Mulch is organic material applied to the soil surface to provide a protective layer. Whether mulch is beneficial for grass depends entirely on the material and the method of application. While one type of mulch is highly recommended for turf health, others used in garden beds can cause significant damage. The material’s composition dictates whether it will nourish the lawn or create a barrier to growth.
The Benefits of Mulching with Grass Clippings
Finely shredded grass clippings returned to the lawn act as a natural, slow-release fertilizer, a process known as grasscycling. Clippings are composed of high amounts of water, allowing them to decompose rapidly when cut small enough. This quick breakdown releases essential nutrients back into the turf’s root zone.
The primary benefit is the return of nitrogen, phosphorus, and potassium. Clippings can provide up to 25 percent of a lawn’s annual nitrogen requirement without synthetic inputs. This recycling process creates a sustainable nutrient cycle, feeding the soil microbes that support grass health.
The thin layer of clippings also aids in moisture regulation. By covering the soil surface, the organic material shields the ground from direct sun and wind exposure. This protective effect significantly reduces the rate of water evaporation from the soil, allowing the turf to remain hydrated longer. Reducing moisture loss conserves water and can extend the time between irrigation cycles, particularly during hot, dry weather.
Separating Mulching from Thatch Buildup
A frequent concern among homeowners is the belief that leaving clippings on the lawn causes a harmful layer of thatch. Thatch is a dense, tightly interwoven layer of dead organic matter that accumulates between the soil surface and the grass blades. This layer is primarily made up of woody plant parts like stems and rhizomes, which contain high amounts of lignin and cellulose that are difficult for microbes to break down quickly.
In contrast, the tender, soft leaf tissue of grass clippings decomposes very rapidly due to its high water content. University research confirms that grass clippings do not contribute significantly to thatch buildup, provided the lawn is mowed correctly. When a mulching mower cuts the grass into fine pieces, these fragments filter down to the soil level where they are consumed by microorganisms.
Excessive thatch buildup, typically greater than half an inch, is usually caused by conditions like over-fertilization, soil compaction, or using turfgrass varieties prone to producing dense runners. Leaving fine clippings on a healthy lawn actually helps feed the microbes that break down existing thatch. However, if the lawn is mowed when the grass is overly long or wet, the resulting large clumps can smother the turf beneath them.
Why Traditional Mulches Are Not for Turf
Materials commonly used in garden beds, such as wood chips, shredded bark, or straw, are detrimental when applied directly to turf. These materials have a high carbon-to-nitrogen (C:N) ratio, meaning they contain a large amount of carbon relative to a small amount of nitrogen. Soil microbes require nitrogen to break down this carbon-rich material.
When these traditional mulches are spread over turf, the microbes draw available nitrogen from the surrounding soil to fuel their decomposition process. This phenomenon, known as nitrogen immobilization, starves the shallow-rooted turfgrass of the nitrogen it needs for growth, leading to yellowing and decline. The grass, whose roots are concentrated in the top few inches of soil, is particularly vulnerable to this surface nitrogen depletion.
Furthermore, applying a deep layer of traditional mulch creates a physical barrier that smothers the grass plants. The layer blocks sunlight and restricts the air circulation necessary for healthy turf growth. This dark, moist environment can also promote the growth of wood-decaying fungi and mold, indicating conditions unsuitable for a healthy lawn ecosystem.