It is possible and often recommended to lay mulch directly over existing grass or turf to kill it. This technique, frequently referred to as sheet mulching or lasagna layering, naturally converts a lawn area into a garden bed or pathway. It provides an efficient alternative to the labor-intensive process of digging up and removing sod. The method relies on layering materials to suppress existing vegetation, allowing for the creation of new planting areas with minimal disruption to the underlying soil structure.
The Mechanism of Smothering
The process of killing grass beneath the mulch layer works primarily by deprivation, targeting the plant’s fundamental biological needs. Placing an impenetrable barrier over the grass prevents sunlight from reaching the blades, immediately halting photosynthesis. Without the ability to produce energy, the grass cannot sustain itself and its roots deplete their stored reserves.
This smothering action also restricts the exchange of air and oxygen at the soil surface, further stressing the root system. The combination of light blockage and reduced gas exchange causes the existing grass and weed roots to die back and decay. As the turf breaks down, it contributes organic matter and nutrients back into the soil, enriching the environment for future plantings. This method avoids the physical disturbance of digging, which can often bring dormant weed seeds to the surface where they can germinate.
Essential Preparation Layers
For the smothering technique to be successful, a mandatory intermediate layer must be placed directly over the grass to create a light barrier. Materials like plain corrugated cardboard or thick layers of newspaper are effective for this purpose. Cardboard is preferred because its rigid structure is more robust and harder for aggressive weeds or grass runners to push through once wet.
If using newspaper, apply at least ten sheets thick to ensure complete opacity and block all incoming light. For either material, remove all plastic tape, staples, and glossy inserts, as these items will not decompose naturally. The chosen material should be laid flat over the entire area, with sheets overlapping by at least six to eight inches at the seams to prevent gaps.
Thoroughly saturate this barrier layer with water immediately after laying it down. This saturation helps the material settle, begins the decomposition process, and prevents it from blowing away.
Applying the Mulch Layer Correctly
Once the light-blocking barrier layer is securely in place, apply a thick, consistent layer of organic mulch on top. The mulch serves multiple functions, including concealing the cardboard barrier and holding it firmly in place. It also acts as an insulator, moderating soil temperature and conserving moisture, which accelerates the decomposition of the underlying grass and cardboard.
The recommended depth for this top layer is between three and four inches for effective grass suppression. Applying less than three inches may allow enough light to penetrate, potentially allowing the grass to survive. For this technique, a dense, moisture-retentive organic mulch, such as finely shredded bark or wood chips, is the preferred choice.
The mulch should never be piled up directly against the trunks of trees or the stems of existing plants. This common mistake, often called a “mulch volcano,” traps excessive moisture against the plant’s base, leading to decay and suffocation of the root collar. Ensure a small gap of one to two inches is left around any existing plant stems to allow for proper air circulation. Maintaining the correct depth and leaving space around plants ensures the mulch suppresses the grass while promoting a healthier environment.