Yes, you can effectively kill existing grass by mulching over it, a technique commonly referred to as sheet mulching or smothering. This organic, no-dig process involves layering specific materials directly on top of the turf to eliminate light and oxygen, which halts the grass’s ability to photosynthesize and ultimately kills it. Sheet mulching is a highly effective, low-labor method for converting a lawn into a new garden bed or pathway while simultaneously building healthy soil structure. This layered approach allows the underlying turf and barrier materials to decompose in place, enriching the soil with organic matter.
Preparing the Area for Smothering
Thorough preparation of the site significantly increases the success rate of the smothering process. Begin by mowing the existing grass to the lowest possible setting on your mower, leaving the short clippings on the ground. This initial step reduces the volume of living material the layers must suppress and allows the decomposition process to begin more quickly.
Next, manually remove any persistent or woody perennial weeds, such as thistle, blackberry, or aggressive vines. These plants possess deep root systems or rhizomes that may survive the smothering process, making manual removal necessary. Once the area is cleared, clearly define the borders of the new bed or path with a shovel or edging material. Digging a small, shallow trench around the perimeter can help anchor the barrier material and prevent grass from creeping in from the edges.
The last step before applying the layers is to deeply water the entire area being treated. Saturation is necessary because the grass and soil organisms beneath the barrier require moisture to begin the decomposition process once the light is cut off. This pre-soaking ensures the soil is damp, which promotes microbial activity, accelerating the breakdown of the turf once the sheet mulch is applied.
The Essential Layers for Grass Suppression
The success of the sheet mulching technique relies on creating a light-blocking, breathable barrier that allows water to penetrate. The foundational layer must be a solid, opaque material, with plain corrugated cardboard being the preferred choice for its strength and high carbon content. To ensure complete light exclusion, the cardboard pieces should be overlapped by at least four to six inches at all seams, and any plastic tape or labels should be removed before application.
As an alternative to cardboard, a thick layer of plain newspaper can be used, requiring a minimum of six to eight sheets to create an effective light block. Once the entire area is covered with the chosen barrier, it must be thoroughly wetted down to help it conform to the ground and prevent it from shifting in the wind. This moisture initiates the softening of the material, making it easier for earthworms and microbes to begin breaking it down.
The final layer is the organic mulch, which must be spread to a depth of four to six inches over the cardboard or newspaper barrier. This top layer, which can consist of wood chips, shredded leaves, or straw, serves to block any remaining sunlight and provides a clean, finished appearance. It is important to use organic materials rather than plastic sheeting, as plastic does not decompose, restricts beneficial gas exchange, and negatively affects the soil life you are trying to cultivate.
Maintaining the Mulched Area
The process of grass suppression is not instantaneous and requires a waiting period for the underlying turf to fully die and decompose. While the grass will begin to die quickly, it may take anywhere from six to twelve months for the materials to fully break down and the area to be ready for planting. The decomposition timeline is influenced by temperature and moisture, with warmer, wetter conditions speeding up the process.
During this period, you may notice small bits of grass or weeds attempting to regrow, often appearing around the outer edges or pushing through small gaps in the cardboard layer. These persistent growths should be pulled out immediately or simply covered with an extra handful of the top mulch material. Regularly checking the moisture level beneath the barrier is important, especially during dry spells, because decomposition requires a consistent level of dampness.
As the months pass, the organic materials will settle and compress. This necessitates replenishing the top mulch layer to maintain an effective depth of at least two to four inches. This routine maintenance ensures that the light-blocking capability is preserved and provides a continuous source of carbon material to enrich the soil. Consistent management of the mulch depth will also prevent new weed seeds from germinating on the surface.