A weed barrier, often called landscape fabric, is a synthetic material designed to be laid over soil and covered with mulch or aggregate. Its primary purpose is to suppress the growth of weeds by blocking sunlight from reaching weed seeds already in the ground. The fabric also serves a secondary function of keeping decorative materials like gravel or wood chips from sinking and mixing into the underlying soil. The question of whether to remove existing grass before installation is a common one that holds the answer to the project’s long-term success. Ignoring the vegetation already present will almost certainly lead to the failure of the weed control system.
The Necessity of Complete Vegetation Removal
The direct answer to whether grass must be removed is a definitive yes, especially if the existing vegetation includes aggressive perennial grasses. Landscape fabric is primarily effective against the germination of new seeds, not the established, robust root systems of perennial plants. These established grasses possess specialized underground structures that allow them to persist and bypass the barrier.
Perennial grasses, such as Bermuda or Johnson grass, often spread using rhizomes or stolons, which are underground and above-ground stems, respectively. These structures store energy and allow the plant to spread horizontally across the landscape. When covered by fabric, these grasses do not simply die; they search for light and moisture, and their sharp-tipped shoots can easily puncture the woven or non-woven material to reach the surface.
If the grass and weeds are merely covered, their decomposition creates a layer of organic material between the soil and the fabric. This trapped, decaying matter eventually forms a nutrient-rich medium for new airborne weed seeds to germinate directly on top of the barrier. The roots of these new weeds then grow down, intertwining with the fabric, which makes future weeding nearly impossible without tearing the barrier.
Detailed Methods for Ground Preparation
Once the necessity of removal is understood, several methods can be used to clear the area, depending on the size of the project and the preference for using chemicals. Manual removal is the most immediate and chemical-free method, involving digging up the sod and grass with a flat-edged shovel or a sod cutter. When removing the grass, it is crucial to lift the top layer of soil, typically two to four inches, to ensure all horizontal rhizomes and root fragments are completely taken out of the area.
Use of a non-selective herbicide chemically kills all vegetation it contacts. These products must be applied according to label instructions, requiring a waiting period of one to three weeks for the grass to fully die back before the dead material is raked away. This approach is highly effective for aggressive grasses but requires careful application to avoid affecting desirable nearby plants.
Smothering, or occultation, is a slower, passive approach that uses opaque materials like black plastic or cardboard to block all sunlight. This deprives the grass of the energy needed for photosynthesis, effectively killing it. This method requires the material to remain in place for at least four to six weeks during the growing season to ensure the entire plant is dead.
A faster, heat-based variation is solarization, which involves covering the area with clear plastic sheeting during the hottest part of the summer. The clear plastic creates a greenhouse effect, raising the soil temperature to levels that can kill not only the existing vegetation but also many weed seeds and pathogens in the upper soil profile. For solarization to be effective, the soil should be moistened before covering, and the plastic edges must be buried to trap the heat, with the process requiring four to eight weeks of consistent heat.
Before laying the barrier, regardless of the removal method used, the ground must be raked smooth, level, and free of any sharp rocks or debris. A smooth surface prevents the fabric from tearing prematurely and ensures a consistent, flat base for the final mulch or aggregate layer.
Ensuring Long-Term Weed Control
Selecting the correct type of landscape fabric is important for the barrier’s performance and longevity after the ground has been prepared. Woven polypropylene fabrics are generally more durable and are often recommended for use under stone, gravel, or pavers where heavy-duty stability is needed. Non-woven fabrics tend to be more permeable, allowing better water and air exchange, making them a better choice for flower beds where soil health is a greater concern.
The lifespan of any weed barrier is heavily dependent on the material used to cover it, as ultraviolet (UV) radiation from the sun causes the plastic or polypropylene fabric to degrade over time. A minimum covering of three inches of organic mulch or stone is necessary to protect the fabric from UV damage and physical wear. This top layer shields the fabric.
A common oversight is failing to understand that the top layer of mulch or aggregate will eventually collect organic debris and wind-blown dust. This accumulation of material creates a new, shallow growing medium on top of the fabric. Weed seeds that land in this new layer will germinate and root above the barrier, requiring regular maintenance to prevent the roots from growing down and into the fabric.