The presence of moss in a lawn or garden is a common frustration, often leading homeowners to seek a quick fix using soil amendments like lime. Moss thrives where turfgrass struggles, indicating underlying soil or environmental issues. While many believe lime is a direct moss killer, the relationship is indirect. Lime’s primary function is to improve soil conditions so that grass can eventually outcompete the moss, not to act as a herbicide.
How Lime Affects Moss Growth
Lime is derived from ground agricultural limestone (calcium carbonate or magnesium carbonate). It is not a moss poison; its function is to raise the soil’s pH level, making the environment less acidic. Moss excels in acidic soil where turfgrass often becomes weak and thin. Turfgrass prefers a slightly acidic to neutral pH (6.0 to 7.0) for optimal nutrient uptake.
When the soil pH drops too low, essential nutrients become less available, weakening the turf and allowing moss to colonize. Applying lime corrects the soil chemistry to favor the grass, allowing it to grow denser and more vigorously. The healthier turf then physically crowds out and shades the moss, preventing its return. The two common types of lime are calcitic (providing calcium) and dolomitic (providing calcium and magnesium).
Conditions That Promote Moss
Moss is a non-vascular plant that thrives in poor soil conditions where turfgrass fails. The appearance of moss indicates the environment is unsuitable for a dense lawn. Excessive shade is a common factor, as moss tolerates low light levels better than most lawn species. Pruning overhanging branches can increase sunlight and discourage moss growth.
Poor drainage and waterlogged soil also contribute significantly to moss patches. Moss thrives in persistently wet areas, while saturated soil suffocates grass roots by limiting oxygen exchange. Heavily compacted soil restricts the movement of air, water, and nutrients, creating a surface moss can easily colonize. Low soil fertility further weakens the lawn. Addressing these physical and nutritional factors must accompany pH correction for long-term moss control.
Applying Lime Safely and Effectively
Before applying lime, perform a professional soil test to accurately measure the current pH and determine the exact amount needed. Guessing the application rate risks raising the pH too high, which can cause nutrient deficiencies and harm the grass. The soil test report provides a recommendation, usually expressed in pounds of lime per 1,000 square feet.
Lime is a slow-acting amendment, taking several months to fully dissolve and change the soil chemistry. The optimal time for application is typically in the fall or early spring, as the freeze-thaw cycles and moisture help the lime break down and move into the soil. Use a drop or rotary spreader to ensure an even application.
For established lawns, pelletized lime is preferred because it is less dusty and easier to distribute than pulverized lime. Wear protective gear, including gloves and a dust mask, when handling lime to avoid irritation or inhalation. If the recommended rate is high, split it into two applications—half in the spring and half in the fall—to avoid applying more than 50 pounds per 1,000 square feet at one time.
Immediate Solutions for Eliminating Existing Moss
Since lime is a slow-acting preventative measure, a method is often required to quickly eliminate existing moss coverage. The most common and effective fast-acting chemical treatment is iron sulfate (ferrous sulfate). This product rapidly desiccates the moss, turning it black or orange within hours or days. Iron sulfate is selective, killing the moss without harming the surrounding turfgrass, and the iron content provides a temporary greening effect on the lawn.
Once the moss is dead, it must be physically removed using a rake or a dethatching machine. Physical removal is necessary because dead moss left in place blocks sunlight and prevents new grass seed from establishing. For small patches, a simple non-chemical alternative is a mixture of water and dish soap sprayed onto the moss. However, these immediate solutions only address the symptom and do not fix underlying issues like poor drainage, shade, or low pH. For long-term success, killing the existing moss must be followed by cultural improvements, including aeration and the necessary application of lime.