Flattened cardboard boxes are a readily available material increasingly repurposed by gardeners. Using cardboard sheets beneath mulch is an effective, organic strategy for controlling unwanted plants in landscape beds and garden areas. This technique, known as sheet mulching, prepares new planting areas by suppressing existing vegetation without chemical herbicides or intensive digging. It uses a simple, physical barrier to manage weeds, offering an environmentally conscious alternative to traditional gardening practices.
How Cardboard Blocks Weed Growth
Cardboard is an effective weed suppressant primarily because it creates a complete barrier to light. Plants require light for photosynthesis, which produces the energy needed for growth and survival. Placing a thick, opaque layer of cardboard over weeds deprives them of sunlight, causing them to exhaust stored energy reserves and eventually die back.
The physical barrier also acts as a smothering agent, preventing existing weed shoots from pushing through to the surface. It also prevents weed seeds already in the soil from germinating, as many require light to sprout. Furthermore, the cardboard layer helps moderate soil temperature and significantly reduces water evaporation, benefiting desirable plants once established.
Essential Steps for Proper Application
Effective application requires careful preparation of the cardboard material. Before laying the sheets, remove all non-biodegradable elements, such as plastic tape, shipping labels, and metal staples. Use plain, brown corrugated cardboard free of glossy finishes or colored inks to avoid introducing unwanted substances into the soil. Laying a single, extra-thick layer or two standard layers ensures the barrier is opaque enough to block all light.
When placing the cardboard, overlap sheets generously (six to eight inches) to prevent gaps that weeds could exploit and to account for shifting. Thoroughly saturate the sheets with water to make them pliable, secure them against wind, and initiate decomposition. Finally, cover the entire layer with several inches of organic mulch, such as wood chips or compost, to hold the cardboard down and suppress surface-germinating weed seeds.
Comparing Cardboard to Synthetic Barriers
Cardboard offers distinct advantages over non-biodegradable alternatives like landscape fabric or plastic sheeting because of its organic nature. Unlike synthetic barriers that remain in the soil indefinitely, cardboard naturally decomposes. This breakdown usually occurs within six months to two years, depending on climate and thickness. Decomposition enriches the soil by contributing organic matter, which enhances soil structure and fertility.
Synthetic barriers impede the necessary exchange of air and water, potentially creating an unhealthy environment for beneficial soil organisms. Landscape fabrics do not break down and can eventually shred, leaving synthetic fragments in the garden and making removal difficult. Cardboard promotes decomposition, inviting earthworms and other soil fauna that naturally aerate the soil and improve its tilth. Using recycled cardboard offers a zero-cost, sustainable solution, diverting waste from landfills while improving the garden ecosystem.