A raised garden bed is a contained structure, typically made of wood, metal, or stone, that sits directly on the ground and is filled with specialized soil mix. This elevated setup allows gardeners to control soil quality, improve drainage, and reduce compaction, offering an advantage over in-ground planting. Whether a liner should be installed inside the frame depends on the specific materials used for the bed and the conditions of the ground beneath it. Deciding involves balancing the soil’s need for drainage with requirements for pest control and contamination protection.
When Lining is Not Necessary
For most installations on healthy, native soil, the best practice is to avoid lining the sides or the bottom of the bed. Leaving the bottom open allows unimpeded contact between the imported garden soil and the subsoil beneath. This connection promotes optimal drainage, allowing excess water to filter naturally into the earth below and preventing waterlogging that can lead to root disease. An open bottom also allows beneficial organisms, such as earthworms and other soil microbes, to migrate freely between the native ground and the rich organic matter within the bed. If the bed is constructed from rot-resistant, untreated lumber, like cedar or redwood, lining the interior is counterproductive. These wood types naturally resist decay, and adding a liner can trap moisture against the wood, potentially accelerating its deterioration.
Specific Situations That Require a Liner
Despite the general preference for an unlined bed, certain circumstances require the installation of a barrier for the safety of the plants and the longevity of the structure. A liner is strongly recommended if the bed is situated on ground with a known history of chemical or heavy metal contamination. In urban environments, for example, the native soil may contain elevated levels of lead or other toxins, and a physical barrier is needed to prevent plant roots from accessing these contaminants. This barrier acts as a protective shield, isolating the clean, imported soil from the polluted ground below.
Lining the sides is also necessary when using certain construction materials for the bed itself. If the frame is built with pressure-treated lumber, especially older wood treated with chromated copper arsenate (CCA), a liner creates a physical separation to prevent chemicals from leaching into the edible soil. Even newer treatments, which often use less toxic copper compounds like alkaline copper quaternary (ACQ), benefit from a liner to reduce the migration of these preservatives into the growing medium. In these cases, the liner is a preventative measure to ensure the food safety of the harvest.
For the bottom of the bed, a barrier is essential to mitigate the threat posed by sub-surface pests such as gophers, moles, and voles. These burrowing animals can quickly destroy root systems from below, but a durable, open-mesh material provides an effective deterrent. Additionally, if the bed is built from standard, untreated pine or other non-durable wood, lining the interior sides with a moisture-resistant material can significantly delay the onset of rot.
Selecting Safe and Effective Liner Materials
When a liner is deemed necessary, the material selection must be carefully considered to avoid creating new problems, such as poor drainage or chemical contamination. For side lining, which protects the wood or acts as a barrier against treated lumber, a durable, water-permeable landscape fabric is a safe and effective choice. This material allows moisture to pass through while preventing soil particles from washing out or contacting the frame, which helps extend the life of the wood. Natural materials, such as untreated burlap or heavy-duty coir mats, can also be used as side liners, offering a biodegradable option that maintains breathability.
For the bottom barrier used to deter burrowing pests, a material like half-inch galvanized hardware cloth or stainless steel mesh is the preferred choice. This sturdy wire mesh is durable enough to withstand chewing and burrowing, while its open structure ensures that excess water can drain freely into the ground below.
Materials to strictly avoid for any part of the bed include non-porous items that trap water and materials that may leach chemicals:
- Thin plastic sheeting or garbage bags, which are non-porous and encourage root rot.
- Old carpeting.
- Roofing felt.
- Vinyl siding, as these materials can contain chemicals, glues, or asphalt that may leach into the soil.