A “Weed and Feed” product is a combination treatment designed to simplify lawn care by simultaneously nourishing the grass and controlling weeds. It contains fertilizer (with nutrients like nitrogen, phosphorus, and potassium) and a selective herbicide intended to kill broadleaf weeds without harming the grass. Homeowners often apply these products hoping for a single solution to address common lawn issues, including moss.
The Effect of Weed and Feed on Moss
Standard Weed and Feed products are typically ineffective against moss because the herbicides they contain are designed to target a different type of plant. The active ingredients, such as 2,4-D, MCPP, and Dicamba, are synthetic auxins that act as growth regulators. These chemicals cause uncontrolled, destructive growth in broadleaf weeds, which are dicots, ultimately killing them.
Moss is a non-vascular plant (bryophyte) that lacks the complex sap-conducting systems needed to transport these chemicals. Since the herbicides cannot be effectively circulated or disrupt the simple cellular structure of the moss, the weed-killing component fails to work. The fertilizer portion may even encourage moss growth by feeding the surrounding grass, which then allows the moss to receive more light.
An exception exists when a product is specifically marketed as a “Weed, Feed, and Moss Killer.” These formulations include an additional, non-herbicide ingredient, typically a high concentration of iron, such as ferrous sulfate. The iron compound is the true moss killer in these combination products, working through a different mechanism than the broadleaf herbicides.
Effective Moss Eradication Methods
The most reliable treatment for existing moss is the application of an iron-based product like ferrous sulfate. This compound acts as a contact killer, dehydrating the moss tissue and causing it to quickly turn black, often within hours or days. The iron also provides a deep greening effect to the surrounding grass, making it a dual-purpose treatment.
When using iron sulfate, dissolve the granules in water and apply the solution evenly, avoiding hot, dry conditions which can scorch the grass. Iron will permanently stain concrete, paving stones, and masonry a rust-colored orange or brown. After the moss has blackened and died, it must be physically removed through dethatching or vigorous raking (scarification).
For smaller, isolated patches, a non-traditional treatment using diluted liquid dish soap can be effective. A mixture of two ounces of gentle dish soap per gallon of water can be sprayed directly onto the moss, causing it to dry out and die. While this is a less toxic option, it only acts as a contact killer and does not address the underlying causes of the moss growth.
Highly acidic solutions like vinegar are not recommended for broad lawn application. They can harm the surrounding grass and drastically alter the soil’s pH balance.
Addressing the Root Causes of Moss
To prevent moss from returning, the environmental conditions that favor its growth must be corrected. Moss does not compete well with healthy turf and typically colonizes areas where grass is struggling. Common causes include poor soil drainage, excessive shade, and overly acidic soil.
Improving drainage and relieving soil compaction is achieved through core aeration, which involves pulling small plugs of soil from the lawn. This action increases oxygen flow to the grass roots and allows water to percolate deeper instead of pooling on the surface.
For areas with heavy shade, pruning tree limbs and shrubs can increase the amount of sunlight and improve air circulation. This makes the area less hospitable to moss growth.
Moss thrives in acidic soil, which is often unsuitable for many turfgrass varieties. A soil test can determine the current pH level, and if it is below the ideal range of 6.0 to 6.8, an application of dolomitic or calcitic lime is recommended. Lime raises the soil pH, creating a more favorable environment for grass to flourish and outcompete the moss.