Laying mulch directly over an existing lawn, a technique often called sheet mulching, provides an effective, no-dig method for transforming turf into a garden bed. This process works by smothering the underlying grass and weeds, denying them the sunlight required for photosynthesis and survival. The organic materials used in the layers slowly decompose, adding beneficial carbon and nutrients to the soil beneath. This approach improves soil structure and fertility over time without the physical labor of sod removal. The resulting deep layer of organic matter conserves moisture and creates an environment where new plants can thrive.
Essential Materials and Supplies
The sheet mulching method requires simple tools and specific organic materials for the barrier and top layer. Basic gardening tools, such as a sturdy wheelbarrow, a shovel, and a garden hose with a spray nozzle, are needed for preparation and application.
For the barrier layer, plain corrugated cardboard or large sheets of black-and-white newspaper are the preferred materials. Remove all plastic tape, staples, and glossy labels from the cardboard, as these will not decompose and can impede water and air flow. The top layer is composed of organic mulch, with suitable options including wood chips, shredded bark, or arborist chips, which provide a dense, light-blocking cover. To accurately cover the area to the proper depth, calculate the required volume of mulch based on the square footage of the planned area.
Preparing the Grass and Edges
Before applying the materials, the area must be properly defined and the existing turf reduced to maximize the effectiveness of the smothering layers. Start by clearly marking the boundaries of the new garden bed using stakes and string or a landscape spray paint. This step ensures that materials are contained and the final shape is intentional.
The existing grass should be mowed to the lowest possible setting to reduce the amount of plant material the barrier must cover. Aggressive perennial weeds, such as dandelions or thistle, should be manually dug out and removed, roots and all, before the mulching process begins.
Creating a physical edge helps contain the deep layers of cardboard and mulch. Digging a shallow trench, 3 to 6 inches deep, around the perimeter provides space for the layers to settle and prevents the top mulch from creeping onto the surrounding lawn.
Layering the Mulch Over Turf
The application process begins with laying the barrier material directly onto the prepared, low-cut grass. Corrugated cardboard is ideal, and pieces should be unfolded to their largest size to minimize seams. The sheets must be overlapped by at least 6 to 8 inches at all seams to prevent light from penetrating and allowing grass or weeds to grow through.
Immediately after the cardboard or newspaper barrier is laid down, it must be thoroughly wetted with a hose. Soaking the layer secures the lightweight material against the wind and initiates decomposition by attracting soil microbes and earthworms. The wet barrier will also lay flatter and conform better to the soil surface.
Once the barrier is saturated, the primary mulch layer is applied on top. The goal is to achieve a consistent depth of 3 to 4 inches of wood chips or shredded bark across the entire area. This depth ensures the grass is effectively smothered by blocking all sunlight. If the area includes existing trees, keep the mulch pulled back a few inches from the base of the trunk to prevent moisture buildup and decay at the root flare.
Initial Watering and Ongoing Care
After the mulch is spread, a final, deep watering is necessary to settle the entire layered structure and begin the moisture exchange between the mulch and the soil beneath. This saturation helps the wood chips or bark knit together, making the whole layer less prone to shifting from wind or rain. The moisture also encourages the microbial activity needed to break down the cardboard and the turf below.
Over the following weeks, monitor the area, particularly around the edges and seams, for any signs of grass or weed breakthrough. If small shoots are observed, they can be easily addressed by adding a small patch of wetted cardboard and a handful of additional mulch on top.
As the organic material breaks down over several months, the depth of the mulch layer will naturally decrease. Check the depth annually, typically in the spring, and replenish the layer with a top-dressing of one or two inches to maintain weed suppression and aesthetic benefit.