Grass clippings are an excellent material for composting, provided they are managed correctly. They are a common, readily available input that transforms into a rich soil amendment. Successfully incorporating grass requires understanding its high-nitrogen and high-moisture properties and mitigating problems from improper handling. With simple preparation techniques, grass clippings accelerate decomposition rather than becoming a source of odor and slime.
Grass as the Nitrogen Source (The Green Component)
Grass clippings are categorized as a “green” material, signifying their high concentration of nitrogen. This nitrogen is necessary for the microorganisms, primarily bacteria, that drive the decomposition process. They use it to build proteins and reproduce rapidly. The role of grass is to supply this nitrogen, which fuels the microbial activity that breaks down carbon-rich “brown” materials.
The efficiency of a compost pile is measured by its Carbon-to-Nitrogen (C:N) ratio. Fresh grass clippings typically have a low C:N ratio (12:1 to 25:1), while the ideal ratio for rapid decomposition is approximately 30:1. When mixed with high-carbon materials, the nitrogen fuels a rapid population boom of microbes. This intense activity generates heat, causing the pile temperature to rise, which is a desirable sign of effective decomposition.
Management Issues: Matting, Moisture, and Odor
The high-nitrogen content that makes grass valuable also contributes to common problems. Fresh clippings are composed of 80 to 85 percent water, which quickly saturates a compost pile. This excess moisture is a primary cause of failure, filling air pockets and leading to compaction.
When fresh grass is added in a thick layer, the fine, wet blades compress into a dense, impenetrable mat. This mat blocks the flow of oxygen, suffocating the aerobic (oxygen-requiring) microorganisms. The pile then shifts to anaerobic decomposition, carried out by microbes that do not require oxygen.
Anaerobic conditions dramatically slow the breakdown process and produce undesirable byproducts. The most noticeable result is a foul, putrid odor, often smelling of sulfur or rotten eggs, caused by the release of compounds like hydrogen sulfide. This shift is the main reason many people mistakenly believe grass clippings are unsuitable for composting.
Essential Preparation Techniques for Success
The problems associated with fresh grass clippings are easily avoided through simple preparation and mixing techniques. A successful approach involves pre-treating the clippings to reduce moisture and ensuring they are thoroughly integrated with carbon-rich materials. The first technique is to allow the fresh clippings to wilt or dry for 24 to 48 hours before adding them. Spreading them in a thin layer allows excess moisture to evaporate, significantly reducing the risk of creating a soggy, compacted mat.
The most important step is to never add large quantities of grass in thick, unmixed layers. Clippings should be spread thinly, no more than one to two inches deep, and immediately mixed with “brown” materials. Brown materials like dry leaves, shredded cardboard, or straw provide the necessary carbon and create the structure for air pockets, maintaining porosity.
A general rule is to mix the grass with brown materials at a ratio of about one part green to two parts brown by volume. This mixing achieves the optimal C:N ratio and prevents matting. Frequent turning of the pile is necessary to reintroduce oxygen, manage heat and moisture, and ensure aerobic microbes continue working effectively.
The Risk of Chemical Residues
A serious concern when composting grass clippings is the potential presence of persistent herbicides. These chemicals, often used in broadleaf weed killers, are designed to resist microbial breakdown, allowing them to remain active for a long time. Herbicides like clopyralid and aminopyralid can survive the composting process and remain in the finished product.
Finished compost contaminated with these residues can severely damage or kill sensitive garden plants, such as tomatoes, beans, and potatoes, when applied to the garden soil. Symptoms often include twisted, cupped leaves, stunted growth, and misshapen fruit. To avoid this, only use clippings from lawns where the chemical treatment history is known to be clean. If there is any doubt about the source, the clippings should be avoided to prevent long-term soil contamination.