Smothering grass is a non-chemical technique for lawn removal that prepares the ground for a new garden bed or landscape. This method uses layers of material to block light and air, causing existing grass and weeds to die and decompose naturally. It is a practical, cost-effective approach that avoids the physical labor of digging up sod and improves underlying soil health. The organic matter from the lawn remains in place, contributing nutrients to the new garden space.
Understanding How Smothering Works
Smothering operates by directly interfering with the grass’s biological processes, primarily photosynthesis. Photosynthesis is the mechanism by which plants use sunlight, water, and carbon dioxide to create sugars for energy and growth. By completely blocking sunlight with an opaque layer, the grass is prevented from producing the food it needs to survive. The plant must then rely on stored energy reserves in the roots and crown, which are eventually depleted, leading to starvation and decline.
The materials used for smothering can also restrict the necessary exchange of gases, impacting cellular respiration in the roots. Respiration is the process plants use to convert stored sugars into usable energy, requiring oxygen. By starving the plant of light and sometimes oxygen, the smothering process ensures the grass canopy and root structure eventually die off.
The Sheet Mulch Technique
The sheet mulch technique, often called lasagna gardening, is a highly effective, organic method of smothering grass. Begin by cutting the existing grass as low as possible; the clippings can be left in place to contribute nitrogen to the soil. The first layer is a light-blocking barrier, typically consisting of overlapping cardboard or a thick layer of newspaper. Remove all tape and labels from the cardboard, and use at least 10 to 15 sheets of newspaper to form a sufficient barrier.
The edges of the cardboard or newspaper must overlap by at least six inches to prevent grass or weeds from growing through the gaps. Once this light-suppressing layer is in place, thoroughly saturate it with water until it is completely soggy. Wetting the barrier helps it stick to the ground, prevents it from blowing away, and jump-starts the decomposition process by supporting soil microorganisms.
Next, apply a heavy layer of organic mulch on top of the wet barrier. This layer can be a mix of compost, leaves, straw, or wood chips, spread to a depth of four to eight inches. The organic layer holds the cardboard down, retains moisture, and will eventually decompose into rich soil for the new garden. This multi-layered structure suppresses the grass underneath while simultaneously building a nutrient-rich environment for future planting.
Using Opaque Plastic or Tarps
This alternative method uses non-porous synthetic materials, such as opaque black plastic sheeting or tarps. Unlike sheet mulching, this technique relies on complete light blockage and heat buildup, a process known as solarization. The dark material absorbs solar radiation, raising the temperature of the soil beneath it. This heat effectively sterilizes the top layer of soil, killing grass, weeds, and sometimes pathogens.
To employ this method, mow the grass very short and water the area thoroughly. Lay the opaque plastic or tarp directly over the prepared area, ensuring the material extends beyond the planned garden bed edges. Secure the edges of the plastic tightly to the ground using heavy rocks, bricks, or by burying the edges in a shallow trench. This seal prevents light from entering and traps the heat and moisture inside.
The duration for this technique is shorter than sheet mulching, often requiring six to eight weeks of consistent, intense sun and heat for the grass to fully die off. This method is highly effective for complete grass removal, though it does not contribute organic matter to the soil. Once the grass is dead, the plastic must be removed and properly disposed of, as it does not break down naturally.
Preparing the Soil After Smothering
Once the smothering period is complete and the grass has fully died, the soil is ready for final preparation. Inspect the area closely for any tenacious weeds or grass runners that may have survived the process. Any remaining live vegetation should be manually pulled out or spot-treated to prevent regrowth in the new garden.
If the sheet mulch technique was used, the decomposed cardboard and organic layers can be left in place and lightly incorporated into the topsoil. This natural breakdown has already added valuable organic matter, improving the soil structure and fertility. For areas treated with plastic, or if decomposition is incomplete, a layer of quality compost should be spread across the surface.
Incorporating additional amendments, such as balanced organic fertilizer or a soil conditioner, can further enrich the planting area. The goal is to create a loose, aerated, and nutrient-rich environment for the new plants. Finally, the surface should be raked smooth and level, making it ready for the immediate planting of seeds or seedlings.