Many gardeners seek methods to clear areas of turf and weeds without relying on chemical herbicides. A popular and environmentally sound technique involves using a layered approach, often called sheet mulching. This process utilizes a temporary barrier placed directly over the existing vegetation before adding an organic top layer. This strategy effectively prepares the soil for new plantings while simultaneously eliminating undesirable growth.
The Biological Principle of Smothering
The efficacy of this method relies on interrupting the fundamental biological process of photosynthesis. By completely blocking sunlight, the barrier prevents the grass blades from converting light energy into chemical energy necessary for growth and survival. This immediate cessation of energy production is the primary mechanism of grass suppression.
Furthermore, the physical barrier restricts gas exchange and significantly alters the microclimate directly beneath it. While some moisture is retained, the lack of air circulation and light places immense stress on the plant roots and rhizomes. The grass is forced to utilize its stored carbohydrates to try and push shoots around the barrier.
When deprived of light for an extended period, the plant’s stored energy reserves become depleted in a futile effort to reach the surface. Once these reserves are exhausted, the grass dies and begins to decompose in place. This decomposition naturally adds organic matter, improving the soil structure and nutrient profile for future use.
Barrier Material Options for Grass Suppression
Unwaxed corrugated cardboard is generally considered the most effective biodegradable barrier for killing grass. Its thickness provides superior light blockage, ensuring that even faint light cannot penetrate to sustain photosynthesis. Cardboard also breaks down relatively slowly, often lasting a full growing season, which is sufficient to eliminate persistent turfgrass varieties.
An additional benefit is that cardboard retains moisture, creating a damp environment that encourages soil microorganisms and earthworms to begin the decomposition process. Before application, all plastic tapes, staples, and glossy labels must be removed to ensure the material remains fully biodegradable and non-toxic to the soil.
Newspaper is a viable alternative, though it requires significantly more layering than cardboard to achieve the same level of light exclusion. A minimum of six to ten sheets of newspaper should be used to create an opaque layer capable of suppressing turf. The ink used in modern newspapers is generally soy-based and safe for garden use.
The primary drawback of newspaper is its rapid rate of breakdown, especially in wet conditions. While this quick decomposition benefits the soil structure, it may necessitate reapplication if dealing with aggressive perennial weeds or rhizomatous grasses.
Synthetic materials like landscape fabric or plastic sheeting offer immediate and near-total light blockage. However, these options present long-term disadvantages for establishing a healthy garden bed. Plastic sheeting inhibits water and air from reaching the soil, negatively impacting beneficial soil fauna and microorganisms. Landscape fabric, while porous, often becomes embedded in the soil over time and must be removed later, potentially disrupting the soil ecosystem. For permanent planting areas, biodegradable barriers are preferred over synthetic ones.
Step-by-Step Application Sequence
The first step in preparing the area involves reducing the height of the existing vegetation. Tall grass and weeds should be cut down as close to the soil surface as possible, using a lawnmower or string trimmer. Do not remove the clippings, as they will add organic matter to the soil as they decompose beneath the barrier.
This initial reduction minimizes the chance of the grass piercing the barrier layer as it makes its initial attempt to grow toward the light. The area should be relatively level and free of large debris before the barrier material is introduced.
Once the area is prepped, the chosen barrier material, such as cardboard or layered newspaper, should be laid directly over the ground. The most frequent failure point is inadequate coverage, allowing grass to escape through the seams. To prevent this, each piece of cardboard or section of newspaper must overlap the adjacent piece by a minimum of six to eight inches. This substantial overlap ensures a continuous, light-proof seal across the entire area, preventing lateral growth from finding a gap.
Immediately after the barrier is positioned, it must be thoroughly saturated with water. Wetting the material helps it conform to the contours of the ground, prevents it from blowing away, and initiates the decomposition process more quickly.
Before applying the final mulch, an optional but beneficial step is to spread a thin layer of compost or aged manure directly onto the wet barrier. This intermediate layer helps introduce beneficial microbes and nutrients, accelerating the breakdown of the cardboard and enriching the soil beneath.
The final layer of organic mulch, such as wood chips or shredded bark, should then be spread over the entire area. This layer must be applied at a depth of four to six inches. This substantial depth is necessary to weigh down the barrier, insulate the soil, and maintain a consistent dark, moist environment underneath. The success of this smothering technique is not immediate; the grass requires time to fully exhaust its energy reserves. Depending on the type of turfgrass and local climate conditions, complete elimination typically occurs over the course of one full growing season.