Weed barrier, often referred to as landscape fabric, is a geotextile material used to suppress unwanted growth by blocking sunlight from reaching the soil beneath it. The material is designed to be porous, allowing water and air to pass through to the soil. While it is physically possible to place a weed barrier directly over a weedy area, success depends on proper preparation and understanding the fabric’s limitations.
The Immediate Answer: Smothering Existing Weeds
Placing a weed barrier over existing weeds works by utilizing the principle of light deprivation, effectively smothering the plants by cutting off their energy source. The fabric blocks the sunlight required for photosynthesis, which causes the vegetation underneath to eventually die back. This mechanism is most successful when targeting small, newly germinated annual weeds that rely on a single growing season and have shallow root systems.
The effectiveness changes when dealing with established perennial weeds, which are characterized by deep, extensive root systems, rhizomes, or tubers. Aggressive perennial weeds, such as bindweed or certain grasses, have stored energy reserves that allow them to persist for long periods without sunlight. These plants may push through the fabric’s weave, exploit any small tear, or simply grow around the edges of the barrier to reach the light. In these cases, the fabric only offers a temporary setback to the plant. The process of smothering is accelerated when the fabric is covered with a layer of mulch or gravel, which further blocks light.
Essential Preparation for Success
Successful installation of a weed barrier over existing growth requires significant preparation to maximize the fabric’s smothering effect. Although the goal is to avoid extensive root removal, any large, established vegetation must be reduced before the fabric is laid. The first step involves cutting down all tall weeds and grasses to ground level using a string trimmer or shears. This prevents the plant material from creating large, uneven mounds beneath the barrier, which could make the fabric installation difficult.
Next, the area must be cleared of any sharp debris, including sticks, rocks, or construction remnants, as these items can easily puncture the fabric. The soil surface should then be raked smooth to create a flat foundation for the barrier. Leveling the soil ensures the fabric lies flat and makes consistent contact with the ground across the entire area, which is important for uniform light exclusion. This pre-installation work reduces physical obstacles, promoting better contact with the soil for a more effective smothering action.
Installation Techniques Over Existing Weeds
Once the area is prepared, the physical act of laying the fabric must be meticulous to prevent weeds from exploiting seams or edges. The landscape fabric should be unrolled and stretched tautly across the prepared ground, ensuring there are no large wrinkles or folds. When multiple pieces of fabric are needed to cover the area, the seams must be overlapped by a minimum of 6 to 12 inches. Overlapping the edges significantly reduces the chance of underground weeds finding a path to light.
The perimeter and all overlapping seams must be secured using U-shaped landscape staples, typically 6-inch steel pins. These staples should be driven through the fabric and into the ground every few feet along the edges and down the center of the seams to prevent shifting or lifting. If the area contains desirable plants, the fabric must be carefully cut to fit around them, using an X-shaped incision to create an opening for the plant’s trunk or stem. The cut fabric flaps should be folded in toward the base of the plant to minimize the exposed soil area, and the flaps should be secured with additional staples.
Long-Term Drawbacks and Limitations
While a weed barrier may offer immediate control, its use over weeds introduces several long-term limitations that can eventually undermine its effectiveness. Even after being cut down, persistent perennial weeds can still attempt to grow, often piercing the fabric with sharp shoots or growing along the edges to find an exit point. When these weeds become entangled in the fabric’s weave, removal becomes extremely difficult, sometimes requiring the entire section of fabric to be cut out.
Over time, the barrier negatively impacts the health of the soil beneath it, as it acts as a separator between the ground and organic matter. Leaves, dust, and decomposed mulch accumulate on top of the fabric, preventing the natural incorporation of organic material into the soil. This lack of organic matter can inhibit the activity of beneficial soil organisms like earthworms, leading to increased soil compaction. The accumulation of debris on top of the barrier creates a seedbed for new weed growth, allowing wind-blown seeds to germinate in the fine layer of organic matter above the fabric.