Depressions or low spots in a lawn are common issues that create tripping hazards and make mowing difficult. Many homeowners consider using readily available grass clippings as a free material to fill these uneven areas. Understanding how organic matter decomposes is necessary to determine if this simple solution is effective for achieving a smooth, level lawn surface.
Why Grass Clippings Are Not Recommended for Volume Filling
Fresh grass clippings are problematic as a bulk filler due to their composition and rapid biological breakdown. Clippings are 80 to 85 percent water and have a low Carbon-to-Nitrogen (C:N) ratio (12:1 to 25:1). This combination of high moisture and nitrogen causes the material to decompose extremely quickly once piled.
When a large volume of clippings is placed into a hole, rapid decomposition leads to a significant reduction in volume. This settling means the initial level surface will soon sink, recreating the depression and defeating the purpose of the fill. Furthermore, a deep pile of wet clippings quickly becomes compacted, restricting oxygen flow.
The resulting lack of oxygen promotes anaerobic decomposition, a process carried out by microorganisms that do not require air. This anaerobic breakdown generates organic acids and sulfur compounds, producing a foul odor and transforming the clippings into a dense, slimy muck. This wet, airless environment is toxic to turfgrass roots, preventing the lawn from re-establishing and resulting in a dead patch with poor soil structure.
Proper Techniques for Leveling Lawn Depressions
For a permanent, stable lawn surface, a specialized leveling mix should be used instead of organic material. The ideal mixture consists of screened topsoil, fine sand, and compost, often blended in a ratio of 40% topsoil, 40% sand, and 20% compost. The sand improves drainage, the topsoil provides bulk, and the compost introduces organic matter and nutrients to support new grass growth.
To prepare the area, the lawn should be mowed to a low height, making it easier to work the mixture into the existing turf. The leveling mix is then applied in thin layers over the depression, not exceeding half an inch deep at a time. This thin application rate prevents the underlying grass from being smothered.
The mixture should be carefully worked into the grass canopy using a leveling rake, push broom, or the back of a garden rake, ensuring the tips of the grass blades remain exposed. Allowing the grass to grow through the layer before reapplying more mix ensures the turf remains healthy. For holes deeper than two inches, it is beneficial to fill the area in multiple stages over several weeks or gently compact the first layer of soil before adding the final topdressing layer.
Safely Incorporating Clippings into Lawn Soil
While they are unsuitable as a bulk filler, grass clippings remain a valuable resource that can be returned to the lawn ecosystem. The simplest method is grasscycling, where small, finely cut clippings are left on the lawn surface during regular mowing. These pieces quickly filter down to the soil, decomposing rapidly and acting as a natural, slow-release fertilizer.
Clippings contain a substantial amount of nutrients, including up to 4% nitrogen, 2.5% to 3.5% potassium, and 0.5% phosphorus by dry weight. Returning these clippings can reduce the need for commercial fertilizer applications by up to 25 percent. Successful grasscycling requires mowing frequently so that no more than one-third of the grass blade height is removed in a single cutting.
For excess clippings, composting is the appropriate use, provided they are managed correctly. Since clippings are a high-nitrogen “green” material, they must be mixed with a high-carbon “brown” material, such as dry leaves or shredded cardboard, to achieve a proper C:N ratio for aerobic decomposition. They can also be dried and used in a thin layer (one to two inches deep) as a garden mulch to retain soil moisture and suppress weeds.