A raised bed is a large container placed directly on the ground, filled with imported soil mix to create an optimal growing environment for plants. The decision to install a physical barrier underneath depends heavily on the native soil conditions and the gardener’s primary goals. There is no universal answer, as a barrier that prevents weed growth may also restrict beneficial soil processes. The choice involves balancing short-term weed control with the long-term health of the garden bed ecosystem.
The Case for Using a Barrier
A physical barrier provides an advantage by blocking aggressive weeds and deterring burrowing pests. In areas with tenacious, deep-rooted perennial weeds like Bermuda grass or nut sedge, a barrier prevents these invaders from growing upward into the new soil.
Preventing subterranean pests is a primary reason to install a lining. If gophers, voles, or moles are an issue, a physical barrier is necessary to protect plant roots and tubers. For this purpose, hardware cloth, a woven metal mesh, is used instead of fabric. The small, rigid openings in the mesh block animals while still permitting water and desirable plant roots to penetrate the native soil below.
Potential Negative Effects of Barriers
Using non-biodegradable, synthetic materials like landscape fabric or plastic sheeting creates several long-term problems. These materials often impede the natural movement of water, causing pooling or runoff, which can lead to soggy or compacted soil. Over time, fine pores in the fabric clog with silt and organic matter, reducing air and water permeability.
A synthetic barrier isolates the raised bed soil from the native ground, hindering soil health. Beneficial organisms like earthworms are prevented from migrating upward to aerate and enrich the imported soil mix. Although the fabric initially suppresses weeds, wind-blown seeds germinate on the soil layer accumulating on top of the fabric. Weeds whose roots grow into the fabric become difficult to pull without tearing the material and scattering plastic fragments.
Effective Alternatives to Fabric
Biodegradable materials offer a sustainable alternative to synthetic fabrics. They provide initial weed suppression while eventually decomposing to improve soil structure. Corrugated cardboard is a recommended option, as it is thick enough to block light and smother existing vegetation. Use a single layer of cardboard, ensuring all plastic tape and labels are removed before installation.
Newspaper can also be used as a barrier, provided it is applied in a thick layer, typically eight to twelve sheets deep, using only black-and-white print. Both cardboard and newspaper function by cutting off light to the plants beneath, but they remain permeable to water. As these materials break down, earthworms consume them, allowing the raised bed soil to integrate with the native soil and adding organic matter.
Preparing the Ground Surface
Proper site preparation is necessary before placing any raised bed, regardless of whether a barrier is used. The chosen area should be leveled to ensure the bed rests evenly, which prevents soil washout and maintains uniform water distribution. Remove any large rocks, sticks, or debris that could interfere with the bed’s base.
Aggressive perennial weeds should be cut back or dug out entirely before the bed is set in place. While a barrier helps, rhizomatous roots from weeds like quackgrass or bindweed can often find a way around the edges of a liner. For compacted or poorly draining native soil, use a digging fork to loosen the surface before installation to improve long-term root growth.