Mulch is a protective layer of material placed over the soil surface, serving multiple functions in a landscape. It primarily works to retain soil moisture by reducing evaporation and helps to moderate soil temperatures, keeping roots cooler in summer and warmer in winter. While it provides an aesthetic benefit, its most recognized practical use is suppressing weed growth. The material placed beneath the mulch determines the longevity of weed control and impacts the health of the soil below.
The Role of Underlayment
Placing a barrier beneath the mulch is a direct strategy for enhancing weed control by blocking sunlight, which prevents the germination of weed seeds. The underlayment also separates the organic mulch from the underlying soil. This separation helps prevent the mulch from rapidly mixing into the soil, which is desirable in permanent planting areas. Depending on the material chosen, the barrier can also contribute to moisture retention.
Synthetic Barriers: Fabric and Plastic
Synthetic materials offer durable, long-term options for weed suppression in areas like shrub borders or around trees. Landscape fabric, typically a woven or non-woven geotextile, is designed to be permeable, allowing water, air, and nutrients to pass through to the soil. It is often preferred over plastic for permanent plantings because it prevents the root systems of desirable plants from being choked off. Securing the fabric with staples and cutting small X-shaped holes for plants helps maximize its effectiveness as a barrier.
The major long-term issue is that decomposing organic mulch creates a layer of material on top of the fabric where new weed seeds germinate. These weeds establish roots directly into the fabric, making them difficult to pull out without tearing the barrier. Over time, the fabric can impede the natural exchange of gasses and water, and established plant roots may become entangled, potentially leading to stress and poor growth.
Plastic sheeting, often a thin polyethylene film, offers superior initial weed blocking because it is completely impermeable. It also traps heat effectively, which can be beneficial for warming the soil to encourage heat-loving crops in short-term vegetable gardens. However, this impermeability is a significant drawback in most permanent landscape settings.
By blocking the movement of water and air into the soil, plastic can lead to anaerobic conditions that harm beneficial soil microbes and plant roots. Water tends to run off the edges or pool on the surface, which increases the risk of disease and localized flooding. Plastic sheeting is not recommended for use in flower beds or around perennial plants, as it compromises the long-term health of the soil ecosystem.
Biodegradable Options: Cardboard and Paper
Biodegradable materials like corrugated cardboard and newspaper offer an environmentally responsible alternative to synthetic barriers. These options are effective for smothering existing weeds and grass when establishing a new planting bed. Composed primarily of cellulose, they decompose over one or two seasons, adding organic carbon material to the soil structure and attracting earthworms.
When using cardboard, remove all plastic tape, staples, and glossy labels, as these materials do not break down. The cardboard should be laid directly over the area, with edges overlapped by at least six to eight inches to prevent weeds from pushing through the seams. For newspaper, a thickness of about ten sheets is required to create an effective light barrier.
After laying down the material, it must be thoroughly wetted down to prevent it from blowing away and to initiate decomposition. The mulch is then applied on top, holding the paper or cardboard in place. This method provides temporary weed suppression while simultaneously improving the soil’s composition and aeration.
Direct Application: Mulching on Bare Soil
In many gardening scenarios, especially in vegetable gardens and annual flower beds, the best practice is to forgo an underlayment and apply organic mulch directly to the bare soil. This approach prioritizes long-term soil health and fertility. As the mulch decomposes, it integrates into the top layer of soil, providing organic matter that improves drainage and encourages beneficial microorganisms.
When mulching directly on soil, the depth of the layer becomes the sole mechanism for weed suppression. A layer of organic mulch, such as wood chips or shredded bark, should be maintained at a depth of three to four inches. This thickness effectively blocks sunlight and physically impedes weed seedlings from emerging.
While this method requires more consistent monitoring and replenishment of the mulch layer each year, it ensures that water and nutrients penetrate the soil without impedance. The long-term benefits of enhanced soil structure and reduced need for external fertilizers often outweigh the slightly increased maintenance of managing surface weeds that may sprout in the decomposing mulch layer.