Moss in the lawn is considered an invasive weed by many homeowners. While not capable of overtaking turf grass, moss is an opportunistic weed that will grow where grass is absent or thin. Treating affected lawn areas with a weed killer helps control moss. To permanently eradicate moss in lawns, though, a homeowner should understand the basic cultural requirements of both grass and moss.
Moss and turf grass have opposite cultural needs. The presence of moss, therefore, indicates that cultural conditions of the lawn are not suitable for growing healthy grass. Gaining control of moss will improve the overall health and quality of the lawn.
Most mosses grow in shady, damp conditions. They favor acidic soils that are poor in fertility. Grass, on the other hand, requires full or part sun, well-drained soil, neutral to slightly alkaline soil, and rich nutrition. Where turf grass thrives, moss will not, and vice versa.
Chemical control of moss should only take place if cultural concerns are also addressed. In late winter or early spring, use a moss-killing product containing iron sulfate or iron ammonium sulfate. Be aware that these chemicals can acidify soil, so use according to label instructions to avoid overuse. For a more natural control, use a moss- and algae-killing soap.
After killing the moss in early spring, vigorously rake it off the lawn and address the lawn's cultural needs. Get a soil test done so you can address soil pH and nutrient needs. High pH can be modified with lime. If the soil is poor or damp, amend it with organic matter to improve drainage and hold soil nutrients.
Keep lawns in good health with regular fertilization. Irrigate as needed. Maintain proper mowing height--usually about 2 1/2 to 3 1/2 inches, depending on the grass variety. Keep lawns sunny by thinning out overhead branches. In shady areas, grow shade-tolerant grasses that will thrive and choke out moss.