Cardboard mulching, often called sheet mulching, is a popular, low-cost technique used to suppress unwanted vegetation and prepare new garden beds. This method involves laying sheets of cardboard over a designated area, covering them with organic material, and allowing them to break down naturally. Cardboard is highly effective as a temporary, organic barrier that aids in soil improvement. It provides initial weed control, setting the stage for a healthier planting area without synthetic materials or chemicals.
The Mechanism of Weed Suppression
Cardboard’s effectiveness as a weed barrier stems from its ability to block light, which is essential for photosynthesis. By creating an opaque layer over the soil, the cardboard smothers existing weeds and turf by depriving them of sunlight. This process is useful for killing off large areas of grass or weeds before establishing a new garden bed.
The physical barrier also prevents weed seeds already in the soil from germinating and blocks new airborne seeds from reaching the soil. For this technique to be successful, it is important to select the right material. Plain brown corrugated cardboard is the most suitable choice, as it is composed of natural cellulose fibers and starch-based glues. Avoid glossy, heavily dyed, or wax-coated cardboard, as these contain plasticizers or compounds that impede decomposition or introduce chemicals. All plastic tapes, staples, and shipping labels must be removed before use, as they are not biodegradable.
Proper Installation Techniques
Preparing the area involves cutting or mowing existing vegetation down to the ground, but typically does not require labor-intensive weeding or tilling. Lay the cardboard directly over the flattened weeds or lawn, ensuring pieces overlap by at least six to eight inches at the seams. This generous overlap prevents persistent weeds from finding a path to light between the sheets.
Once positioned, the cardboard must be thoroughly saturated with water until it is heavy and pliable. Soaking the material prevents it from becoming hydrophobic, which causes water to run off instead of penetrating the soil if dry cardboard is exposed to rain. Finally, apply a thick layer of organic material, such as wood chips, straw, or compost, immediately on top of the wet cardboard. This covering holds the cardboard in place against the wind, conceals the material, and aids decomposition. A layer of three to four inches of organic mulch is generally recommended for optimal results.
Environmental Trade-offs and Decomposition
Cardboard is a temporary weed barrier, but its decomposition offers significant long-term benefits to soil health, a major advantage over non-biodegradable landscape fabrics. As a wood product, cardboard is rich in carbon, a necessary nutrient for soil microorganisms. As the material breaks down, it integrates into the soil structure, improving aeration and water infiltration.
The dark, moist environment beneath the cardboard attracts earthworms and other beneficial soil organisms, which accelerate breakdown. Earthworms consume the decomposing cellulose and aerate the soil as they move, leaving behind nutrient-rich castings. Cardboard also helps retain soil moisture by reducing evaporation, which is beneficial in dry or hot climates. However, the high carbon content may temporarily cause soil microbes to draw nitrogen from the surrounding soil to fuel decomposition, a phenomenon known as temporary nitrogen immobilization. This potential drawback can be countered by applying nitrogen-rich materials like compost on top of the cardboard layer.
Practical Limitations and Troubleshooting
While effective, the cardboard barrier is not a permanent solution and typically lasts between six and twelve months, depending on material thickness and environmental moisture levels. This temporary nature means the process may need to be repeated or supplemented with other mulching techniques over time. Aggressive perennial weeds, such as bindweed or nutsedges, can be persistent and may push through the barrier, especially if the overlap is insufficient.
Uncovered sections of cardboard are susceptible to being lifted by strong winds, which compromises the barrier and exposes underlying weeds to light. A heavy top layer of mulch is required primarily to prevent this issue. In consistently wet conditions, the dark, moist environment under the cardboard can unintentionally provide habitat for pests like slugs and snails. Because cardboard is cellulose-based, placing the material directly against wooden structures, such as a house foundation or deck, is generally not recommended as it could attract termites.