Mosses are nonparasitic, primitive green plants that have fine branched, threadlike stems with tiny leaves. They reproduce by means of wind-blown spores. Mosses typically form a thick, green mat on the soil surface. They produce their own food and do not kill grass plants but rather fill in the spaces in the lawn where the grass is not growing.
If mosses are present in your lawn, it indicates that conditions are not favorable for the growth of a healthy stand of grass but are favorable for the growth of mosses. The conditions that favor mosses over grass include excessive shade, acidic soil, poor drainage, compacted soil, excessive irrigation, low soil fertility, or some combination of these conditions.
Mosses can be removed mechanically or killed chemically, but unless the underlying conditions are altered to culturally favor turfgrass production, any solution that is attempted will only be temporary. The conditions that are commonly associated with moss growth and their cultural controls are described below.
Algae are unicellular or multicellular, threadlike green plants that can form a dense coating or scum over the soil surface. This scum can form a tough black crust on the soil when it becomes dry, and this will act as a barrier to water movement into the soil. Algal scum can be found on waterlogged and compacted soils, especially during sunny, warm, and humid conditions.
Dense Shade: In general, mosses can tolerate more shade than grasses can. If the mosses present in the lawn are limited to shady areas, the options include:
- Plant a more shade-tolerant grass (keeping in mind that an area receiving less than four hours of full sunlight per day is too shady for grasses).
- Mow grasses in shady areas at the top of their recommended mowing height range to encourage deep rooting and provide maximum leaf surface area to intercept more sunlight to manufacture food. For more information on recommended mowing practices, see HGIC 1205, Mowing Lawns.
- Plant a shade-tolerant groundcover rather than grass.
- Cover the area with mulch (pine straw, bark, ground-up leaves, etc.).
- Prune some limbs, especially lower ones, from shrubs and trees to allow in more light.
- Remove some of the trees and shrubs.
- Enjoy the moss as a low-maintenance groundcover that requires little, if any, watering, no fertilizer or mowing, and stays green all year, so long as it does not receive much traffic.
Acidic Soil: Mosses prefer acidic soil. Except for centipedegrass, most lawn grasses prefer a pH of 6.0 to 6.5. Centipede prefers a pH between 5.5 and 6.0. If the pH of the lawn soil is lower than what is recommended for the turfgrass species, raise the pH by adding pelletized, dolomitic limestone. The higher soil pH will not kill the moss but will favor the growth of the grass.
To determine the pH of the soil, take a soil sample to your county Cooperative Extension office. For more information on how to take soil samples, see HGIC 1652, Soil Testing.
Poor Drainage: While mosses will grow in well-drained soil, they grow better in wet soil than grasses. Excessively wet soil is one of the more difficult conditions to alter. Possible solutions include:
- Alter the contour or trench the area to drain water away.
- Alter the elevation of the area (if the wet area is small) by adding sand or soil.
- Install French or tile drains (if the wet area is more extensive).
- Dig a sump (a pit that receives drainage) in the low spots, backfilling with sand.
Compacted Soil: Soil compaction prevents a lawn from thriving. It also prevents internal drainage and provides wet conditions favorable to mosses. When the top 4 inches of soil are compacted, movement of air, water, and nutrients to the grass roots is adversely affected. In addition, it is difficult for grass roots to penetrate compacted soil. These factors stress the grass and decrease its ability to recuperate from damage.
To determine if the lawn soil is compacted, use a soil probe to take a few plugs of soil. This can be done with a piece of ½- to ¾-inch diameter metal pipe. The probe should easily penetrate the top 4 to 6 inches of soil. Use a smaller diameter rod to push the plug of soil out of the pipe. Examine the plugs for the fine white roots. Grasses growing in compacted soil tend to be shallow-rooted. The roots may not extend further than 1 inch into the soil when they should extend 6 or more inches.
Core aeration or aerification will help alleviate soil compaction and improve growing conditions for the grass. This process involves physically removing cores of soil from the lawn, and for some lawns, it should be repeated annually. Aerification is not expensive and can be accomplished in small lawns with a spading fork or a manual sod-coring tool. For larger lawns, use a power-driven core aerator. For more information about aerating lawns, see HGIC 1200, Aerating Lawns.
Excessive Irrigation: Automatic watering on a too frequent schedule tends to promote problems with excessive moisture, especially in shady or poorly drained areas. Since these areas are already prone to moss problems, excess moisture just adds to the problem.
It is best to water the lawn only when it is needed. When signs of moisture stress are observed, such as grass with a dull, bluish-gray cast, footprinting becomes apparent (the presence of footprints after walking on the lawn late in the day), or wilting or folding grass leaves, then irrigate with ¾- to 1-inch of water. This amount will moisten the soil to a depth of 4 to 6 inches. The key to proper watering of the lawn is to water infrequently but deeply. For more information on properly watering lawns, see HGIC 1207, Watering Lawns.
Low Soil Fertility: If the required nutrients in the soil are limited, lawn growth and quality may also be limited. Nutrients that are essential for growth are added to the soil through fertilizer applications. In addition to providing the pH of your soil, a typical soil sample analysis will determine the levels of phosphorus, potassium, calcium, magnesium, zinc, manganese, and boron. The nitrogen requirements of the turfgrass cannot be reliably evaluated by a soil test. As a result, the soil test will not contain a nitrogen recommendation. For recommended nitrogen applications for each turfgrass species, see HGIC 1201, Fertilizing Lawns.
Moss: Once as many of the underlying conditions as possible have been altered, remove the moss by hand raking or by applying chemicals to kill it, such as one of the following products:
1) Ferrous sulfate monohydrate
- Lilly Miller Moss Out! Lawn Granules
- Lilly Miller Moss Out! Spot Treater For Lawns & Flower Beds
- Jonathan Green Lawn Moss Control Granules,
- Scott’s Moss EX Moss Control for Lawns Granules
- Bonide Moss Max Lawn Granules
2) Potassium or ammonium salts of fatty acids
- Safer Brand Moss & Algae Killer & Surface Cleaner Ready to Spray (RTS); & RTU,
- Bayer BioAdvanced 2-in-1 Moss & Algae Killer & Cleaner RTS
- Scott’s 3-in-1 Moss Control Ready-Spray (RTS)
- Bonide Moss Max RTS
- Lilly Miller Moss Out! For Roof & Walks
- Worry Free Moss & Algae Control Ready to Spray
4) Iron (Fe HEDTA)
- Bonide Weed Beater Fe Concentrate & RTU
5) Ferric Sulfate, Anhydrous
- Lilly Miller Moss Out! For Lawns Concentrate
- Lilly Miller Hose N Go Moss Out for Lawns RTS
6) Another moss control option is the use of copper sulfate or ferrous (iron) sulfate mixed at the rate of 5 ounces in 4 gallons of water sprayed over 1000 square feet. After the moss has been killed, apply 5 to 10 pounds of ground limestone to inactivate the copper sulfate prior to reseeding with grass, which may be toxic to grass seedlings.
7) Other options include ground limestone (75 to 100 pounds per 1000 square feet) or hydrated lime (2 to 3 pounds per 1000 square feet in 3 gallons of water), or diluted bleach (2 to 4 ounces per gallon of water), or dishwashing detergent (2 to 4 ounces per gallon of water). Note: one ounce equals two tablespoons.
These materials kill the moss because they act as desiccants (drying agents). They should be applied over the moss-infested areas during winter through early spring (December through April) when moss is actively growing, temperatures are cool, and when the lawn soil is moist. To be effective, they need a 24-hour rain-free period after application. As the moss starts to die, it will turn orange-brown or golden brown. Once the moss is dead, remove it by raking.
Algae: Algae can be controlled by using sprays of the following products:
1) Potassium salts of fatty acids
- Safer Brand Moss & Algae Killer & Surface Cleaner RTS; & RTU
- Bayer BioAdvanced 2-in-1 Moss & Algae Killer Cleaner RTS,
- Scott’s 3-in-1 Moss Control Ready-Spray (RTS),
- Bonide Moss Max RTS,
2) Iron (Fe HEDTA)
- Bonide Weed Beater Fe Concentrate & RTU
- Worry Free Moss & Algae Control Ready To Spray
4) Another algae control is to mix copper sulfate at the rate of 2 to 3 ounces (1½ tablespoons) in 8 gallons of water sprayed over 1000 square feet. After the algae has been killed, apply 5 to 10 pounds of ground limestone to inactivate the copper sulfate before reseeding with grass, which may be toxic to grass seedlings. The dead algal crust may have to be removed by raking.
If large areas remain bare after removing the moss or algae, renovate the area by following the recommendations provided in HGIC 1204, Lawn Renovation.
Always read and follow all product label directions and precautions. Note: RTS means the product is ready to spray, which is a hose-end bottle.