Moss is a primitive, non-vascular plant that absorbs water and nutrients directly through its leaves rather than developing true roots. It flourishes in conditions inhospitable to turfgrass, often appearing in weakened or struggling lawns. Standard broadleaf weed killers are ineffective because moss lacks the internal structure of grass. Effective moss control requires a selective treatment that targets the moss’s unique biology while encouraging the health and growth of the surrounding lawn.
Iron-Based Treatments and Selective Moss Killers
Ferrous sulfate, commonly sold as iron sulfate, is the most effective and widely used selective moss killer. This treatment is highly selective because it leverages the fundamental biological difference between moss and turfgrass. When applied, the iron compound rapidly oxidizes the moss cells, causing them to turn a dark brown or black color within hours as the moss dies.
The same iron compound acts as a micronutrient for turfgrass, which is why lawn treatments often include iron for a rapid green-up effect. At the concentration used to kill moss, the iron is damaging to the non-vascular moss structure but serves as a beneficial supplement for the grass. The grass utilizes the iron to enhance chlorophyll production, resulting in a deeper green color and stronger growth that can help fill in the newly cleared areas.
A less common chemical alternative is copper sulfate, which also damages the cell walls of moss. Iron sulfate is favored for lawn applications because of its dual action, providing a direct nutrient boost to the grass while simultaneously killing the moss. It is important to understand that these selective chemical treatments only kill the existing moss and do not address the underlying environmental issues that allowed the moss to grow.
Correcting Environmental Factors for Long-Term Control
Sustained moss control requires correcting the three primary environmental conditions that favor its growth over turfgrass: soil acidity, poor drainage, and excessive shade. Moss thrives in acidic soil, often below a pH of 5.5, while most turfgrass varieties prefer a slightly acidic to neutral range, typically between a pH of 6.0 and 7.0. A soil test is the first step to confirm the pH level and determine the necessary corrective measure.
If the soil is too acidic, an application of lime is required to raise the pH level. The two common forms are calcitic lime (high in calcium) and dolomitic lime (containing magnesium). Choosing between them depends on whether your soil test indicates a magnesium deficiency, as both are effective at neutralizing soil acidity. Pelletized forms are generally recommended as they are easier to spread evenly across the lawn surface.
Moss also flourishes in soil that is constantly moist and lacks proper air circulation, a condition often caused by soil compaction. Addressing this requires mechanical aeration, with core aeration being the most beneficial method. A core aerator removes small cylindrical plugs of soil, which physically reduces compaction and allows air, water, and nutrients to penetrate the root zone of the grass. This is significantly more effective than spike aeration, which can increase compaction in clay-heavy soils.
In areas with heavy tree cover, grass struggles due to insufficient light for photosynthesis, allowing the shade-tolerant moss to take over. Increasing light penetration by selectively pruning lower tree branches is an immediate solution. If pruning is not an option, choosing a turf variety with a higher tolerance for low light is necessary. Cool-season choices often include fine fescue and tall fescue, while warm-season options may involve certain varieties of Buffalo or Zoysia grass.
Applying Treatments and Removing Dead Moss
The best time to apply iron-based moss killer is during the cool, moist conditions of late fall or early spring, when moss is actively growing. Treating the moss when it is metabolically active ensures it absorbs the compound most effectively. Application should be avoided during periods of high heat or drought, as this can cause the iron to scorch the turfgrass.
Treatments are available in both liquid and granular forms, and the choice depends on the size of the area and desired speed of results. Liquid applications, typically mixed with water and applied via a sprayer, offer the fastest visible results, often blackening the moss within a day. Granular products are easier to spread over large areas, but they require a light watering or morning dew to activate the iron.
Once the moss has turned completely black, indicating it is dead, it must be physically removed from the lawn. Allowing the dead moss to remain creates a dense, water-retaining layer known as thatch, which blocks new grass growth and encourages future moss problems. The removal process, called scarification, involves using a spring-tined rake or a specialized dethatching machine to lift the dead material from the soil surface.
Scarification should be thorough to expose the bare soil, but not so aggressive that it damages the healthy turf. A necessary safety consideration when using iron sulfate is its tendency to cause rust-colored stains on concrete, pavers, and other hardscapes. Sweep or wash any granules or liquid overspray immediately from these surfaces to prevent permanent staining.