A raised garden bed is a frame, typically made of wood, metal, or stone, that sits directly on the ground, containing soil above the native grade. Lining a raised bed involves placing a barrier material between the soil and the frame or between the soil and the ground beneath the bed. The decision to install this barrier is not a simple yes or no answer; it depends entirely on the construction materials and the conditions of the location where it is placed. Understanding the site-specific threats is the only way to determine if a liner is appropriate for your gardening project.
Specific Scenarios That Require Lining
Lining becomes necessary when the materials used for the bed or the environment beneath it pose a threat to the soil or the plants. One common reason to line a bed is to prevent chemical contamination from the frame material. For instance, older treated lumber or repurposed railroad ties often contain chemicals like chromated copper arsenate, which can leach into the soil and potentially be absorbed by edible crops. Lining the interior sides of these beds with an impermeable barrier creates a separation zone between the treated wood and the growing medium.
Another reason to use a liner relates to perimeter pest control, especially against burrowing rodents. Gophers, voles, and moles can easily enter a raised bed from below and destroy root systems, making a bottom liner a required defense. This type of lining must be a strong, hardware-based material that these animals cannot chew through or push aside. The physical barrier prevents the pests from accessing the soil column from the ground level.
A third scenario involves structural soil containment. Beds made from rough-cut lumber, loosely stacked stone, or cinder blocks can allow fine soil particles to wash out through the sides over time. A liner placed along the inner perimeter prevents this erosion, keeping the soil volume stable and maintaining the bed’s structural integrity. This maintains the soil and nutrient levels within the confined space.
Potential Drawbacks of Using a Liner
While liners address specific risks, their improper use can introduce problems related to water management. Impermeable materials, such as plastic sheeting, can severely impede drainage if placed across the bottom of the raised bed. Trapped excess water creates an anaerobic environment that leads to root rot and starves the plant roots of oxygen, which is detrimental to the health of most garden vegetables.
Lining the wooden sides of a raised bed can accelerate its decay. When a liner is placed directly against the wood, moisture that seeps between the soil and the liner becomes trapped against the frame. This trapped moisture cannot evaporate easily, maintaining a constantly damp condition that encourages fungal growth and speeds up the decomposition process of the wood.
Most plastic liners will eventually degrade due to exposure to soil, water, and UV radiation. As they break down, they introduce microplastics into the soil, which is an environmental concern within the garden ecosystem. Material degradation also adds to the ongoing cost and labor of maintaining the bed.
Selecting the Appropriate Lining Material
Choosing the correct material depends on the specific problem you are trying to solve. For perimeter pest control against burrowing animals, a bottom liner of galvanized hardware cloth is the standard solution. This material should have a mesh size of no more than one-half inch to effectively block voles and gophers. The wire gauge should be heavy enough, typically 18 to 20 gauge, to resist chewing and corrosion, and the mesh construction ensures water can drain freely without obstruction.
When lining the sides to prevent soil washout or to block chemical leaching from the frame, a breathable, non-woven landscape fabric is the preferred option. This fabric allows for the passage of air and water vapor, which helps the wooden frame dry out and prevents the creation of a perpetually damp environment. Landscape fabric effectively contains fine soil particles while still permitting healthy gas exchange within the soil column.
Impermeable plastic sheeting should be reserved only for lining the sides of beds constructed from chemically treated wood, where blocking contaminant leaching is the primary goal. If plastic is used, it should be a food-grade or UV-stabilized material and must be secured only to the interior sides of the bed, stopping well short of the bottom.