate/reduce the conditions which favor moss growth: 1) Fertilize to improve soil fertility and stimulate vigorous grass growth. 2) Correct the soil pH so that it is in the pH 6 to 7 range. 3) Reduce the amount and/or frequency of watering to allow some drying between irrigations. 4) Core or slice your lawn to open up the soil to increase aeration and drainage. In severe circumstances, the lawn may need to be recontoured, or drain tiles may need to be installed so drainage is improved. 5) If possible, reduce the shade by pruning trees to allow more air and light penetration.
In situations where the physical environment cannot be altered sufficiently to eliminate moss, control can be achieved with chemicals. Because moss is a primitive plant, it can be killed by chemicals that will not harm turf grasses or higher plants. Several copper-containing compounds may be used. Mix two to three ounces of these chemicals in four gallons of water and use this volume to treat 1,000 square feet of turf or other moss-infested areas. In cool, humid regions like the Central Coast, the three-ounce rate is suggested. Also several applications at 7 to 10-day intervals may be necessary to achieve control.
Once control is achieved, the dead moss should be broken up or removed, the soil worked up and the area reseeded or resodded. Mosses reproduce by spores, which are microscopic and readily airborne. Therefore, if you do not improve the adverse conditions mentioned at the beginning of this answer, the moss will return in a short period of time. Remember that vigorous, healthy, well-maintained turf is the best way to prevent moss establishment.