Moss in a lawn is an indication that the turf is not growing well. It is important to consider that moss does not kill the grass; it simply thrives on growing conditions that are unfavorable for grass, such as deep shade, poor drainage, poor fertility, or compacted soil. These conditions, not the moss, ultimately cause the grass to die out. If you want to eliminate moss from a lawn, focus on improving conditions for growing grass, and don't worry about the moss -- it will disappear on its own as the grass gains vigor.
Herbicides and chemical control have only short term effects on moss. If herbicide use is not accompanied by proper environmental and physical controls, then the initial effect will be bare dirt or mud. Mosses will eventually return because the conditions that favored the moss and disfavored the grass still exist. When herbicides alone are used, the end result, not the cause, of a mossy lawn are being treated. Furthermore, many of the common herbicides, such as glyphosate, are ineffective against mosses, at least in some conditions (Woodfill 1999; Roberts and Ziegenhagen 1999). Therefore, if you perceive of the moss in your lawn as a problem, improve conditions for growing grass, rather than using herbicides.
Photo at right: Lawn area with poor vigor of grasses and mosses in the open spaces.
One of the most common herbicides in use today is glyphosate (for example in Round-Up). This herbicide is often not effective against mosses (Woodfill 1999; Roberts and Ziegenhagen 1999; Newmaster et al. 1999) but does kill mosses in other cases (Newmaster et al. 1999). This chemical is absorbed through the leaves, becomes tightly bound to the soil, and is degraded by microbes. The failure of many common herbicides against mosses can be seen dramatically in some christmas tree plantations and other perennial crops where competing higher plants have been killed by herbicides, leaving a green carpet of mosses and other bryophytes. The situations in which glyphosate does or does not kill mosses remain unclear.
Ammonium sulfate and Copper sulfate
Iron sets back mosses while having little effect on the grass. However, iron will not always kill the moss. It may "burn" the moss severely and render the plant very weak. If the moss is to be eliminated, it must be removed and grass seed sown into its place for a thick turf. Even so, if the conditions that favor the moss are not changed, the moss will recover over time, and you are back to where you started.
Moss can be controlled with products containing Iron Sulfate, and Ferrous Ammonium Sulfate. A good mixture is approximately 3 ounces of Iron Sulfate in 5 gallons of water. A five-gallon mixture should be sprayed over a 1000 sq. ft. area. Products such as Moss-Out (ferrous ammonium sulfate) can be used to control moss. Note that it is best to keep the Moss-Out product away from sidewalks and driveway concrete, because it will stain the concrete.
Moss can also be controlled by spraying with copper sulfate at a rate of 2 to 5 ounces in 4 gallons of water. The 4 gallons will be sufficient to cover 1000 square feet. However, the material could stain your hands and clothing, and may be caustic to metal containers. Copper sulfate in some cases has negative effects on aquatic ecosystems and its use should be avoided if possible.
Lime is a good product to control acidic conditions in your lawn. Over time, the acidic conditions can become detrimental to grass health by binding up the availability of important nutrients. By liming the turf, especially with a calcium-based lime, one can neutralize the acidity, building a better lawn and a stronger competitor for weeds and moss. The ideal soil pH for most lawns is 'neutral', about 6.5 to 7. A pH below 6 is considered 'acidic' and over 7 is 'alkaline'. Acid soil will often be associated with poor fertility, and may encourage moss growth in bare areas. Crushed limestone is a common remedy used to reduce soil acidity. If soil tests show pH 4 or 5, then applying limestone twice each growing season, in addition to regular fertilizer applications, should significantly increase the pH level. Do not add lime to control a moss problem unless a soil test indicates a need for lime.
Poor soil fertility can be a cause for lawn moss growth. If moss grows in areas of your lawn that appear dry and sunny, then the appearance of moss is probably caused by poor soil fertility. To see if low soil fertility is contributing to a moss problem, take a representative soil sample from the areas where moss usually grows and have it tested. If tests show deficiencies in certain nutrients, addition of those nutrients could alleviate the problem. Only apply fertilizers if it is needed. Over fertilization can cause other problems, including pest infestations and possibly groundwater contamination. At the same time your moss problem remains unaffected if low soil fertility was not the cause to begin with.
High nitrogen fertilizers can have a significant effect on moss reduction, and supports the growth of healthy turf. Monthly applications of iron and potassium, in combination with nitrogen, are also helpful. Specific moss control fertilizers are available that contain nitrogen, potassium and iron. These are most effective in a four-application per year program, with applications in early spring, late spring, mid-summer, and early fall. As with all fertilizers, carefully read and follow the directions on the product.
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