Applying a thick layer of organic mulch helps retain moisture and regulate temperature in garden beds. However, mulch alone is often insufficient to suppress tenacious perennial grasses and established weeds. The primary purpose of using an underlayment is to create a physical barrier that denies sunlight to the underlying vegetation, halting photosynthesis. This light deprivation forces existing grass and weed roots to starve and decompose. Selecting the appropriate barrier material determines the long-term health of the soil and future maintenance frequency.
Essential Site Preparation
The success of any underlayment relies on thorough preparation before installation. Existing turf and weeds must be physically removed to eliminate the bulk of the vegetation and its root systems. For small areas, use a sharp spade or grub hoe to slice just beneath the shallow root zone, typically one to two inches deep. This manual technique removes the sod layer while minimizing damage to deeper roots. For larger spaces, a rented gas-powered sod cutter can shear off the turf in manageable strips, significantly reducing labor.
Once vegetation is cleared, the soil surface should be raked smooth and level. This prevents the barrier material from bridging over uneven spots. Gaps or folds in the barrier allow weeds to find light and moisture, compromising the installation. Defining a clear, deep edge around the mulched area is also necessary to prevent lateral invasion. This physical trench or integrated edging material blocks grass runners (stolons) from creeping over or under the barrier from the adjacent lawn.
Temporary and Organic Underlayments
Materials that naturally decompose offer weed suppression while simultaneously improving the underlying soil structure. This method, often called sheet mulching, uses organic materials to smother unwanted plants by blocking incoming light. The most common biodegradable barriers are plain corrugated cardboard and newspaper.
Cardboard
When using cardboard, select non-glossy sheets and remove all plastic packing tape, staples, and shipping labels, as these items will not break down. The sheets must be overlapped by a minimum of six inches at all seams to ensure no light penetrates the barrier. Once laid, thoroughly wetting the cardboard helps it conform tightly to the soil surface and prevents displacement by wind during mulch application.
Newspaper
Newspaper provides a similar light-blocking function but requires a much thicker application. A minimum of eight to ten sheets must be layered together to achieve the necessary opacity to halt photosynthesis. The layers should be overlapped generously and immediately saturated with water to hold them in place and initiate decomposition.
The decomposition of these organic barriers is their main advantage, adding carbon and organic matter to the soil. This improves water retention and feeds beneficial soil organisms like earthworms. Cardboard typically breaks down within 9 to 12 months, while newspaper decomposes faster. Their temporary nature means the underlayment needs refreshing every one to two years, which maintains a healthy soil environment.
Permanent Synthetic Barriers
For long-term installations where minimal maintenance is desired, manufactured materials such as landscape fabric and plastic sheeting are considered. Landscape fabric is a permeable geotextile, typically made from woven or non-woven polypropylene or polyethylene. This material is designed with small pores that allow water, air, and some nutrients to pass through to the soil beneath.
Landscape Fabric
The permeability of landscape fabric helps maintain a relatively healthy soil ecosystem compared to solid plastic. It is secured to the ground using U-shaped metal staples, ensuring a tight fit. However, organic matter from the overlying mulch inevitably decomposes and settles on the fabric surface. This accumulation creates a new, thin layer of soil above the fabric where airborne weed seeds can germinate and establish roots, making subsequent weeding difficult.
Plastic Sheeting
Plastic sheeting, usually black polyethylene film, is a non-permeable material that provides absolute light and moisture blockage. While highly effective at smothering existing growth, this impermeability is detrimental to long-term soil health. The lack of air and water exchange can lead to anaerobic conditions beneath the plastic, decreasing the microbial activity necessary for nutrient cycling.
Water is unable to soak into the ground where the plastic is laid, leading to increased surface runoff and potentially depriving nearby established plant roots. If used in planted areas, holes must be manually punctured for plants and drainage, creating points where weeds can still emerge. While plastic sheeting offers a greater initial weed-killing effect, its negative impact on the soil ecosystem makes landscape fabric the preferred synthetic option for permanent beds.