The technique of using cardboard to eliminate unwanted vegetation is a highly effective, organic method known as sheet mulching. This process works by denying weeds the sunlight necessary for photosynthesis, effectively smothering them. Unlike chemical herbicides, this approach is environmentally sound, as the cardboard and subsequent layers naturally decompose, adding beneficial carbon material to the earth over time. The goal is to create a dense, opaque barrier that starves existing plant life and prevents new weed seeds from germinating.
Selecting the Right Materials and Site Preparation
Choosing the correct cardboard is the first step toward a successful sheet mulching project; plain brown, corrugated material is the best option. Avoid glossy cardboard, which may contain plastic films, and wax-coated boxes, as these materials will not break down easily. Before laying down any material, meticulously remove all non-paper elements, including plastic packing tape, shipping labels, and metal staples.
Preparing the site properly will maximize the effectiveness of the smothering process. If the existing weeds or grass are tall, cutting them down close to the soil surface is recommended, though complete removal is not necessary. Slightly moistening the ground before application encourages decomposition and ensures a better seal between the cardboard and the soil. This moisture also helps secure the first sheets of cardboard while you work.
Laying the Cardboard for Effective Weed Suppression
The key to a successful weed barrier is total light exclusion, requiring careful layering of the cardboard. Use at least two to three layers of standard-thickness cardboard to ensure no light penetrates the barrier once it is covered. To prevent aggressive weeds from emerging, the edges of the individual pieces must overlap significantly, ideally by six to eight inches.
Any gaps or seams that are not securely overlapped become failure points where weeds will quickly emerge. When working around permanent obstacles like existing plants or trees, rip and fold the cardboard to fit tightly against the base. Immediately after laying the cardboard, thoroughly soak it with water. Soaking makes the material pliable, heavy, and resistant to wind, while also initiating the breakdown process and appealing to beneficial soil organisms.
Covering the Cardboard and Monitoring Results
Once the cardboard layer is securely in place and saturated, cover it with a thick layer of organic material, such as wood chips, straw, or compost. This covering serves several purposes: it holds the cardboard down, improves aesthetics, and provides a continuous food source for microbes. A dense layer of three to six inches of organic mulch is recommended to ensure the cardboard stays moist and fully obscured.
The process of weed suppression takes time, with results typically visible within three to six months as the smothered vegetation dies back. Monitor the area for any weeds that push through small tears or gaps. These breakthroughs can be spot-treated by placing an additional piece of cardboard and mulch over the area. If the area is intended for planting, create a space for a new plant by cutting an “X” shape through the mulch and cardboard layers.