Landscape fabric is used widely in landscaping and gardening projects. Its primary function is to act as a physical separator, stabilizing ground cover materials while suppressing the growth of unwanted plants beneath. A common question among homeowners is whether a layer of soil should be placed directly on top of this fabric. Applying standard topsoil or garden soil to the surface is generally counterproductive to the fabric’s intended function. Understanding how landscape fabric works reveals why other materials are better suited for the top layer.
The Primary Function of Landscape Fabric
Landscape fabric serves two main purposes: weed suppression and material separation. It creates a physical barrier that blocks sunlight from reaching the soil beneath, preventing dormant weed seeds from germinating and growing upward. The material is manufactured to be porous, allowing water, air, and nutrients to pass through to the soil and plant roots below.
The fabric also functions as a separation layer, preventing the subsoil from mixing with the decorative covering material placed on top. This is useful in pathways and areas covered with gravel or rock, where it keeps the stone clean and prevents it from sinking into the underlying soil. Different types of fabric exist, such as woven polypropylene for stabilization and non-woven fabrics for filtration and drainage in planting beds.
Why Soil Above Fabric Defeats the Purpose
Adding a layer of soil above the fabric introduces a new medium for weed growth, effectively bypassing the barrier’s function. Airborne weed seeds, carried by wind or animals, will land in this new soil layer and find favorable conditions for germination. Once these surface weeds establish roots, they grow downward and penetrate the fabric’s weave or pores.
When surface roots are intertwined with the fabric, pulling the weeds becomes nearly impossible without tearing the barrier. Tearing the fabric creates holes for new weeds to emerge. Furthermore, the soil particles on top will begin to clog the fabric’s pores over time, impeding the downward flow of water and air. This clogging can lead to water pooling on the surface or cause the soil beneath the fabric to dry out and become compacted due to restricted gas exchange. The moisture retention from the soil layer on top can also accelerate the fabric’s breakdown, reducing its lifespan.
Recommended Materials for Covering Landscape Fabric
The fabric should be covered with materials that suppress weeds by blocking light but do not provide a rich rooting medium. Organic mulches, such as wood chips, shredded bark, or pine needles, are effective and allow water to percolate through to the fabric below. These materials should be applied in a layer between two and three inches deep to block light effectively.
Inorganic materials, including gravel, crushed stone, or river rock, are excellent choices for covering the fabric, particularly in pathways or low-maintenance areas. A layer of one to two inches is sufficient to provide coverage and prevent the fabric from being exposed to sunlight. Both organic and inorganic cover materials shield the fabric from damaging ultraviolet light, extending its functional life. Organic mulches will decompose over several years and require periodic replenishment to maintain the effective depth.
Specific Scenarios for Placing Soil Over Fabric
While not recommended for passive weed control, there are specialized applications where placing soil over landscape fabric is intentional. In these cases, the fabric is used purely as a separation layer rather than as the primary weed barrier. For instance, in the construction of raised garden beds, a layer of fabric is often placed at the bottom before adding the growing medium.
The fabric prevents the more expensive, lighter potting mix from sifting into the native subsoil below or escaping through gaps in the bed frame. Another specialized use is on very steep slopes, where a thin layer of fabric can be used beneath soil or compost to temporarily stabilize the surface. This technique helps prevent immediate erosion while new groundcover plants establish their root systems. In these specific circumstances, the intent is controlled planting, not long-term weed suppression.